Report errors in external files (#6308)

Co-authored-by: Laurenz <laurmaedje@gmail.com>

25 files changed, 995 insertions, 524 deletions

diff --git a/crates/typst-cli/src/compile.rs b/crates/typst-cli/src/compile.rs
index 4edb4c32..207bb7d0 100644
--- a/crates/typst-cli/src/compile.rs
+++ b/crates/typst-cli/src/compile.rs
@@ -16,7 +16,7 @@ use typst::diag::{
 use typst::foundations::{Datetime, Smart};
 use typst::html::HtmlDocument;
 use typst::layout::{Frame, Page, PageRanges, PagedDocument};
-use typst::syntax::{FileId, Source, Span};
+use typst::syntax::{FileId, Lines, Span};
 use typst::WorldExt;
 use typst_pdf::{PdfOptions, PdfStandards, Timestamp};
 
@@ -696,7 +696,7 @@ fn label(world: &SystemWorld, span: Span) -> Option<Label<FileId>> {
 impl<'a> codespan_reporting::files::Files<'a> for SystemWorld {
     type FileId = FileId;
     type Name = String;
-    type Source = Source;
+    type Source = Lines<String>;
 
     fn name(&'a self, id: FileId) -> CodespanResult<Self::Name> {
         let vpath = id.vpath();
diff --git a/crates/typst-cli/src/timings.rs b/crates/typst-cli/src/timings.rs
index 9f017dc1..3d10bbc6 100644
--- a/crates/typst-cli/src/timings.rs
+++ b/crates/typst-cli/src/timings.rs
@@ -85,6 +85,6 @@ fn resolve_span(world: &SystemWorld, span: Span) -> Option<(String, u32)> {
     let id = span.id()?;
     let source = world.source(id).ok()?;
     let range = source.range(span)?;
-    let line = source.byte_to_line(range.start)?;
+    let line = source.lines().byte_to_line(range.start)?;
     Some((format!("{id:?}"), line as u32 + 1))
 }
diff --git a/crates/typst-cli/src/world.rs b/crates/typst-cli/src/world.rs
index 2da03d4d..f63d34b6 100644
--- a/crates/typst-cli/src/world.rs
+++ b/crates/typst-cli/src/world.rs
@@ -9,7 +9,7 @@ use ecow::{eco_format, EcoString};
 use parking_lot::Mutex;
 use typst::diag::{FileError, FileResult};
 use typst::foundations::{Bytes, Datetime, Dict, IntoValue};
-use typst::syntax::{FileId, Source, VirtualPath};
+use typst::syntax::{FileId, Lines, Source, VirtualPath};
 use typst::text::{Font, FontBook};
 use typst::utils::LazyHash;
 use typst::{Library, World};
@@ -181,10 +181,20 @@ impl SystemWorld {
         }
     }
 
-    /// Lookup a source file by id.
+    /// Lookup line metadata for a file by id.
     #[track_caller]
-    pub fn lookup(&self, id: FileId) -> Source {
-        self.source(id).expect("file id does not point to any source file")
+    pub fn lookup(&self, id: FileId) -> Lines<String> {
+        self.slot(id, |slot| {
+            if let Some(source) = slot.source.get() {
+                let source = source.as_ref().expect("file is not valid");
+                source.lines()
+            } else if let Some(bytes) = slot.file.get() {
+                let bytes = bytes.as_ref().expect("file is not valid");
+                Lines::try_from(bytes).expect("file is not valid utf-8")
+            } else {
+                panic!("file id does not point to any source file");
+            }
+        })
     }
 }
 
@@ -339,6 +349,11 @@ impl<T: Clone> SlotCell<T> {
         self.accessed = false;
     }
 
+    /// Gets the contents of the cell.
+    fn get(&self) -> Option<&FileResult<T>> {
+        self.data.as_ref()
+    }
+
     /// Gets the contents of the cell or initialize them.
     fn get_or_init(
         &mut self,
diff --git a/crates/typst-layout/Cargo.toml b/crates/typst-layout/Cargo.toml
index 438e09e4..cc355a3d 100644
--- a/crates/typst-layout/Cargo.toml
+++ b/crates/typst-layout/Cargo.toml
@@ -30,6 +30,7 @@ icu_provider_adapters = { workspace = true }
 icu_provider_blob = { workspace = true }
 icu_segmenter = { workspace = true }
 kurbo = { workspace = true }
+memchr = { workspace = true }
 rustybuzz = { workspace = true }
 smallvec = { workspace = true }
 ttf-parser = { workspace = true }
diff --git a/crates/typst-layout/src/image.rs b/crates/typst-layout/src/image.rs
index 8136a25a..a8f4a0c8 100644
--- a/crates/typst-layout/src/image.rs
+++ b/crates/typst-layout/src/image.rs
@@ -1,6 +1,6 @@
 use std::ffi::OsStr;
 
-use typst_library::diag::{warning, At, SourceResult, StrResult};
+use typst_library::diag::{warning, At, LoadedWithin, SourceResult, StrResult};
 use typst_library::engine::Engine;
 use typst_library::foundations::{Bytes, Derived, Packed, Smart, StyleChain};
 use typst_library::introspection::Locator;
@@ -27,17 +27,17 @@ pub fn layout_image(
 
     // Take the format that was explicitly defined, or parse the extension,
     // or try to detect the format.
-    let Derived { source, derived: data } = &elem.source;
+    let Derived { source, derived: loaded } = &elem.source;
     let format = match elem.format(styles) {
         Smart::Custom(v) => v,
-        Smart::Auto => determine_format(source, data).at(span)?,
+        Smart::Auto => determine_format(source, &loaded.data).at(span)?,
     };
 
     // Warn the user if the image contains a foreign object. Not perfect
     // because the svg could also be encoded, but that's an edge case.
     if format == ImageFormat::Vector(VectorFormat::Svg) {
         let has_foreign_object =
-            data.as_str().is_ok_and(|s| s.contains("<foreignObject"));
+            memchr::memmem::find(&loaded.data, b"<foreignObject").is_some();
 
         if has_foreign_object {
             engine.sink.warn(warning!(
@@ -53,7 +53,7 @@ pub fn layout_image(
     let kind = match format {
         ImageFormat::Raster(format) => ImageKind::Raster(
             RasterImage::new(
-                data.clone(),
+                loaded.data.clone(),
                 format,
                 elem.icc(styles).as_ref().map(|icc| icc.derived.clone()),
             )
@@ -61,11 +61,11 @@ pub fn layout_image(
         ),
         ImageFormat::Vector(VectorFormat::Svg) => ImageKind::Svg(
             SvgImage::with_fonts(
-                data.clone(),
+                loaded.data.clone(),
                 engine.world,
                 &families(styles).map(|f| f.as_str()).collect::<Vec<_>>(),
             )
-            .at(span)?,
+            .within(loaded)?,
         ),
     };
 
diff --git a/crates/typst-library/Cargo.toml b/crates/typst-library/Cargo.toml
index b210637a..f4b21988 100644
--- a/crates/typst-library/Cargo.toml
+++ b/crates/typst-library/Cargo.toml
@@ -66,6 +66,7 @@ unicode-normalization = { workspace = true }
 unicode-segmentation = { workspace = true }
 unscanny = { workspace = true }
 usvg = { workspace = true }
+utf8_iter = { workspace = true }
 wasmi = { workspace = true }
 xmlwriter = { workspace = true }
 
diff --git a/crates/typst-library/src/diag.rs b/crates/typst-library/src/diag.rs
index 49cbd02c..41b92ed6 100644
--- a/crates/typst-library/src/diag.rs
+++ b/crates/typst-library/src/diag.rs
@@ -1,17 +1,20 @@
 //! Diagnostics.
 
-use std::fmt::{self, Display, Formatter};
+use std::fmt::{self, Display, Formatter, Write as _};
 use std::io;
 use std::path::{Path, PathBuf};
 use std::str::Utf8Error;
 use std::string::FromUtf8Error;
 
+use az::SaturatingAs;
 use comemo::Tracked;
 use ecow::{eco_vec, EcoVec};
 use typst_syntax::package::{PackageSpec, PackageVersion};
-use typst_syntax::{Span, Spanned, SyntaxError};
+use typst_syntax::{Lines, Span, Spanned, SyntaxError};
+use utf8_iter::ErrorReportingUtf8Chars;
 
 use crate::engine::Engine;
+use crate::loading::{LoadSource, Loaded};
 use crate::{World, WorldExt};
 
 /// Early-return with a [`StrResult`] or [`SourceResult`].
@@ -148,7 +151,7 @@ pub struct Warned<T> {
     pub warnings: EcoVec<SourceDiagnostic>,
 }
 
-/// An error or warning in a source file.
+/// An error or warning in a source or text file.
 ///
 /// The contained spans will only be detached if any of the input source files
 /// were detached.
@@ -568,31 +571,287 @@ impl From<PackageError> for EcoString {
     }
 }
 
+/// A result type with a data-loading-related error.
+pub type LoadResult<T> = Result<T, LoadError>;
+
+/// A call site independent error that occurred during data loading. This avoids
+/// polluting the memoization with [`Span`]s and [`FileId`]s from source files.
+/// Can be turned into a [`SourceDiagnostic`] using the [`LoadedWithin::within`]
+/// method available on [`LoadResult`].
+///
+/// [`FileId`]: typst_syntax::FileId
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct LoadError {
+    /// The position in the file at which the error occured.
+    pos: ReportPos,
+    /// Must contain a message formatted like this: `"failed to do thing (cause)"`.
+    message: EcoString,
+}
+
+impl LoadError {
+    /// Creates a new error from a position in a file, a base message
+    /// (e.g. `failed to parse JSON`) and a concrete error (e.g. `invalid
+    /// number`)
+    pub fn new(
+        pos: impl Into<ReportPos>,
+        message: impl std::fmt::Display,
+        error: impl std::fmt::Display,
+    ) -> Self {
+        Self {
+            pos: pos.into(),
+            message: eco_format!("{message} ({error})"),
+        }
+    }
+}
+
+impl From<Utf8Error> for LoadError {
+    fn from(err: Utf8Error) -> Self {
+        let start = err.valid_up_to();
+        let end = start + err.error_len().unwrap_or(0);
+        LoadError::new(
+            start..end,
+            "failed to convert to string",
+            "file is not valid utf-8",
+        )
+    }
+}
+
+/// Convert a [`LoadResult`] to a [`SourceResult`] by adding the [`Loaded`]
+/// context.
+pub trait LoadedWithin<T> {
+    /// Report an error, possibly in an external file.
+    fn within(self, loaded: &Loaded) -> SourceResult<T>;
+}
+
+impl<T, E> LoadedWithin<T> for Result<T, E>
+where
+    E: Into<LoadError>,
+{
+    fn within(self, loaded: &Loaded) -> SourceResult<T> {
+        self.map_err(|err| {
+            let LoadError { pos, message } = err.into();
+            load_err_in_text(loaded, pos, message)
+        })
+    }
+}
+
+/// Report an error, possibly in an external file. This will delegate to
+/// [`load_err_in_invalid_text`] if the data isn't valid utf-8.
+fn load_err_in_text(
+    loaded: &Loaded,
+    pos: impl Into<ReportPos>,
+    mut message: EcoString,
+) -> EcoVec<SourceDiagnostic> {
+    let pos = pos.into();
+    // This also does utf-8 validation. Only report an error in an external
+    // file if it is human readable (valid utf-8), otherwise fall back to
+    // `load_err_in_invalid_text`.
+    let lines = Lines::try_from(&loaded.data);
+    match (loaded.source.v, lines) {
+        (LoadSource::Path(file_id), Ok(lines)) => {
+            if let Some(range) = pos.range(&lines) {
+                let span = Span::from_range(file_id, range);
+                return eco_vec![SourceDiagnostic::error(span, message)];
+            }
+
+            // Either `ReportPos::None` was provided, or resolving the range
+            // from the line/column failed. If present report the possibly
+            // wrong line/column in the error message anyway.
+            let span = Span::from_range(file_id, 0..loaded.data.len());
+            if let Some(pair) = pos.line_col(&lines) {
+                message.pop();
+                let (line, col) = pair.numbers();
+                write!(&mut message, " at {line}:{col})").ok();
+            }
+            eco_vec![SourceDiagnostic::error(span, message)]
+        }
+        (LoadSource::Bytes, Ok(lines)) => {
+            if let Some(pair) = pos.line_col(&lines) {
+                message.pop();
+                let (line, col) = pair.numbers();
+                write!(&mut message, " at {line}:{col})").ok();
+            }
+            eco_vec![SourceDiagnostic::error(loaded.source.span, message)]
+        }
+        _ => load_err_in_invalid_text(loaded, pos, message),
+    }
+}
+
+/// Report an error (possibly from an external file) that isn't valid utf-8.
+fn load_err_in_invalid_text(
+    loaded: &Loaded,
+    pos: impl Into<ReportPos>,
+    mut message: EcoString,
+) -> EcoVec<SourceDiagnostic> {
+    let line_col = pos.into().try_line_col(&loaded.data).map(|p| p.numbers());
+    match (loaded.source.v, line_col) {
+        (LoadSource::Path(file), _) => {
+            message.pop();
+            if let Some(package) = file.package() {
+                write!(
+                    &mut message,
+                    " in {package}{}",
+                    file.vpath().as_rooted_path().display()
+                )
+                .ok();
+            } else {
+                write!(&mut message, " in {}", file.vpath().as_rootless_path().display())
+                    .ok();
+            };
+            if let Some((line, col)) = line_col {
+                write!(&mut message, ":{line}:{col}").ok();
+            }
+            message.push(')');
+        }
+        (LoadSource::Bytes, Some((line, col))) => {
+            message.pop();
+            write!(&mut message, " at {line}:{col})").ok();
+        }
+        (LoadSource::Bytes, None) => (),
+    }
+    eco_vec![SourceDiagnostic::error(loaded.source.span, message)]
+}
+
+/// A position at which an error was reported.
+#[derive(Clone, Debug, Default, PartialEq, Eq, Hash)]
+pub enum ReportPos {
+    /// Contains a range, and a line/column pair.
+    Full(std::ops::Range<u32>, LineCol),
+    /// Contains a range.
+    Range(std::ops::Range<u32>),
+    /// Contains a line/column pair.
+    LineCol(LineCol),
+    #[default]
+    None,
+}
+
+impl From<std::ops::Range<usize>> for ReportPos {
+    fn from(value: std::ops::Range<usize>) -> Self {
+        Self::Range(value.start.saturating_as()..value.end.saturating_as())
+    }
+}
+
+impl From<LineCol> for ReportPos {
+    fn from(value: LineCol) -> Self {
+        Self::LineCol(value)
+    }
+}
+
+impl ReportPos {
+    /// Creates a position from a pre-existing range and line-column pair.
+    pub fn full(range: std::ops::Range<usize>, pair: LineCol) -> Self {
+        let range = range.start.saturating_as()..range.end.saturating_as();
+        Self::Full(range, pair)
+    }
+
+    /// Tries to determine the byte range for this position.
+    fn range(&self, lines: &Lines<String>) -> Option<std::ops::Range<usize>> {
+        match self {
+            ReportPos::Full(range, _) => Some(range.start as usize..range.end as usize),
+            ReportPos::Range(range) => Some(range.start as usize..range.end as usize),
+            &ReportPos::LineCol(pair) => {
+                let i =
+                    lines.line_column_to_byte(pair.line as usize, pair.col as usize)?;
+                Some(i..i)
+            }
+            ReportPos::None => None,
+        }
+    }
+
+    /// Tries to determine the line/column for this position.
+    fn line_col(&self, lines: &Lines<String>) -> Option<LineCol> {
+        match self {
+            &ReportPos::Full(_, pair) => Some(pair),
+            ReportPos::Range(range) => {
+                let (line, col) = lines.byte_to_line_column(range.start as usize)?;
+                Some(LineCol::zero_based(line, col))
+            }
+            &ReportPos::LineCol(pair) => Some(pair),
+            ReportPos::None => None,
+        }
+    }
+
+    /// Either gets the line/column pair, or tries to compute it from possibly
+    /// invalid utf-8 data.
+    fn try_line_col(&self, bytes: &[u8]) -> Option<LineCol> {
+        match self {
+            &ReportPos::Full(_, pair) => Some(pair),
+            ReportPos::Range(range) => {
+                LineCol::try_from_byte_pos(range.start as usize, bytes)
+            }
+            &ReportPos::LineCol(pair) => Some(pair),
+            ReportPos::None => None,
+        }
+    }
+}
+
+/// A line/column pair.
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub struct LineCol {
+    /// The 0-based line.
+    line: u32,
+    /// The 0-based column.
+    col: u32,
+}
+
+impl LineCol {
+    /// Constructs the line/column pair from 0-based indices.
+    pub fn zero_based(line: usize, col: usize) -> Self {
+        Self {
+            line: line.saturating_as(),
+            col: col.saturating_as(),
+        }
+    }
+
+    /// Constructs the line/column pair from 1-based numbers.
+    pub fn one_based(line: usize, col: usize) -> Self {
+        Self::zero_based(line.saturating_sub(1), col.saturating_sub(1))
+    }
+
+    /// Try to compute a line/column pair from possibly invalid utf-8 data.
+    pub fn try_from_byte_pos(pos: usize, bytes: &[u8]) -> Option<Self> {
+        let bytes = &bytes[..pos];
+        let mut line = 0;
+        #[allow(clippy::double_ended_iterator_last)]
+        let line_start = memchr::memchr_iter(b'\n', bytes)
+            .inspect(|_| line += 1)
+            .last()
+            .map(|i| i + 1)
+            .unwrap_or(bytes.len());
+
+        let col = ErrorReportingUtf8Chars::new(&bytes[line_start..]).count();
+        Some(LineCol::zero_based(line, col))
+    }
+
+    /// Returns the 0-based line/column indices.
+    pub fn indices(&self) -> (usize, usize) {
+        (self.line as usize, self.col as usize)
+    }
+
+    /// Returns the 1-based line/column numbers.
+    pub fn numbers(&self) -> (usize, usize) {
+        (self.line as usize + 1, self.col as usize + 1)
+    }
+}
+
 /// Format a user-facing error message for an XML-like file format.
-pub fn format_xml_like_error(format: &str, error: roxmltree::Error) -> EcoString {
-    match error {
-        roxmltree::Error::UnexpectedCloseTag(expected, actual, pos) => {
-            eco_format!(
-                "failed to parse {format} (found closing tag '{actual}' \
-                 instead of '{expected}' in line {})",
-                pos.row
-            )
+pub fn format_xml_like_error(format: &str, error: roxmltree::Error) -> LoadError {
+    let pos = LineCol::one_based(error.pos().row as usize, error.pos().col as usize);
+    let message = match error {
+        roxmltree::Error::UnexpectedCloseTag(expected, actual, _) => {
+            eco_format!("failed to parse {format} (found closing tag '{actual}' instead of '{expected}')")
         }
-        roxmltree::Error::UnknownEntityReference(entity, pos) => {
-            eco_format!(
-                "failed to parse {format} (unknown entity '{entity}' in line {})",
-                pos.row
-            )
+        roxmltree::Error::UnknownEntityReference(entity, _) => {
+            eco_format!("failed to parse {format} (unknown entity '{entity}')")
         }
-        roxmltree::Error::DuplicatedAttribute(attr, pos) => {
-            eco_format!(
-                "failed to parse {format} (duplicate attribute '{attr}' in line {})",
-                pos.row
-            )
+        roxmltree::Error::DuplicatedAttribute(attr, _) => {
+            eco_format!("failed to parse {format} (duplicate attribute '{attr}')")
         }
         roxmltree::Error::NoRootNode => {
             eco_format!("failed to parse {format} (missing root node)")
         }
         err => eco_format!("failed to parse {format} ({err})"),
-    }
+    };
+
+    LoadError { pos: pos.into(), message }
 }
diff --git a/crates/typst-library/src/foundations/bytes.rs b/crates/typst-library/src/foundations/bytes.rs
index d633c99a..180dcdad 100644
--- a/crates/typst-library/src/foundations/bytes.rs
+++ b/crates/typst-library/src/foundations/bytes.rs
@@ -7,6 +7,7 @@ use std::sync::Arc;
 
 use ecow::{eco_format, EcoString};
 use serde::{Serialize, Serializer};
+use typst_syntax::Lines;
 use typst_utils::LazyHash;
 
 use crate::diag::{bail, StrResult};
@@ -286,6 +287,16 @@ impl Serialize for Bytes {
     }
 }
 
+impl TryFrom<&Bytes> for Lines<String> {
+    type Error = Utf8Error;
+
+    #[comemo::memoize]
+    fn try_from(value: &Bytes) -> Result<Lines<String>, Utf8Error> {
+        let text = value.as_str()?;
+        Ok(Lines::new(text.to_string()))
+    }
+}
+
 /// Any type that can back a byte buffer.
 trait Bytelike: Send + Sync {
     fn as_bytes(&self) -> &[u8];
diff --git a/crates/typst-library/src/foundations/plugin.rs b/crates/typst-library/src/foundations/plugin.rs
index 31f8cd73..a04443bf 100644
--- a/crates/typst-library/src/foundations/plugin.rs
+++ b/crates/typst-library/src/foundations/plugin.rs
@@ -151,8 +151,8 @@ pub fn plugin(
     /// A [path]($syntax/#paths) to a WebAssembly file or raw WebAssembly bytes.
     source: Spanned<DataSource>,
 ) -> SourceResult<Module> {
-    let data = source.load(engine.world)?;
-    Plugin::module(data).at(source.span)
+    let loaded = source.load(engine.world)?;
+    Plugin::module(loaded.data).at(source.span)
 }
 
 #[scope]
diff --git a/crates/typst-library/src/loading/cbor.rs b/crates/typst-library/src/loading/cbor.rs
index aa14c5c7..d95f7384 100644
--- a/crates/typst-library/src/loading/cbor.rs
+++ b/crates/typst-library/src/loading/cbor.rs
@@ -23,8 +23,8 @@ pub fn cbor(
     /// A [path]($syntax/#paths) to a CBOR file or raw CBOR bytes.
     source: Spanned<DataSource>,
 ) -> SourceResult<Value> {
-    let data = source.load(engine.world)?;
-    ciborium::from_reader(data.as_slice())
+    let loaded = source.load(engine.world)?;
+    ciborium::from_reader(loaded.data.as_slice())
         .map_err(|err| eco_format!("failed to parse CBOR ({err})"))
         .at(source.span)
 }
diff --git a/crates/typst-library/src/loading/csv.rs b/crates/typst-library/src/loading/csv.rs
index 6afb5bae..d5b54a06 100644
--- a/crates/typst-library/src/loading/csv.rs
+++ b/crates/typst-library/src/loading/csv.rs
@@ -1,7 +1,7 @@
-use ecow::{eco_format, EcoString};
+use az::SaturatingAs;
 use typst_syntax::Spanned;
 
-use crate::diag::{bail, At, SourceResult};
+use crate::diag::{bail, LineCol, LoadError, LoadedWithin, ReportPos, SourceResult};
 use crate::engine::Engine;
 use crate::foundations::{cast, func, scope, Array, Dict, IntoValue, Type, Value};
 use crate::loading::{DataSource, Load, Readable};
@@ -44,7 +44,7 @@ pub fn csv(
     #[default(RowType::Array)]
     row_type: RowType,
 ) -> SourceResult<Array> {
-    let data = source.load(engine.world)?;
+    let loaded = source.load(engine.world)?;
 
     let mut builder = ::csv::ReaderBuilder::new();
     let has_headers = row_type == RowType::Dict;
@@ -53,7 +53,7 @@ pub fn csv(
 
     // Counting lines from 1 by default.
     let mut line_offset: usize = 1;
-    let mut reader = builder.from_reader(data.as_slice());
+    let mut reader = builder.from_reader(loaded.data.as_slice());
     let mut headers: Option<::csv::StringRecord> = None;
 
     if has_headers {
@@ -62,9 +62,9 @@ pub fn csv(
         headers = Some(
             reader
                 .headers()
+                .cloned()
                 .map_err(|err| format_csv_error(err, 1))
-                .at(source.span)?
-                .clone(),
+                .within(&loaded)?,
         );
     }
 
@@ -74,7 +74,7 @@ pub fn csv(
         // incorrect with `has_headers` set to `false`. See issue:
         // https://github.com/BurntSushi/rust-csv/issues/184
         let line = line + line_offset;
-        let row = result.map_err(|err| format_csv_error(err, line)).at(source.span)?;
+        let row = result.map_err(|err| format_csv_error(err, line)).within(&loaded)?;
         let item = if let Some(headers) = &headers {
             let mut dict = Dict::new();
             for (field, value) in headers.iter().zip(&row) {
@@ -164,15 +164,23 @@ cast! {
 }
 
 /// Format the user-facing CSV error message.
-fn format_csv_error(err: ::csv::Error, line: usize) -> EcoString {
+fn format_csv_error(err: ::csv::Error, line: usize) -> LoadError {
+    let msg = "failed to parse CSV";
+    let pos = (err.kind().position())
+        .map(|pos| {
+            let start = pos.byte().saturating_as();
+            ReportPos::from(start..start)
+        })
+        .unwrap_or(LineCol::one_based(line, 1).into());
     match err.kind() {
-        ::csv::ErrorKind::Utf8 { .. } => "file is not valid utf-8".into(),
+        ::csv::ErrorKind::Utf8 { .. } => {
+            LoadError::new(pos, msg, "file is not valid utf-8")
+        }
         ::csv::ErrorKind::UnequalLengths { expected_len, len, .. } => {
-            eco_format!(
-                "failed to parse CSV (found {len} instead of \
-                 {expected_len} fields in line {line})"
-            )
+            let err =
+                format!("found {len} instead of {expected_len} fields in line {line}");
+            LoadError::new(pos, msg, err)
         }
-        _ => eco_format!("failed to parse CSV ({err})"),
+        _ => LoadError::new(pos, "failed to parse CSV", err),
     }
 }
diff --git a/crates/typst-library/src/loading/json.rs b/crates/typst-library/src/loading/json.rs
index aa908cca..7d0732ba 100644
--- a/crates/typst-library/src/loading/json.rs
+++ b/crates/typst-library/src/loading/json.rs
@@ -1,7 +1,7 @@
 use ecow::eco_format;
 use typst_syntax::Spanned;
 
-use crate::diag::{At, SourceResult};
+use crate::diag::{At, LineCol, LoadError, LoadedWithin, SourceResult};
 use crate::engine::Engine;
 use crate::foundations::{func, scope, Str, Value};
 use crate::loading::{DataSource, Load, Readable};
@@ -54,10 +54,13 @@ pub fn json(
     /// A [path]($syntax/#paths) to a JSON file or raw JSON bytes.
     source: Spanned<DataSource>,
 ) -> SourceResult<Value> {
-    let data = source.load(engine.world)?;
-    serde_json::from_slice(data.as_slice())
-        .map_err(|err| eco_format!("failed to parse JSON ({err})"))
-        .at(source.span)
+    let loaded = source.load(engine.world)?;
+    serde_json::from_slice(loaded.data.as_slice())
+        .map_err(|err| {
+            let pos = LineCol::one_based(err.line(), err.column());
+            LoadError::new(pos, "failed to parse JSON", err)
+        })
+        .within(&loaded)
 }
 
 #[scope]
diff --git a/crates/typst-library/src/loading/mod.rs b/crates/typst-library/src/loading/mod.rs
index c57e0288..67f4be83 100644
--- a/crates/typst-library/src/loading/mod.rs
+++ b/crates/typst-library/src/loading/mod.rs
@@ -17,7 +17,7 @@ mod yaml_;
 
 use comemo::Tracked;
 use ecow::EcoString;
-use typst_syntax::Spanned;
+use typst_syntax::{FileId, Spanned};
 
 pub use self::cbor_::*;
 pub use self::csv_::*;
@@ -74,39 +74,44 @@ pub trait Load {
 }
 
 impl Load for Spanned<DataSource> {
-    type Output = Bytes;
+    type Output = Loaded;
 
-    fn load(&self, world: Tracked<dyn World + '_>) -> SourceResult<Bytes> {
+    fn load(&self, world: Tracked<dyn World + '_>) -> SourceResult<Self::Output> {
         self.as_ref().load(world)
     }
 }
 
 impl Load for Spanned<&DataSource> {
-    type Output = Bytes;
+    type Output = Loaded;
 
-    fn load(&self, world: Tracked<dyn World + '_>) -> SourceResult<Bytes> {
+    fn load(&self, world: Tracked<dyn World + '_>) -> SourceResult<Self::Output> {
         match &self.v {
             DataSource::Path(path) => {
                 let file_id = self.span.resolve_path(path).at(self.span)?;
-                world.file(file_id).at(self.span)
+                let data = world.file(file_id).at(self.span)?;
+                let source = Spanned::new(LoadSource::Path(file_id), self.span);
+                Ok(Loaded::new(source, data))
+            }
+            DataSource::Bytes(data) => {
+                let source = Spanned::new(LoadSource::Bytes, self.span);
+                Ok(Loaded::new(source, data.clone()))
             }
-            DataSource::Bytes(bytes) => Ok(bytes.clone()),
         }
     }
 }
 
 impl Load for Spanned<OneOrMultiple<DataSource>> {
-    type Output = Vec<Bytes>;
+    type Output = Vec<Loaded>;
 
-    fn load(&self, world: Tracked<dyn World + '_>) -> SourceResult<Vec<Bytes>> {
+    fn load(&self, world: Tracked<dyn World + '_>) -> SourceResult<Self::Output> {
         self.as_ref().load(world)
     }
 }
 
 impl Load for Spanned<&OneOrMultiple<DataSource>> {
-    type Output = Vec<Bytes>;
+    type Output = Vec<Loaded>;
 
-    fn load(&self, world: Tracked<dyn World + '_>) -> SourceResult<Vec<Bytes>> {
+    fn load(&self, world: Tracked<dyn World + '_>) -> SourceResult<Self::Output> {
         self.v
             .0
             .iter()
@@ -115,6 +120,28 @@ impl Load for Spanned<&OneOrMultiple<DataSource>> {
     }
 }
 
+/// Data loaded from a [`DataSource`].
+#[derive(Clone, Debug, PartialEq, Eq, Hash)]
+pub struct Loaded {
+    /// Details about where `data` was loaded from.
+    pub source: Spanned<LoadSource>,
+    /// The loaded data.
+    pub data: Bytes,
+}
+
+impl Loaded {
+    pub fn new(source: Spanned<LoadSource>, bytes: Bytes) -> Self {
+        Self { source, data: bytes }
+    }
+}
+
+/// A loaded [`DataSource`].
+#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
+pub enum LoadSource {
+    Path(FileId),
+    Bytes,
+}
+
 /// A value that can be read from a file.
 #[derive(Debug, Clone, PartialEq, Hash)]
 pub enum Readable {
diff --git a/crates/typst-library/src/loading/read.rs b/crates/typst-library/src/loading/read.rs
index 32dadc79..91e6e436 100644
--- a/crates/typst-library/src/loading/read.rs
+++ b/crates/typst-library/src/loading/read.rs
@@ -1,11 +1,10 @@
 use ecow::EcoString;
 use typst_syntax::Spanned;
 
-use crate::diag::{At, FileError, SourceResult};
+use crate::diag::{LoadedWithin, SourceResult};
 use crate::engine::Engine;
 use crate::foundations::{func, Cast};
-use crate::loading::Readable;
-use crate::World;
+use crate::loading::{DataSource, Load, Readable};
 
 /// Reads plain text or data from a file.
 ///
@@ -36,14 +35,10 @@ pub fn read(
     #[default(Some(Encoding::Utf8))]
     encoding: Option<Encoding>,
 ) -> SourceResult<Readable> {
-    let Spanned { v: path, span } = path;
-    let id = span.resolve_path(&path).at(span)?;
-    let data = engine.world.file(id).at(span)?;
+    let loaded = path.map(DataSource::Path).load(engine.world)?;
     Ok(match encoding {
-        None => Readable::Bytes(data),
-        Some(Encoding::Utf8) => {
-            Readable::Str(data.to_str().map_err(FileError::from).at(span)?)
-        }
+        None => Readable::Bytes(loaded.data),
+        Some(Encoding::Utf8) => Readable::Str(loaded.data.to_str().within(&loaded)?),
     })
 }
 
diff --git a/crates/typst-library/src/loading/toml.rs b/crates/typst-library/src/loading/toml.rs
index f04b2e74..a4252fec 100644
--- a/crates/typst-library/src/loading/toml.rs
+++ b/crates/typst-library/src/loading/toml.rs
@@ -1,7 +1,7 @@
-use ecow::{eco_format, EcoString};
-use typst_syntax::{is_newline, Spanned};
+use ecow::eco_format;
+use typst_syntax::Spanned;
 
-use crate::diag::{At, FileError, SourceResult};
+use crate::diag::{At, LoadError, LoadedWithin, ReportPos, SourceResult};
 use crate::engine::Engine;
 use crate::foundations::{func, scope, Str, Value};
 use crate::loading::{DataSource, Load, Readable};
@@ -32,11 +32,9 @@ pub fn toml(
     /// A [path]($syntax/#paths) to a TOML file or raw TOML bytes.
     source: Spanned<DataSource>,
 ) -> SourceResult<Value> {
-    let data = source.load(engine.world)?;
-    let raw = data.as_str().map_err(FileError::from).at(source.span)?;
-    ::toml::from_str(raw)
-        .map_err(|err| format_toml_error(err, raw))
-        .at(source.span)
+    let loaded = source.load(engine.world)?;
+    let raw = loaded.data.as_str().within(&loaded)?;
+    ::toml::from_str(raw).map_err(format_toml_error).within(&loaded)
 }
 
 #[scope]
@@ -71,15 +69,7 @@ impl toml {
 }
 
 /// Format the user-facing TOML error message.
-fn format_toml_error(error: ::toml::de::Error, raw: &str) -> EcoString {
-    if let Some(head) = error.span().and_then(|range| raw.get(..range.start)) {
-        let line = head.lines().count();
-        let column = 1 + head.chars().rev().take_while(|&c| !is_newline(c)).count();
-        eco_format!(
-            "failed to parse TOML ({} at line {line} column {column})",
-            error.message(),
-        )
-    } else {
-        eco_format!("failed to parse TOML ({})", error.message())
-    }
+fn format_toml_error(error: ::toml::de::Error) -> LoadError {
+    let pos = error.span().map(ReportPos::from).unwrap_or_default();
+    LoadError::new(pos, "failed to parse TOML", error.message())
 }
diff --git a/crates/typst-library/src/loading/xml.rs b/crates/typst-library/src/loading/xml.rs
index e76c4e9c..0023c5df 100644
--- a/crates/typst-library/src/loading/xml.rs
+++ b/crates/typst-library/src/loading/xml.rs
@@ -1,8 +1,7 @@
-use ecow::EcoString;
 use roxmltree::ParsingOptions;
 use typst_syntax::Spanned;
 
-use crate::diag::{format_xml_like_error, At, FileError, SourceResult};
+use crate::diag::{format_xml_like_error, LoadError, LoadedWithin, SourceResult};
 use crate::engine::Engine;
 use crate::foundations::{dict, func, scope, Array, Dict, IntoValue, Str, Value};
 use crate::loading::{DataSource, Load, Readable};
@@ -61,14 +60,14 @@ pub fn xml(
     /// A [path]($syntax/#paths) to an XML file or raw XML bytes.
     source: Spanned<DataSource>,
 ) -> SourceResult<Value> {
-    let data = source.load(engine.world)?;
-    let text = data.as_str().map_err(FileError::from).at(source.span)?;
+    let loaded = source.load(engine.world)?;
+    let text = loaded.data.as_str().within(&loaded)?;
     let document = roxmltree::Document::parse_with_options(
         text,
         ParsingOptions { allow_dtd: true, ..Default::default() },
     )
     .map_err(format_xml_error)
-    .at(source.span)?;
+    .within(&loaded)?;
     Ok(convert_xml(document.root()))
 }
 
@@ -111,6 +110,6 @@ fn convert_xml(node: roxmltree::Node) -> Value {
 }
 
 /// Format the user-facing XML error message.
-fn format_xml_error(error: roxmltree::Error) -> EcoString {
+fn format_xml_error(error: roxmltree::Error) -> LoadError {
     format_xml_like_error("XML", error)
 }
diff --git a/crates/typst-library/src/loading/yaml.rs b/crates/typst-library/src/loading/yaml.rs
index 3f48113e..0edf1f90 100644
--- a/crates/typst-library/src/loading/yaml.rs
+++ b/crates/typst-library/src/loading/yaml.rs
@@ -1,7 +1,7 @@
 use ecow::eco_format;
 use typst_syntax::Spanned;
 
-use crate::diag::{At, SourceResult};
+use crate::diag::{At, LineCol, LoadError, LoadedWithin, ReportPos, SourceResult};
 use crate::engine::Engine;
 use crate::foundations::{func, scope, Str, Value};
 use crate::loading::{DataSource, Load, Readable};
@@ -44,10 +44,10 @@ pub fn yaml(
     /// A [path]($syntax/#paths) to a YAML file or raw YAML bytes.
     source: Spanned<DataSource>,
 ) -> SourceResult<Value> {
-    let data = source.load(engine.world)?;
-    serde_yaml::from_slice(data.as_slice())
-        .map_err(|err| eco_format!("failed to parse YAML ({err})"))
-        .at(source.span)
+    let loaded = source.load(engine.world)?;
+    serde_yaml::from_slice(loaded.data.as_slice())
+        .map_err(format_yaml_error)
+        .within(&loaded)
 }
 
 #[scope]
@@ -76,3 +76,16 @@ impl yaml {
             .at(span)
     }
 }
+
+/// Format the user-facing YAML error message.
+pub fn format_yaml_error(error: serde_yaml::Error) -> LoadError {
+    let pos = error
+        .location()
+        .map(|loc| {
+            let line_col = LineCol::one_based(loc.line(), loc.column());
+            let range = loc.index()..loc.index();
+            ReportPos::full(range, line_col)
+        })
+        .unwrap_or_default();
+    LoadError::new(pos, "failed to parse YAML", error)
+}
diff --git a/crates/typst-library/src/model/bibliography.rs b/crates/typst-library/src/model/bibliography.rs
index 51e3b03b..11435657 100644
--- a/crates/typst-library/src/model/bibliography.rs
+++ b/crates/typst-library/src/model/bibliography.rs
@@ -19,7 +19,10 @@ use smallvec::{smallvec, SmallVec};
 use typst_syntax::{Span, Spanned};
 use typst_utils::{Get, ManuallyHash, NonZeroExt, PicoStr};
 
-use crate::diag::{bail, error, At, FileError, HintedStrResult, SourceResult, StrResult};
+use crate::diag::{
+    bail, error, At, HintedStrResult, LoadError, LoadResult, LoadedWithin, ReportPos,
+    SourceResult, StrResult,
+};
 use crate::engine::{Engine, Sink};
 use crate::foundations::{
     elem, Bytes, CastInfo, Content, Derived, FromValue, IntoValue, Label, NativeElement,
@@ -31,7 +34,7 @@ use crate::layout::{
     BlockBody, BlockElem, Em, GridCell, GridChild, GridElem, GridItem, HElem, PadElem,
     Sides, Sizing, TrackSizings,
 };
-use crate::loading::{DataSource, Load};
+use crate::loading::{format_yaml_error, DataSource, Load, LoadSource, Loaded};
 use crate::model::{
     CitationForm, CiteGroup, Destination, FootnoteElem, HeadingElem, LinkElem, ParElem,
     Url,
@@ -294,24 +297,21 @@ impl Bibliography {
         world: Tracked<dyn World + '_>,
         sources: Spanned<OneOrMultiple<DataSource>>,
     ) -> SourceResult<Derived<OneOrMultiple<DataSource>, Self>> {
-        let data = sources.load(world)?;
-        let bibliography = Self::decode(&sources.v, &data).at(sources.span)?;
+        let loaded = sources.load(world)?;
+        let bibliography = Self::decode(&loaded)?;
         Ok(Derived::new(sources.v, bibliography))
     }
 
     /// Decode a bibliography from loaded data sources.
     #[comemo::memoize]
     #[typst_macros::time(name = "load bibliography")]
-    fn decode(
-        sources: &OneOrMultiple<DataSource>,
-        data: &[Bytes],
-    ) -> StrResult<Bibliography> {
+    fn decode(data: &[Loaded]) -> SourceResult<Bibliography> {
         let mut map = IndexMap::new();
         let mut duplicates = Vec::<EcoString>::new();
 
         // We might have multiple bib/yaml files
-        for (source, data) in sources.0.iter().zip(data) {
-            let library = decode_library(source, data)?;
+        for d in data.iter() {
+            let library = decode_library(d)?;
             for entry in library {
                 match map.entry(Label::new(PicoStr::intern(entry.key()))) {
                     indexmap::map::Entry::Vacant(vacant) => {
@@ -325,7 +325,11 @@ impl Bibliography {
         }
 
         if !duplicates.is_empty() {
-            bail!("duplicate bibliography keys: {}", duplicates.join(", "));
+            // TODO: Store spans of entries for duplicate key error messages.
+            // Requires hayagriva entries to store their location, which should
+            // be fine, since they are 1kb anyway.
+            let span = data.first().unwrap().source.span;
+            bail!(span, "duplicate bibliography keys: {}", duplicates.join(", "));
         }
 
         Ok(Bibliography(Arc::new(ManuallyHash::new(map, typst_utils::hash128(data)))))
@@ -351,36 +355,47 @@ impl Debug for Bibliography {
 }
 
 /// Decode on library from one data source.
-fn decode_library(source: &DataSource, data: &Bytes) -> StrResult<Library> {
-    let src = data.as_str().map_err(FileError::from)?;
+fn decode_library(loaded: &Loaded) -> SourceResult<Library> {
+    let data = loaded.data.as_str().within(loaded)?;
 
-    if let DataSource::Path(path) = source {
+    if let LoadSource::Path(file_id) = loaded.source.v {
         // If we got a path, use the extension to determine whether it is
         // YAML or BibLaTeX.
-        let ext = Path::new(path.as_str())
+        let ext = file_id
+            .vpath()
+            .as_rooted_path()
             .extension()
             .and_then(OsStr::to_str)
             .unwrap_or_default();
 
         match ext.to_lowercase().as_str() {
-            "yml" | "yaml" => hayagriva::io::from_yaml_str(src)
-                .map_err(|err| eco_format!("failed to parse YAML ({err})")),
-            "bib" => hayagriva::io::from_biblatex_str(src)
-                .map_err(|errors| format_biblatex_error(src, Some(path), errors)),
-            _ => bail!("unknown bibliography format (must be .yml/.yaml or .bib)"),
+            "yml" | "yaml" => hayagriva::io::from_yaml_str(data)
+                .map_err(format_yaml_error)
+                .within(loaded),
+            "bib" => hayagriva::io::from_biblatex_str(data)
+                .map_err(format_biblatex_error)
+                .within(loaded),
+            _ => bail!(
+                loaded.source.span,
+                "unknown bibliography format (must be .yml/.yaml or .bib)"
+            ),
         }
     } else {
         // If we just got bytes, we need to guess. If it can be decoded as
         // hayagriva YAML, we'll use that.
-        let haya_err = match hayagriva::io::from_yaml_str(src) {
+        let haya_err = match hayagriva::io::from_yaml_str(data) {
             Ok(library) => return Ok(library),
             Err(err) => err,
         };
 
         // If it can be decoded as BibLaTeX, we use that isntead.
-        let bib_errs = match hayagriva::io::from_biblatex_str(src) {
-            Ok(library) => return Ok(library),
-            Err(err) => err,
+        let bib_errs = match hayagriva::io::from_biblatex_str(data) {
+            // If the file is almost valid yaml, but contains no `@` character
+            // it will be successfully parsed as an empty BibLaTeX library,
+            // since BibLaTeX does support arbitrary text outside of entries.
+            Ok(library) if !library.is_empty() => return Ok(library),
+            Ok(_) => None,
+            Err(err) => Some(err),
         };
 
         // If neither decoded correctly, check whether `:` or `{` appears
@@ -388,7 +403,7 @@ fn decode_library(source: &DataSource, data: &Bytes) -> StrResult<Library> {
         // and emit the more appropriate error.
         let mut yaml = 0;
         let mut biblatex = 0;
-        for c in src.chars() {
+        for c in data.chars() {
             match c {
                 ':' => yaml += 1,
                 '{' => biblatex += 1,
@@ -396,37 +411,33 @@ fn decode_library(source: &DataSource, data: &Bytes) -> StrResult<Library> {
             }
         }
 
-        if yaml > biblatex {
-            bail!("failed to parse YAML ({haya_err})")
-        } else {
-            Err(format_biblatex_error(src, None, bib_errs))
+        match bib_errs {
+            Some(bib_errs) if biblatex >= yaml => {
+                Err(format_biblatex_error(bib_errs)).within(loaded)
+            }
+            _ => Err(format_yaml_error(haya_err)).within(loaded),
         }
     }
 }
 
 /// Format a BibLaTeX loading error.
-fn format_biblatex_error(
-    src: &str,
-    path: Option<&str>,
-    errors: Vec<BibLaTeXError>,
-) -> EcoString {
-    let Some(error) = errors.first() else {
-        return match path {
-            Some(path) => eco_format!("failed to parse BibLaTeX file ({path})"),
-            None => eco_format!("failed to parse BibLaTeX"),
-        };
+fn format_biblatex_error(errors: Vec<BibLaTeXError>) -> LoadError {
+    // TODO: return multiple errors?
+    let Some(error) = errors.into_iter().next() else {
+        // TODO: can this even happen, should we just unwrap?
+        return LoadError::new(
+            ReportPos::None,
+            "failed to parse BibLaTeX",
+            "something went wrong",
+        );
     };
 
-    let (span, msg) = match error {
-        BibLaTeXError::Parse(error) => (&error.span, error.kind.to_string()),
-        BibLaTeXError::Type(error) => (&error.span, error.kind.to_string()),
+    let (range, msg) = match error {
+        BibLaTeXError::Parse(error) => (error.span, error.kind.to_string()),
+        BibLaTeXError::Type(error) => (error.span, error.kind.to_string()),
     };
 
-    let line = src.get(..span.start).unwrap_or_default().lines().count();
-    match path {
-        Some(path) => eco_format!("failed to parse BibLaTeX file ({path}:{line}: {msg})"),
-        None => eco_format!("failed to parse BibLaTeX ({line}: {msg})"),
-    }
+    LoadError::new(range, "failed to parse BibLaTeX", msg)
 }
 
 /// A loaded CSL style.
@@ -442,8 +453,8 @@ impl CslStyle {
         let style = match &source {
             CslSource::Named(style) => Self::from_archived(*style),
             CslSource::Normal(source) => {
-                let data = Spanned::new(source, span).load(world)?;
-                Self::from_data(data).at(span)?
+                let loaded = Spanned::new(source, span).load(world)?;
+                Self::from_data(&loaded.data).within(&loaded)?
             }
         };
         Ok(Derived::new(source, style))
@@ -464,16 +475,18 @@ impl CslStyle {
 
     /// Load a CSL style from file contents.
     #[comemo::memoize]
-    pub fn from_data(data: Bytes) -> StrResult<CslStyle> {
-        let text = data.as_str().map_err(FileError::from)?;
+    pub fn from_data(bytes: &Bytes) -> LoadResult<CslStyle> {
+        let text = bytes.as_str()?;
         citationberg::IndependentStyle::from_xml(text)
             .map(|style| {
                 Self(Arc::new(ManuallyHash::new(
                     style,
-                    typst_utils::hash128(&(TypeId::of::<Bytes>(), data)),
+                    typst_utils::hash128(&(TypeId::of::<Bytes>(), bytes)),
                 )))
             })
-            .map_err(|err| eco_format!("failed to load CSL style ({err})"))
+            .map_err(|err| {
+                LoadError::new(ReportPos::None, "failed to load CSL style", err)
+            })
     }
 
     /// Get the underlying independent style.
diff --git a/crates/typst-library/src/text/raw.rs b/crates/typst-library/src/text/raw.rs
index d5c07424..f2485e16 100644
--- a/crates/typst-library/src/text/raw.rs
+++ b/crates/typst-library/src/text/raw.rs
@@ -3,15 +3,17 @@ use std::ops::Range;
 use std::sync::{Arc, LazyLock};
 
 use comemo::Tracked;
-use ecow::{eco_format, EcoString, EcoVec};
-use syntect::highlighting as synt;
-use syntect::parsing::{SyntaxDefinition, SyntaxSet, SyntaxSetBuilder};
+use ecow::{EcoString, EcoVec};
+use syntect::highlighting::{self as synt};
+use syntect::parsing::{ParseSyntaxError, SyntaxDefinition, SyntaxSet, SyntaxSetBuilder};
 use typst_syntax::{split_newlines, LinkedNode, Span, Spanned};
 use typst_utils::ManuallyHash;
 use unicode_segmentation::UnicodeSegmentation;
 
 use super::Lang;
-use crate::diag::{At, FileError, SourceResult, StrResult};
+use crate::diag::{
+    LineCol, LoadError, LoadResult, LoadedWithin, ReportPos, SourceResult,
+};
 use crate::engine::Engine;
 use crate::foundations::{
     cast, elem, scope, Bytes, Content, Derived, NativeElement, OneOrMultiple, Packed,
@@ -539,40 +541,29 @@ impl RawSyntax {
         world: Tracked<dyn World + '_>,
         sources: Spanned<OneOrMultiple<DataSource>>,
     ) -> SourceResult<Derived<OneOrMultiple<DataSource>, Vec<RawSyntax>>> {
-        let data = sources.load(world)?;
-        let list = sources
-            .v
-            .0
+        let loaded = sources.load(world)?;
+        let list = loaded
             .iter()
-            .zip(&data)
-            .map(|(source, data)| Self::decode(source, data))
-            .collect::<StrResult<_>>()
-            .at(sources.span)?;
+            .map(|data| Self::decode(&data.data).within(data))
+            .collect::<SourceResult<_>>()?;
         Ok(Derived::new(sources.v, list))
     }
 
     /// Decode a syntax from a loaded source.
     #[comemo::memoize]
     #[typst_macros::time(name = "load syntaxes")]
-    fn decode(source: &DataSource, data: &Bytes) -> StrResult<RawSyntax> {
-        let src = data.as_str().map_err(FileError::from)?;
-        let syntax = SyntaxDefinition::load_from_str(src, false, None).map_err(
-            |err| match source {
-                DataSource::Path(path) => {
-                    eco_format!("failed to parse syntax file `{path}` ({err})")
-                }
-                DataSource::Bytes(_) => {
-                    eco_format!("failed to parse syntax ({err})")
-                }
-            },
-        )?;
+    fn decode(bytes: &Bytes) -> LoadResult<RawSyntax> {
+        let str = bytes.as_str()?;
+
+        let syntax = SyntaxDefinition::load_from_str(str, false, None)
+            .map_err(format_syntax_error)?;
 
         let mut builder = SyntaxSetBuilder::new();
         builder.add(syntax);
 
         Ok(RawSyntax(Arc::new(ManuallyHash::new(
             builder.build(),
-            typst_utils::hash128(data),
+            typst_utils::hash128(bytes),
         ))))
     }
 
@@ -582,6 +573,24 @@ impl RawSyntax {
     }
 }
 
+fn format_syntax_error(error: ParseSyntaxError) -> LoadError {
+    let pos = syntax_error_pos(&error);
+    LoadError::new(pos, "failed to parse syntax", error)
+}
+
+fn syntax_error_pos(error: &ParseSyntaxError) -> ReportPos {
+    match error {
+        ParseSyntaxError::InvalidYaml(scan_error) => {
+            let m = scan_error.marker();
+            ReportPos::full(
+                m.index()..m.index(),
+                LineCol::one_based(m.line(), m.col() + 1),
+            )
+        }
+        _ => ReportPos::None,
+    }
+}
+
 /// A loaded syntect theme.
 #[derive(Debug, Clone, PartialEq, Hash)]
 pub struct RawTheme(Arc<ManuallyHash<synt::Theme>>);
@@ -592,18 +601,18 @@ impl RawTheme {
         world: Tracked<dyn World + '_>,
         source: Spanned<DataSource>,
     ) -> SourceResult<Derived<DataSource, Self>> {
-        let data = source.load(world)?;
-        let theme = Self::decode(&data).at(source.span)?;
+        let loaded = source.load(world)?;
+        let theme = Self::decode(&loaded.data).within(&loaded)?;
         Ok(Derived::new(source.v, theme))
     }
 
     /// Decode a theme from bytes.
     #[comemo::memoize]
-    fn decode(data: &Bytes) -> StrResult<RawTheme> {
-        let mut cursor = std::io::Cursor::new(data.as_slice());
-        let theme = synt::ThemeSet::load_from_reader(&mut cursor)
-            .map_err(|err| eco_format!("failed to parse theme ({err})"))?;
-        Ok(RawTheme(Arc::new(ManuallyHash::new(theme, typst_utils::hash128(data)))))
+    fn decode(bytes: &Bytes) -> LoadResult<RawTheme> {
+        let mut cursor = std::io::Cursor::new(bytes.as_slice());
+        let theme =
+            synt::ThemeSet::load_from_reader(&mut cursor).map_err(format_theme_error)?;
+        Ok(RawTheme(Arc::new(ManuallyHash::new(theme, typst_utils::hash128(bytes)))))
     }
 
     /// Get the underlying syntect theme.
@@ -612,6 +621,14 @@ impl RawTheme {
     }
 }
 
+fn format_theme_error(error: syntect::LoadingError) -> LoadError {
+    let pos = match &error {
+        syntect::LoadingError::ParseSyntax(err, _) => syntax_error_pos(err),
+        _ => ReportPos::None,
+    };
+    LoadError::new(pos, "failed to parse theme", error)
+}
+
 /// A highlighted line of raw text.
 ///
 /// This is a helper element that is synthesized by [`raw`] elements.
diff --git a/crates/typst-library/src/visualize/image/mod.rs b/crates/typst-library/src/visualize/image/mod.rs
index f9e345e7..f5109798 100644
--- a/crates/typst-library/src/visualize/image/mod.rs
+++ b/crates/typst-library/src/visualize/image/mod.rs
@@ -22,7 +22,7 @@ use crate::foundations::{
     Smart, StyleChain,
 };
 use crate::layout::{BlockElem, Length, Rel, Sizing};
-use crate::loading::{DataSource, Load, Readable};
+use crate::loading::{DataSource, Load, LoadSource, Loaded, Readable};
 use crate::model::Figurable;
 use crate::text::LocalName;
 
@@ -65,10 +65,10 @@ pub struct ImageElem {
     #[required]
     #[parse(
         let source = args.expect::<Spanned<DataSource>>("source")?;
-        let data = source.load(engine.world)?;
-        Derived::new(source.v, data)
+        let loaded = source.load(engine.world)?;
+        Derived::new(source.v, loaded)
     )]
-    pub source: Derived<DataSource, Bytes>,
+    pub source: Derived<DataSource, Loaded>,
 
     /// The image's format.
     ///
@@ -154,8 +154,8 @@ pub struct ImageElem {
     /// to `{auto}`, Typst will try to extract an ICC profile from the image.
     #[parse(match args.named::<Spanned<Smart<DataSource>>>("icc")? {
         Some(Spanned { v: Smart::Custom(source), span }) => Some(Smart::Custom({
-            let data = Spanned::new(&source, span).load(engine.world)?;
-            Derived::new(source, data)
+            let loaded = Spanned::new(&source, span).load(engine.world)?;
+            Derived::new(source, loaded.data)
         })),
         Some(Spanned { v: Smart::Auto, .. }) => Some(Smart::Auto),
         None => None,
@@ -173,7 +173,7 @@ impl ImageElem {
     pub fn decode(
         span: Span,
         /// The data to decode as an image. Can be a string for SVGs.
-        data: Readable,
+        data: Spanned<Readable>,
         /// The image's format. Detected automatically by default.
         #[named]
         format: Option<Smart<ImageFormat>>,
@@ -193,8 +193,10 @@ impl ImageElem {
         #[named]
         scaling: Option<Smart<ImageScaling>>,
     ) -> StrResult<Content> {
-        let bytes = data.into_bytes();
-        let source = Derived::new(DataSource::Bytes(bytes.clone()), bytes);
+        let bytes = data.v.into_bytes();
+        let loaded =
+            Loaded::new(Spanned::new(LoadSource::Bytes, data.span), bytes.clone());
+        let source = Derived::new(DataSource::Bytes(bytes), loaded);
         let mut elem = ImageElem::new(source);
         if let Some(format) = format {
             elem.push_format(format);
diff --git a/crates/typst-library/src/visualize/image/svg.rs b/crates/typst-library/src/visualize/image/svg.rs
index 9bf1ead0..1a3f6d47 100644
--- a/crates/typst-library/src/visualize/image/svg.rs
+++ b/crates/typst-library/src/visualize/image/svg.rs
@@ -3,10 +3,9 @@ use std::hash::{Hash, Hasher};
 use std::sync::{Arc, Mutex};
 
 use comemo::Tracked;
-use ecow::EcoString;
 use siphasher::sip128::{Hasher128, SipHasher13};
 
-use crate::diag::{format_xml_like_error, StrResult};
+use crate::diag::{format_xml_like_error, LoadError, LoadResult, ReportPos};
 use crate::foundations::Bytes;
 use crate::layout::Axes;
 use crate::text::{
@@ -30,7 +29,7 @@ impl SvgImage {
     /// Decode an SVG image without fonts.
     #[comemo::memoize]
     #[typst_macros::time(name = "load svg")]
-    pub fn new(data: Bytes) -> StrResult<SvgImage> {
+    pub fn new(data: Bytes) -> LoadResult<SvgImage> {
         let tree =
             usvg::Tree::from_data(&data, &base_options()).map_err(format_usvg_error)?;
         Ok(Self(Arc::new(Repr { data, size: tree_size(&tree), font_hash: 0, tree })))
@@ -43,7 +42,7 @@ impl SvgImage {
         data: Bytes,
         world: Tracked<dyn World + '_>,
         families: &[&str],
-    ) -> StrResult<SvgImage> {
+    ) -> LoadResult<SvgImage> {
         let book = world.book();
         let resolver = Mutex::new(FontResolver::new(world, book, families));
         let tree = usvg::Tree::from_data(
@@ -125,16 +124,15 @@ fn tree_size(tree: &usvg::Tree) -> Axes<f64> {
 }
 
 /// Format the user-facing SVG decoding error message.
-fn format_usvg_error(error: usvg::Error) -> EcoString {
-    match error {
-        usvg::Error::NotAnUtf8Str => "file is not valid utf-8".into(),
-        usvg::Error::MalformedGZip => "file is not compressed correctly".into(),
-        usvg::Error::ElementsLimitReached => "file is too large".into(),
-        usvg::Error::InvalidSize => {
-            "failed to parse SVG (width, height, or viewbox is invalid)".into()
-        }
-        usvg::Error::ParsingFailed(error) => format_xml_like_error("SVG", error),
-    }
+fn format_usvg_error(error: usvg::Error) -> LoadError {
+    let error = match error {
+        usvg::Error::NotAnUtf8Str => "file is not valid utf-8",
+        usvg::Error::MalformedGZip => "file is not compressed correctly",
+        usvg::Error::ElementsLimitReached => "file is too large",
+        usvg::Error::InvalidSize => "width, height, or viewbox is invalid",
+        usvg::Error::ParsingFailed(error) => return format_xml_like_error("SVG", error),
+    };
+    LoadError::new(ReportPos::None, "failed to parse SVG", error)
 }
 
 /// Provides Typst's fonts to usvg.
diff --git a/crates/typst-syntax/Cargo.toml b/crates/typst-syntax/Cargo.toml
index 263595bd..c20f6a08 100644
--- a/crates/typst-syntax/Cargo.toml
+++ b/crates/typst-syntax/Cargo.toml
@@ -15,6 +15,7 @@ readme = { workspace = true }
 [dependencies]
 typst-timing = { workspace = true }
 typst-utils = { workspace = true }
+comemo = { workspace = true }
 ecow = { workspace = true }
 serde = { workspace = true }
 toml = { workspace = true }
diff --git a/crates/typst-syntax/src/lib.rs b/crates/typst-syntax/src/lib.rs
index 5e7b710f..1249f88e 100644
--- a/crates/typst-syntax/src/lib.rs
+++ b/crates/typst-syntax/src/lib.rs
@@ -7,6 +7,7 @@ mod file;
 mod highlight;
 mod kind;
 mod lexer;
+mod lines;
 mod node;
 mod parser;
 mod path;
@@ -22,6 +23,7 @@ pub use self::lexer::{
     is_id_continue, is_id_start, is_ident, is_newline, is_valid_label_literal_id,
     link_prefix, split_newlines,
 };
+pub use self::lines::Lines;
 pub use self::node::{LinkedChildren, LinkedNode, Side, SyntaxError, SyntaxNode};
 pub use self::parser::{parse, parse_code, parse_math};
 pub use self::path::VirtualPath;
diff --git a/crates/typst-syntax/src/lines.rs b/crates/typst-syntax/src/lines.rs
new file mode 100644
index 00000000..fa1e7756
--- /dev/null
+++ b/crates/typst-syntax/src/lines.rs
@@ -0,0 +1,402 @@
+use std::hash::{Hash, Hasher};
+use std::iter::zip;
+use std::ops::Range;
+use std::sync::Arc;
+
+use crate::is_newline;
+
+/// A text buffer and metadata about lines.
+#[derive(Clone)]
+pub struct Lines<S>(Arc<Repr<S>>);
+
+#[derive(Clone)]
+struct Repr<T> {
+    lines: Vec<Line>,
+    text: T,
+}
+
+/// Metadata about a line.
+#[derive(Debug, Copy, Clone, Eq, PartialEq)]
+pub struct Line {
+    /// The UTF-8 byte offset where the line starts.
+    byte_idx: usize,
+    /// The UTF-16 codepoint offset where the line starts.
+    utf16_idx: usize,
+}
+
+impl<T: AsRef<str>> Lines<T> {
+    /// Create from the text buffer and compute the line metadata.
+    pub fn new(text: T) -> Self {
+        let lines = lines(text.as_ref());
+        Lines(Arc::new(Repr { lines, text }))
+    }
+
+    /// The text as a string slice.
+    pub fn text(&self) -> &str {
+        self.0.text.as_ref()
+    }
+
+    /// Get the length of the file in UTF-8 encoded bytes.
+    pub fn len_bytes(&self) -> usize {
+        self.0.text.as_ref().len()
+    }
+
+    /// Get the length of the file in UTF-16 code units.
+    pub fn len_utf16(&self) -> usize {
+        let last = self.0.lines.last().unwrap();
+        last.utf16_idx + len_utf16(&self.text()[last.byte_idx..])
+    }
+
+    /// Get the length of the file in lines.
+    pub fn len_lines(&self) -> usize {
+        self.0.lines.len()
+    }
+
+    /// Return the index of the UTF-16 code unit at the byte index.
+    pub fn byte_to_utf16(&self, byte_idx: usize) -> Option<usize> {
+        let line_idx = self.byte_to_line(byte_idx)?;
+        let line = self.0.lines.get(line_idx)?;
+        let head = self.text().get(line.byte_idx..byte_idx)?;
+        Some(line.utf16_idx + len_utf16(head))
+    }
+
+    /// Return the index of the line that contains the given byte index.
+    pub fn byte_to_line(&self, byte_idx: usize) -> Option<usize> {
+        (byte_idx <= self.text().len()).then(|| {
+            match self.0.lines.binary_search_by_key(&byte_idx, |line| line.byte_idx) {
+                Ok(i) => i,
+                Err(i) => i - 1,
+            }
+        })
+    }
+
+    /// Return the index of the column at the byte index.
+    ///
+    /// The column is defined as the number of characters in the line before the
+    /// byte index.
+    pub fn byte_to_column(&self, byte_idx: usize) -> Option<usize> {
+        let line = self.byte_to_line(byte_idx)?;
+        let start = self.line_to_byte(line)?;
+        let head = self.text().get(start..byte_idx)?;
+        Some(head.chars().count())
+    }
+
+    /// Return the index of the line and column at the byte index.
+    pub fn byte_to_line_column(&self, byte_idx: usize) -> Option<(usize, usize)> {
+        let line = self.byte_to_line(byte_idx)?;
+        let start = self.line_to_byte(line)?;
+        let head = self.text().get(start..byte_idx)?;
+        let col = head.chars().count();
+        Some((line, col))
+    }
+
+    /// Return the byte index at the UTF-16 code unit.
+    pub fn utf16_to_byte(&self, utf16_idx: usize) -> Option<usize> {
+        let line = self.0.lines.get(
+            match self.0.lines.binary_search_by_key(&utf16_idx, |line| line.utf16_idx) {
+                Ok(i) => i,
+                Err(i) => i - 1,
+            },
+        )?;
+
+        let text = self.text();
+        let mut k = line.utf16_idx;
+        for (i, c) in text[line.byte_idx..].char_indices() {
+            if k >= utf16_idx {
+                return Some(line.byte_idx + i);
+            }
+            k += c.len_utf16();
+        }
+
+        (k == utf16_idx).then_some(text.len())
+    }
+
+    /// Return the byte position at which the given line starts.
+    pub fn line_to_byte(&self, line_idx: usize) -> Option<usize> {
+        self.0.lines.get(line_idx).map(|line| line.byte_idx)
+    }
+
+    /// Return the range which encloses the given line.
+    pub fn line_to_range(&self, line_idx: usize) -> Option<Range<usize>> {
+        let start = self.line_to_byte(line_idx)?;
+        let end = self.line_to_byte(line_idx + 1).unwrap_or(self.text().len());
+        Some(start..end)
+    }
+
+    /// Return the byte index of the given (line, column) pair.
+    ///
+    /// The column defines the number of characters to go beyond the start of
+    /// the line.
+    pub fn line_column_to_byte(
+        &self,
+        line_idx: usize,
+        column_idx: usize,
+    ) -> Option<usize> {
+        let range = self.line_to_range(line_idx)?;
+        let line = self.text().get(range.clone())?;
+        let mut chars = line.chars();
+        for _ in 0..column_idx {
+            chars.next();
+        }
+        Some(range.start + (line.len() - chars.as_str().len()))
+    }
+}
+
+impl Lines<String> {
+    /// Fully replace the source text.
+    ///
+    /// This performs a naive (suffix/prefix-based) diff of the old and new text
+    /// to produce the smallest single edit that transforms old into new and
+    /// then calls [`edit`](Self::edit) with it.
+    ///
+    /// Returns whether any changes were made.
+    pub fn replace(&mut self, new: &str) -> bool {
+        let Some((prefix, suffix)) = self.replacement_range(new) else {
+            return false;
+        };
+
+        let old = self.text();
+        let replace = prefix..old.len() - suffix;
+        let with = &new[prefix..new.len() - suffix];
+        self.edit(replace, with);
+
+        true
+    }
+
+    /// Returns the common prefix and suffix lengths.
+    /// Returns [`None`] if the old and new strings are equal.
+    pub fn replacement_range(&self, new: &str) -> Option<(usize, usize)> {
+        let old = self.text();
+
+        let mut prefix =
+            zip(old.bytes(), new.bytes()).take_while(|(x, y)| x == y).count();
+
+        if prefix == old.len() && prefix == new.len() {
+            return None;
+        }
+
+        while !old.is_char_boundary(prefix) || !new.is_char_boundary(prefix) {
+            prefix -= 1;
+        }
+
+        let mut suffix = zip(old[prefix..].bytes().rev(), new[prefix..].bytes().rev())
+            .take_while(|(x, y)| x == y)
+            .count();
+
+        while !old.is_char_boundary(old.len() - suffix)
+            || !new.is_char_boundary(new.len() - suffix)
+        {
+            suffix += 1;
+        }
+
+        Some((prefix, suffix))
+    }
+
+    /// Edit the source file by replacing the given range.
+    ///
+    /// Returns the range in the new source that was ultimately reparsed.
+    ///
+    /// The method panics if the `replace` range is out of bounds.
+    #[track_caller]
+    pub fn edit(&mut self, replace: Range<usize>, with: &str) {
+        let start_byte = replace.start;
+        let start_utf16 = self.byte_to_utf16(start_byte).unwrap();
+        let line = self.byte_to_line(start_byte).unwrap();
+
+        let inner = Arc::make_mut(&mut self.0);
+
+        // Update the text itself.
+        inner.text.replace_range(replace.clone(), with);
+
+        // Remove invalidated line starts.
+        inner.lines.truncate(line + 1);
+
+        // Handle adjoining of \r and \n.
+        if inner.text[..start_byte].ends_with('\r') && with.starts_with('\n') {
+            inner.lines.pop();
+        }
+
+        // Recalculate the line starts after the edit.
+        inner.lines.extend(lines_from(
+            start_byte,
+            start_utf16,
+            &inner.text[start_byte..],
+        ));
+    }
+}
+
+impl<S: Hash> Hash for Lines<S> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.0.text.hash(state);
+    }
+}
+
+impl<S: AsRef<str>> AsRef<str> for Lines<S> {
+    fn as_ref(&self) -> &str {
+        self.0.text.as_ref()
+    }
+}
+
+/// Create a line vector.
+fn lines(text: &str) -> Vec<Line> {
+    std::iter::once(Line { byte_idx: 0, utf16_idx: 0 })
+        .chain(lines_from(0, 0, text))
+        .collect()
+}
+
+/// Compute a line iterator from an offset.
+fn lines_from(
+    byte_offset: usize,
+    utf16_offset: usize,
+    text: &str,
+) -> impl Iterator<Item = Line> + '_ {
+    let mut s = unscanny::Scanner::new(text);
+    let mut utf16_idx = utf16_offset;
+
+    std::iter::from_fn(move || {
+        s.eat_until(|c: char| {
+            utf16_idx += c.len_utf16();
+            is_newline(c)
+        });
+
+        if s.done() {
+            return None;
+        }
+
+        if s.eat() == Some('\r') && s.eat_if('\n') {
+            utf16_idx += 1;
+        }
+
+        Some(Line { byte_idx: byte_offset + s.cursor(), utf16_idx })
+    })
+}
+
+/// The number of code units this string would use if it was encoded in
+/// UTF16. This runs in linear time.
+fn len_utf16(string: &str) -> usize {
+    string.chars().map(char::len_utf16).sum()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const TEST: &str = "ä\tcde\nf💛g\r\nhi\rjkl";
+
+    #[test]
+    fn test_source_file_new() {
+        let lines = Lines::new(TEST);
+        assert_eq!(
+            lines.0.lines,
+            [
+                Line { byte_idx: 0, utf16_idx: 0 },
+                Line { byte_idx: 7, utf16_idx: 6 },
+                Line { byte_idx: 15, utf16_idx: 12 },
+                Line { byte_idx: 18, utf16_idx: 15 },
+            ]
+        );
+    }
+
+    #[test]
+    fn test_source_file_pos_to_line() {
+        let lines = Lines::new(TEST);
+        assert_eq!(lines.byte_to_line(0), Some(0));
+        assert_eq!(lines.byte_to_line(2), Some(0));
+        assert_eq!(lines.byte_to_line(6), Some(0));
+        assert_eq!(lines.byte_to_line(7), Some(1));
+        assert_eq!(lines.byte_to_line(8), Some(1));
+        assert_eq!(lines.byte_to_line(12), Some(1));
+        assert_eq!(lines.byte_to_line(21), Some(3));
+        assert_eq!(lines.byte_to_line(22), None);
+    }
+
+    #[test]
+    fn test_source_file_pos_to_column() {
+        let lines = Lines::new(TEST);
+        assert_eq!(lines.byte_to_column(0), Some(0));
+        assert_eq!(lines.byte_to_column(2), Some(1));
+        assert_eq!(lines.byte_to_column(6), Some(5));
+        assert_eq!(lines.byte_to_column(7), Some(0));
+        assert_eq!(lines.byte_to_column(8), Some(1));
+        assert_eq!(lines.byte_to_column(12), Some(2));
+    }
+
+    #[test]
+    fn test_source_file_utf16() {
+        #[track_caller]
+        fn roundtrip(lines: &Lines<&str>, byte_idx: usize, utf16_idx: usize) {
+            let middle = lines.byte_to_utf16(byte_idx).unwrap();
+            let result = lines.utf16_to_byte(middle).unwrap();
+            assert_eq!(middle, utf16_idx);
+            assert_eq!(result, byte_idx);
+        }
+
+        let lines = Lines::new(TEST);
+        roundtrip(&lines, 0, 0);
+        roundtrip(&lines, 2, 1);
+        roundtrip(&lines, 3, 2);
+        roundtrip(&lines, 8, 7);
+        roundtrip(&lines, 12, 9);
+        roundtrip(&lines, 21, 18);
+        assert_eq!(lines.byte_to_utf16(22), None);
+        assert_eq!(lines.utf16_to_byte(19), None);
+    }
+
+    #[test]
+    fn test_source_file_roundtrip() {
+        #[track_caller]
+        fn roundtrip(lines: &Lines<&str>, byte_idx: usize) {
+            let line = lines.byte_to_line(byte_idx).unwrap();
+            let column = lines.byte_to_column(byte_idx).unwrap();
+            let result = lines.line_column_to_byte(line, column).unwrap();
+            assert_eq!(result, byte_idx);
+        }
+
+        let lines = Lines::new(TEST);
+        roundtrip(&lines, 0);
+        roundtrip(&lines, 7);
+        roundtrip(&lines, 12);
+        roundtrip(&lines, 21);
+    }
+
+    #[test]
+    fn test_source_file_edit() {
+        // This tests only the non-parser parts. The reparsing itself is
+        // tested separately.
+        #[track_caller]
+        fn test(prev: &str, range: Range<usize>, with: &str, after: &str) {
+            let reference = Lines::new(after);
+
+            let mut edited = Lines::new(prev.to_string());
+            edited.edit(range.clone(), with);
+            assert_eq!(edited.text(), reference.text());
+            assert_eq!(edited.0.lines, reference.0.lines);
+
+            let mut replaced = Lines::new(prev.to_string());
+            replaced.replace(&{
+                let mut s = prev.to_string();
+                s.replace_range(range, with);
+                s
+            });
+            assert_eq!(replaced.text(), reference.text());
+            assert_eq!(replaced.0.lines, reference.0.lines);
+        }
+
+        // Test inserting at the beginning.
+        test("abc\n", 0..0, "hi\n", "hi\nabc\n");
+        test("\nabc", 0..0, "hi\r", "hi\r\nabc");
+
+        // Test editing in the middle.
+        test(TEST, 4..16, "❌", "ä\tc❌i\rjkl");
+
+        // Test appending.
+        test("abc\ndef", 7..7, "hi", "abc\ndefhi");
+        test("abc\ndef\n", 8..8, "hi", "abc\ndef\nhi");
+
+        // Test appending with adjoining \r and \n.
+        test("abc\ndef\r", 8..8, "\nghi", "abc\ndef\r\nghi");
+
+        // Test removing everything.
+        test(TEST, 0..21, "", "");
+    }
+}
diff --git a/crates/typst-syntax/src/source.rs b/crates/typst-syntax/src/source.rs
index 6ff94c73..514cb9a4 100644
--- a/crates/typst-syntax/src/source.rs
+++ b/crates/typst-syntax/src/source.rs
@@ -2,14 +2,14 @@
 
 use std::fmt::{self, Debug, Formatter};
 use std::hash::{Hash, Hasher};
-use std::iter::zip;
 use std::ops::Range;
 use std::sync::Arc;
 
 use typst_utils::LazyHash;
 
+use crate::lines::Lines;
 use crate::reparser::reparse;
-use crate::{is_newline, parse, FileId, LinkedNode, Span, SyntaxNode, VirtualPath};
+use crate::{parse, FileId, LinkedNode, Span, SyntaxNode, VirtualPath};
 
 /// A source file.
 ///
@@ -24,9 +24,8 @@ pub struct Source(Arc<Repr>);
 #[derive(Clone)]
 struct Repr {
     id: FileId,
-    text: LazyHash<String>,
     root: LazyHash<SyntaxNode>,
-    lines: Vec<Line>,
+    lines: LazyHash<Lines<String>>,
 }
 
 impl Source {
@@ -37,8 +36,7 @@ impl Source {
         root.numberize(id, Span::FULL).unwrap();
         Self(Arc::new(Repr {
             id,
-            lines: lines(&text),
-            text: LazyHash::new(text),
+            lines: LazyHash::new(Lines::new(text)),
             root: LazyHash::new(root),
         }))
     }
@@ -59,8 +57,13 @@ impl Source {
     }
 
     /// The whole source as a string slice.
+    pub fn lines(&self) -> Lines<String> {
+        Lines::clone(&self.0.lines)
+    }
+
+    /// The whole source as a string slice.
     pub fn text(&self) -> &str {
-        &self.0.text
+        self.0.lines.text()
     }
 
     /// Slice out the part of the source code enclosed by the range.
@@ -77,29 +80,12 @@ impl Source {
     /// Returns the range in the new source that was ultimately reparsed.
     pub fn replace(&mut self, new: &str) -> Range<usize> {
         let _scope = typst_timing::TimingScope::new("replace source");
-        let old = self.text();
-
-        let mut prefix =
-            zip(old.bytes(), new.bytes()).take_while(|(x, y)| x == y).count();
 
-        if prefix == old.len() && prefix == new.len() {
+        let Some((prefix, suffix)) = self.0.lines.replacement_range(new) else {
             return 0..0;
-        }
-
-        while !old.is_char_boundary(prefix) || !new.is_char_boundary(prefix) {
-            prefix -= 1;
-        }
-
-        let mut suffix = zip(old[prefix..].bytes().rev(), new[prefix..].bytes().rev())
-            .take_while(|(x, y)| x == y)
-            .count();
-
-        while !old.is_char_boundary(old.len() - suffix)
-            || !new.is_char_boundary(new.len() - suffix)
-        {
-            suffix += 1;
-        }
+        };
 
+        let old = self.text();
         let replace = prefix..old.len() - suffix;
         let with = &new[prefix..new.len() - suffix];
         self.edit(replace, with)
@@ -112,48 +98,28 @@ impl Source {
     /// The method panics if the `replace` range is out of bounds.
     #[track_caller]
     pub fn edit(&mut self, replace: Range<usize>, with: &str) -> Range<usize> {
-        let start_byte = replace.start;
-        let start_utf16 = self.byte_to_utf16(start_byte).unwrap();
-        let line = self.byte_to_line(start_byte).unwrap();
-
         let inner = Arc::make_mut(&mut self.0);
 
-        // Update the text itself.
-        inner.text.replace_range(replace.clone(), with);
-
-        // Remove invalidated line starts.
-        inner.lines.truncate(line + 1);
-
-        // Handle adjoining of \r and \n.
-        if inner.text[..start_byte].ends_with('\r') && with.starts_with('\n') {
-            inner.lines.pop();
-        }
-
-        // Recalculate the line starts after the edit.
-        inner.lines.extend(lines_from(
-            start_byte,
-            start_utf16,
-            &inner.text[start_byte..],
-        ));
+        // Update the text and lines.
+        inner.lines.edit(replace.clone(), with);
 
         // Incrementally reparse the replaced range.
-        reparse(&mut inner.root, &inner.text, replace, with.len())
+        reparse(&mut inner.root, inner.lines.text(), replace, with.len())
     }
 
     /// Get the length of the file in UTF-8 encoded bytes.
     pub fn len_bytes(&self) -> usize {
-        self.text().len()
+        self.0.lines.len_bytes()
     }
 
     /// Get the length of the file in UTF-16 code units.
     pub fn len_utf16(&self) -> usize {
-        let last = self.0.lines.last().unwrap();
-        last.utf16_idx + len_utf16(&self.0.text[last.byte_idx..])
+        self.0.lines.len_utf16()
     }
 
     /// Get the length of the file in lines.
     pub fn len_lines(&self) -> usize {
-        self.0.lines.len()
+        self.0.lines.len_lines()
     }
 
     /// Find the node with the given span.
@@ -171,85 +137,6 @@ impl Source {
     pub fn range(&self, span: Span) -> Option<Range<usize>> {
         Some(self.find(span)?.range())
     }
-
-    /// Return the index of the UTF-16 code unit at the byte index.
-    pub fn byte_to_utf16(&self, byte_idx: usize) -> Option<usize> {
-        let line_idx = self.byte_to_line(byte_idx)?;
-        let line = self.0.lines.get(line_idx)?;
-        let head = self.0.text.get(line.byte_idx..byte_idx)?;
-        Some(line.utf16_idx + len_utf16(head))
-    }
-
-    /// Return the index of the line that contains the given byte index.
-    pub fn byte_to_line(&self, byte_idx: usize) -> Option<usize> {
-        (byte_idx <= self.0.text.len()).then(|| {
-            match self.0.lines.binary_search_by_key(&byte_idx, |line| line.byte_idx) {
-                Ok(i) => i,
-                Err(i) => i - 1,
-            }
-        })
-    }
-
-    /// Return the index of the column at the byte index.
-    ///
-    /// The column is defined as the number of characters in the line before the
-    /// byte index.
-    pub fn byte_to_column(&self, byte_idx: usize) -> Option<usize> {
-        let line = self.byte_to_line(byte_idx)?;
-        let start = self.line_to_byte(line)?;
-        let head = self.get(start..byte_idx)?;
-        Some(head.chars().count())
-    }
-
-    /// Return the byte index at the UTF-16 code unit.
-    pub fn utf16_to_byte(&self, utf16_idx: usize) -> Option<usize> {
-        let line = self.0.lines.get(
-            match self.0.lines.binary_search_by_key(&utf16_idx, |line| line.utf16_idx) {
-                Ok(i) => i,
-                Err(i) => i - 1,
-            },
-        )?;
-
-        let mut k = line.utf16_idx;
-        for (i, c) in self.0.text[line.byte_idx..].char_indices() {
-            if k >= utf16_idx {
-                return Some(line.byte_idx + i);
-            }
-            k += c.len_utf16();
-        }
-
-        (k == utf16_idx).then_some(self.0.text.len())
-    }
-
-    /// Return the byte position at which the given line starts.
-    pub fn line_to_byte(&self, line_idx: usize) -> Option<usize> {
-        self.0.lines.get(line_idx).map(|line| line.byte_idx)
-    }
-
-    /// Return the range which encloses the given line.
-    pub fn line_to_range(&self, line_idx: usize) -> Option<Range<usize>> {
-        let start = self.line_to_byte(line_idx)?;
-        let end = self.line_to_byte(line_idx + 1).unwrap_or(self.0.text.len());
-        Some(start..end)
-    }
-
-    /// Return the byte index of the given (line, column) pair.
-    ///
-    /// The column defines the number of characters to go beyond the start of
-    /// the line.
-    pub fn line_column_to_byte(
-        &self,
-        line_idx: usize,
-        column_idx: usize,
-    ) -> Option<usize> {
-        let range = self.line_to_range(line_idx)?;
-        let line = self.get(range.clone())?;
-        let mut chars = line.chars();
-        for _ in 0..column_idx {
-            chars.next();
-        }
-        Some(range.start + (line.len() - chars.as_str().len()))
-    }
 }
 
 impl Debug for Source {
@@ -261,7 +148,7 @@ impl Debug for Source {
 impl Hash for Source {
     fn hash<H: Hasher>(&self, state: &mut H) {
         self.0.id.hash(state);
-        self.0.text.hash(state);
+        self.0.lines.hash(state);
         self.0.root.hash(state);
     }
 }
@@ -271,176 +158,3 @@ impl AsRef<str> for Source {
         self.text()
     }
 }
-
-/// Metadata about a line.
-#[derive(Debug, Copy, Clone, Eq, PartialEq)]
-struct Line {
-    /// The UTF-8 byte offset where the line starts.
-    byte_idx: usize,
-    /// The UTF-16 codepoint offset where the line starts.
-    utf16_idx: usize,
-}
-
-/// Create a line vector.
-fn lines(text: &str) -> Vec<Line> {
-    std::iter::once(Line { byte_idx: 0, utf16_idx: 0 })
-        .chain(lines_from(0, 0, text))
-        .collect()
-}
-
-/// Compute a line iterator from an offset.
-fn lines_from(
-    byte_offset: usize,
-    utf16_offset: usize,
-    text: &str,
-) -> impl Iterator<Item = Line> + '_ {
-    let mut s = unscanny::Scanner::new(text);
-    let mut utf16_idx = utf16_offset;
-
-    std::iter::from_fn(move || {
-        s.eat_until(|c: char| {
-            utf16_idx += c.len_utf16();
-            is_newline(c)
-        });
-
-        if s.done() {
-            return None;
-        }
-
-        if s.eat() == Some('\r') && s.eat_if('\n') {
-            utf16_idx += 1;
-        }
-
-        Some(Line { byte_idx: byte_offset + s.cursor(), utf16_idx })
-    })
-}
-
-/// The number of code units this string would use if it was encoded in
-/// UTF16. This runs in linear time.
-fn len_utf16(string: &str) -> usize {
-    string.chars().map(char::len_utf16).sum()
-}
-
-#[cfg(test)]
-mod tests {
-    use super::*;
-
-    const TEST: &str = "ä\tcde\nf💛g\r\nhi\rjkl";
-
-    #[test]
-    fn test_source_file_new() {
-        let source = Source::detached(TEST);
-        assert_eq!(
-            source.0.lines,
-            [
-                Line { byte_idx: 0, utf16_idx: 0 },
-                Line { byte_idx: 7, utf16_idx: 6 },
-                Line { byte_idx: 15, utf16_idx: 12 },
-                Line { byte_idx: 18, utf16_idx: 15 },
-            ]
-        );
-    }
-
-    #[test]
-    fn test_source_file_pos_to_line() {
-        let source = Source::detached(TEST);
-        assert_eq!(source.byte_to_line(0), Some(0));
-        assert_eq!(source.byte_to_line(2), Some(0));
-        assert_eq!(source.byte_to_line(6), Some(0));
-        assert_eq!(source.byte_to_line(7), Some(1));
-        assert_eq!(source.byte_to_line(8), Some(1));
-        assert_eq!(source.byte_to_line(12), Some(1));
-        assert_eq!(source.byte_to_line(21), Some(3));
-        assert_eq!(source.byte_to_line(22), None);
-    }
-
-    #[test]
-    fn test_source_file_pos_to_column() {
-        let source = Source::detached(TEST);
-        assert_eq!(source.byte_to_column(0), Some(0));
-        assert_eq!(source.byte_to_column(2), Some(1));
-        assert_eq!(source.byte_to_column(6), Some(5));
-        assert_eq!(source.byte_to_column(7), Some(0));
-        assert_eq!(source.byte_to_column(8), Some(1));
-        assert_eq!(source.byte_to_column(12), Some(2));
-    }
-
-    #[test]
-    fn test_source_file_utf16() {
-        #[track_caller]
-        fn roundtrip(source: &Source, byte_idx: usize, utf16_idx: usize) {
-            let middle = source.byte_to_utf16(byte_idx).unwrap();
-            let result = source.utf16_to_byte(middle).unwrap();
-            assert_eq!(middle, utf16_idx);
-            assert_eq!(result, byte_idx);
-        }
-
-        let source = Source::detached(TEST);
-        roundtrip(&source, 0, 0);
-        roundtrip(&source, 2, 1);
-        roundtrip(&source, 3, 2);
-        roundtrip(&source, 8, 7);
-        roundtrip(&source, 12, 9);
-        roundtrip(&source, 21, 18);
-        assert_eq!(source.byte_to_utf16(22), None);
-        assert_eq!(source.utf16_to_byte(19), None);
-    }
-
-    #[test]
-    fn test_source_file_roundtrip() {
-        #[track_caller]
-        fn roundtrip(source: &Source, byte_idx: usize) {
-            let line = source.byte_to_line(byte_idx).unwrap();
-            let column = source.byte_to_column(byte_idx).unwrap();
-            let result = source.line_column_to_byte(line, column).unwrap();
-            assert_eq!(result, byte_idx);
-        }
-
-        let source = Source::detached(TEST);
-        roundtrip(&source, 0);
-        roundtrip(&source, 7);
-        roundtrip(&source, 12);
-        roundtrip(&source, 21);
-    }
-
-    #[test]
-    fn test_source_file_edit() {
-        // This tests only the non-parser parts. The reparsing itself is
-        // tested separately.
-        #[track_caller]
-        fn test(prev: &str, range: Range<usize>, with: &str, after: &str) {
-            let reference = Source::detached(after);
-
-            let mut edited = Source::detached(prev);
-            edited.edit(range.clone(), with);
-            assert_eq!(edited.text(), reference.text());
-            assert_eq!(edited.0.lines, reference.0.lines);
-
-            let mut replaced = Source::detached(prev);
-            replaced.replace(&{
-                let mut s = prev.to_string();
-                s.replace_range(range, with);
-                s
-            });
-            assert_eq!(replaced.text(), reference.text());
-            assert_eq!(replaced.0.lines, reference.0.lines);
-        }
-
-        // Test inserting at the beginning.
-        test("abc\n", 0..0, "hi\n", "hi\nabc\n");
-        test("\nabc", 0..0, "hi\r", "hi\r\nabc");
-
-        // Test editing in the middle.
-        test(TEST, 4..16, "❌", "ä\tc❌i\rjkl");
-
-        // Test appending.
-        test("abc\ndef", 7..7, "hi", "abc\ndefhi");
-        test("abc\ndef\n", 8..8, "hi", "abc\ndef\nhi");
-
-        // Test appending with adjoining \r and \n.
-        test("abc\ndef\r", 8..8, "\nghi", "abc\ndef\r\nghi");
-
-        // Test removing everything.
-        test(TEST, 0..21, "", "");
-    }
-}