New source loading architecture

19 files changed, 688 insertions, 613 deletions

diff --git a/src/diag.rs b/src/diag.rs
index 397a833f..0fcdef17 100644
--- a/src/diag.rs
+++ b/src/diag.rs
@@ -2,7 +2,7 @@
 
 use serde::{Deserialize, Serialize};
 
-use crate::loading::FileId;
+use crate::source::SourceId;
 use crate::syntax::Span;
 
 /// The result type for typesetting and all its subpasses.
@@ -14,14 +14,14 @@ pub type StrResult<T> = Result<T, String>;
 /// An error in a source file.
 #[derive(Debug, Clone, Eq, PartialEq, Ord, PartialOrd, Serialize, Deserialize)]
 pub struct Error {
-    /// The file that contains the error.
-    pub file: FileId,
+    /// The id of the source file that contains the error.
+    pub source: SourceId,
     /// The erroneous location in the source code.
     pub span: Span,
     /// A diagnostic message describing the problem.
     pub message: String,
     /// The trace of function calls leading to the error.
-    pub trace: Vec<(FileId, Span, Tracepoint)>,
+    pub trace: Vec<(SourceId, Span, Tracepoint)>,
 }
 
 /// A part of an error's [trace](Error::trace).
@@ -35,9 +35,13 @@ pub enum Tracepoint {
 
 impl Error {
     /// Create a new, bare error.
-    pub fn new(file: FileId, span: impl Into<Span>, message: impl Into<String>) -> Self {
+    pub fn new(
+        source: SourceId,
+        span: impl Into<Span>,
+        message: impl Into<String>,
+    ) -> Self {
         Self {
-            file,
+            source,
             span: span.into(),
             trace: vec![],
             message: message.into(),
@@ -47,11 +51,11 @@ impl Error {
     /// Create a boxed vector containing one error. The return value is suitable
     /// as the `Err` variant of a [`TypResult`].
     pub fn boxed(
-        file: FileId,
+        source: SourceId,
         span: impl Into<Span>,
         message: impl Into<String>,
     ) -> Box<Vec<Self>> {
-        Box::new(vec![Self::new(file, span, message)])
+        Box::new(vec![Self::new(source, span, message)])
     }
 
     /// Partially build a vec-boxed error, returning a function that just needs
@@ -60,23 +64,23 @@ impl Error {
     /// This is useful in to convert from [`StrResult`] to a [`TypResult`] using
     /// [`map_err`](Result::map_err).
     pub fn partial(
-        file: FileId,
+        source: SourceId,
         span: impl Into<Span>,
     ) -> impl FnOnce(String) -> Box<Vec<Self>> {
-        move |message| Self::boxed(file, span, message)
+        move |message| Self::boxed(source, span, message)
     }
 }
 
 /// Early-return with a vec-boxed [`Error`].
 #[macro_export]
 macro_rules! bail {
-    ($file:expr, $span:expr, $message:expr $(,)?) => {
+    ($source:expr, $span:expr, $message:expr $(,)?) => {
         return Err(Box::new(vec![$crate::diag::Error::new(
-            $file, $span, $message,
+            $source, $span, $message,
         )]));
     };
 
-    ($file:expr, $span:expr, $fmt:expr, $($arg:expr),+ $(,)?) => {
-        $crate::bail!($file, $span, format!($fmt, $($arg),+));
+    ($source:expr, $span:expr, $fmt:expr, $($arg:expr),+ $(,)?) => {
+        $crate::bail!($source, $span, format!($fmt, $($arg),+));
     };
 }
diff --git a/src/eval/function.rs b/src/eval/function.rs
index b9a168d2..8b1e883f 100644
--- a/src/eval/function.rs
+++ b/src/eval/function.rs
@@ -4,7 +4,7 @@ use std::rc::Rc;
 
 use super::{Cast, EvalContext, Value};
 use crate::diag::{Error, TypResult};
-use crate::loading::FileId;
+use crate::source::SourceId;
 use crate::syntax::{Span, Spanned};
 use crate::util::EcoString;
 
@@ -59,8 +59,8 @@ impl PartialEq for Function {
 /// Evaluated arguments to a function.
 #[derive(Debug, Clone, PartialEq)]
 pub struct FuncArgs {
-    /// The file in which the function was called.
-    pub file: FileId,
+    /// The id of the source file in which the function was called.
+    pub source: SourceId,
     /// The span of the whole argument list.
     pub span: Span,
     /// The positional arguments.
@@ -103,7 +103,7 @@ impl FuncArgs {
     {
         match self.eat() {
             Some(found) => Ok(found),
-            None => bail!(self.file, self.span, "missing argument: {}", what),
+            None => bail!(self.source, self.span, "missing argument: {}", what),
         }
     }
 
@@ -134,14 +134,14 @@ impl FuncArgs {
         let value = self.items.remove(index).value;
         let span = value.span;
 
-        T::cast(value).map(Some).map_err(Error::partial(self.file, span))
+        T::cast(value).map(Some).map_err(Error::partial(self.source, span))
     }
 
     /// Return an "unexpected argument" error if there is any remaining
     /// argument.
     pub fn finish(self) -> TypResult<()> {
         if let Some(arg) = self.items.first() {
-            bail!(self.file, arg.span, "unexpected argument");
+            bail!(self.source, arg.span, "unexpected argument");
         }
         Ok(())
     }
diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index 8f5532eb..22c7c0b4 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -21,30 +21,35 @@ pub use template::*;
 pub use value::*;
 
 use std::collections::HashMap;
+use std::io;
 use std::mem;
-use std::path::Path;
+use std::path::{Path, PathBuf};
 use std::rc::Rc;
 
 use crate::diag::{Error, StrResult, Tracepoint, TypResult};
 use crate::geom::{Angle, Fractional, Length, Relative};
-use crate::image::ImageCache;
-use crate::loading::{FileId, Loader};
+use crate::image::ImageStore;
+use crate::loading::Loader;
 use crate::parse::parse;
-use crate::source::{SourceFile, SourceMap};
+use crate::source::{SourceId, SourceStore};
 use crate::syntax::visit::Visit;
 use crate::syntax::*;
 use crate::util::EcoString;
 use crate::Context;
 
 /// Evaluate a parsed source file into a module.
-pub fn eval(ctx: &mut Context, file: FileId, ast: Rc<SyntaxTree>) -> TypResult<Module> {
-    let mut ctx = EvalContext::new(ctx, file);
+pub fn eval(
+    ctx: &mut Context,
+    source: SourceId,
+    ast: Rc<SyntaxTree>,
+) -> TypResult<Module> {
+    let mut ctx = EvalContext::new(ctx, source);
     let template = ast.eval(&mut ctx)?;
     Ok(Module { scope: ctx.scopes.top, template })
 }
 
 /// Caches evaluated modules.
-pub type ModuleCache = HashMap<FileId, Module>;
+pub type ModuleCache = HashMap<SourceId, Module>;
 
 /// An evaluated module, ready for importing or execution.
 #[derive(Debug, Clone, PartialEq)]
@@ -68,100 +73,99 @@ pub trait Eval {
 pub struct EvalContext<'a> {
     /// The loader from which resources (files and images) are loaded.
     pub loader: &'a dyn Loader,
-    /// The store for source files.
-    pub sources: &'a mut SourceMap,
-    /// The cache for decoded images.
-    pub images: &'a mut ImageCache,
-    /// The cache for loaded modules.
+    /// Stores loaded source files.
+    pub sources: &'a mut SourceStore,
+    /// Stores decoded images.
+    pub images: &'a mut ImageStore,
+    /// Caches evaluated modules.
     pub modules: &'a mut ModuleCache,
     /// The active scopes.
     pub scopes: Scopes<'a>,
-    /// The currently evaluated file.
-    pub file: FileId,
+    /// The id of the currently evaluated source file.
+    pub source: SourceId,
     /// The stack of imported files that led to evaluation of the current file.
-    pub route: Vec<FileId>,
+    pub route: Vec<SourceId>,
     /// The expression map for the currently built template.
     pub map: ExprMap,
 }
 
 impl<'a> EvalContext<'a> {
     /// Create a new evaluation context.
-    pub fn new(ctx: &'a mut Context, file: FileId) -> Self {
+    pub fn new(ctx: &'a mut Context, source: SourceId) -> Self {
         Self {
             loader: ctx.loader.as_ref(),
             sources: &mut ctx.sources,
             images: &mut ctx.images,
             modules: &mut ctx.modules,
             scopes: Scopes::new(Some(&ctx.std)),
-            file,
+            source,
             route: vec![],
             map: ExprMap::new(),
         }
     }
 
-    /// Resolve a path relative to the current file.
-    ///
-    /// Returns an error if the file is not found.
-    pub fn resolve(&mut self, path: &str, span: Span) -> TypResult<FileId> {
-        self.loader
-            .resolve_from(self.file, Path::new(path))
-            .map_err(|_| Error::boxed(self.file, span, "file not found"))
-    }
-
     /// Process an import of a module relative to the current location.
-    pub fn import(&mut self, path: &str, span: Span) -> TypResult<FileId> {
-        let file = self.resolve(path, span)?;
+    pub fn import(&mut self, path: &str, span: Span) -> TypResult<SourceId> {
+        // Load the source file.
+        let full = self.relpath(path);
+        let id = self.sources.load(&full).map_err(|err| {
+            Error::boxed(self.source, span, match err.kind() {
+                io::ErrorKind::NotFound => "file not found".into(),
+                _ => format!("failed to load source file ({})", err),
+            })
+        })?;
 
         // Prevent cyclic importing.
-        if self.file == file || self.route.contains(&file) {
-            bail!(self.file, span, "cyclic import");
+        if self.source == id || self.route.contains(&id) {
+            bail!(self.source, span, "cyclic import");
         }
 
         // Check whether the module was already loaded.
-        if self.modules.get(&file).is_some() {
-            return Ok(file);
+        if self.modules.get(&id).is_some() {
+            return Ok(id);
         }
 
-        // Load the source file.
-        let buffer = self
-            .loader
-            .load_file(file)
-            .map_err(|_| Error::boxed(self.file, span, "failed to load file"))?;
-
-        // Decode UTF-8.
-        let string = String::from_utf8(buffer)
-            .map_err(|_| Error::boxed(self.file, span, "file is not valid utf-8"))?;
-
         // Parse the file.
-        let source = self.sources.insert(SourceFile::new(file, string));
+        let source = self.sources.get(id);
         let ast = parse(&source)?;
 
         // Prepare the new context.
         let new_scopes = Scopes::new(self.scopes.base);
         let old_scopes = mem::replace(&mut self.scopes, new_scopes);
-        self.route.push(self.file);
-        self.file = file;
+        self.route.push(self.source);
+        self.source = id;
 
         // Evaluate the module.
         let result = Rc::new(ast).eval(self);
 
         // Restore the old context.
         let new_scopes = mem::replace(&mut self.scopes, old_scopes);
-        self.file = self.route.pop().unwrap();
+        self.source = self.route.pop().unwrap();
 
         // Add a tracepoint to the errors.
         let template = result.map_err(|mut errors| {
             for error in errors.iter_mut() {
-                error.trace.push((self.file, span, Tracepoint::Import));
+                error.trace.push((self.source, span, Tracepoint::Import));
             }
             errors
         })?;
 
         // Save the evaluated module.
         let module = Module { scope: new_scopes.top, template };
-        self.modules.insert(file, module);
+        self.modules.insert(id, module);
+
+        Ok(id)
+    }
 
-        Ok(file)
+    /// Complete a path that is relative to the current file to be relative to
+    /// the environment's current directory.
+    pub fn relpath(&self, path: impl AsRef<Path>) -> PathBuf {
+        self.sources
+            .get(self.source)
+            .path()
+            .parent()
+            .expect("is a file")
+            .join(path)
     }
 }
 
@@ -231,7 +235,7 @@ impl Eval for Expr {
             Self::Str(_, ref v) => Value::Str(v.clone()),
             Self::Ident(ref v) => match ctx.scopes.get(&v) {
                 Some(slot) => slot.borrow().clone(),
-                None => bail!(ctx.file, v.span, "unknown variable"),
+                None => bail!(ctx.source, v.span, "unknown variable"),
             },
             Self::Array(ref v) => Value::Array(v.eval(ctx)?),
             Self::Dict(ref v) => Value::Dict(v.eval(ctx)?),
@@ -300,7 +304,7 @@ impl Eval for BlockExpr {
         for expr in &self.exprs {
             let value = expr.eval(ctx)?;
             output = ops::join(output, value)
-                .map_err(Error::partial(ctx.file, expr.span()))?;
+                .map_err(Error::partial(ctx.source, expr.span()))?;
         }
 
         if self.scoping {
@@ -321,7 +325,7 @@ impl Eval for UnaryExpr {
             UnOp::Neg => ops::neg(value),
             UnOp::Not => ops::not(value),
         };
-        result.map_err(Error::partial(ctx.file, self.span))
+        result.map_err(Error::partial(ctx.source, self.span))
     }
 }
 
@@ -368,7 +372,7 @@ impl BinaryExpr {
         }
 
         let rhs = self.rhs.eval(ctx)?;
-        op(lhs, rhs).map_err(Error::partial(ctx.file, self.span))
+        op(lhs, rhs).map_err(Error::partial(ctx.source, self.span))
     }
 
     /// Apply an assignment operation.
@@ -380,22 +384,22 @@ impl BinaryExpr {
         let slot = if let Expr::Ident(id) = self.lhs.as_ref() {
             match ctx.scopes.get(id) {
                 Some(slot) => Rc::clone(slot),
-                None => bail!(ctx.file, lspan, "unknown variable"),
+                None => bail!(ctx.source, lspan, "unknown variable"),
             }
         } else {
-            bail!(ctx.file, lspan, "cannot assign to this expression",);
+            bail!(ctx.source, lspan, "cannot assign to this expression",);
         };
 
         let rhs = self.rhs.eval(ctx)?;
         let mut mutable = match slot.try_borrow_mut() {
             Ok(mutable) => mutable,
             Err(_) => {
-                bail!(ctx.file, lspan, "cannot assign to a constant",);
+                bail!(ctx.source, lspan, "cannot assign to a constant",);
             }
         };
 
         let lhs = mem::take(&mut *mutable);
-        *mutable = op(lhs, rhs).map_err(Error::partial(ctx.file, self.span))?;
+        *mutable = op(lhs, rhs).map_err(Error::partial(ctx.source, self.span))?;
 
         Ok(Value::None)
     }
@@ -409,18 +413,18 @@ impl Eval for CallExpr {
             .callee
             .eval(ctx)?
             .cast::<Function>()
-            .map_err(Error::partial(ctx.file, self.callee.span()))?;
+            .map_err(Error::partial(ctx.source, self.callee.span()))?;
 
         let mut args = self.args.eval(ctx)?;
         let returned = callee(ctx, &mut args).map_err(|mut errors| {
             for error in errors.iter_mut() {
                 // Skip errors directly related to arguments.
-                if error.file == ctx.file && self.span.contains(error.span) {
+                if error.source == ctx.source && self.span.contains(error.span) {
                     continue;
                 }
 
                 error.trace.push((
-                    ctx.file,
+                    ctx.source,
                     self.span,
                     Tracepoint::Call(callee.name().map(Into::into)),
                 ));
@@ -439,7 +443,7 @@ impl Eval for CallArgs {
 
     fn eval(&self, ctx: &mut EvalContext) -> TypResult<Self::Output> {
         Ok(FuncArgs {
-            file: ctx.file,
+            source: ctx.source,
             span: self.span,
             items: self
                 .items
@@ -473,7 +477,7 @@ impl Eval for ClosureExpr {
     type Output = Value;
 
     fn eval(&self, ctx: &mut EvalContext) -> TypResult<Self::Output> {
-        let file = ctx.file;
+        let file = ctx.source;
         let params = Rc::clone(&self.params);
         let body = Rc::clone(&self.body);
 
@@ -489,7 +493,7 @@ impl Eval for ClosureExpr {
             // Don't leak the scopes from the call site. Instead, we use the
             // scope of captured variables we collected earlier.
             let prev_scopes = mem::take(&mut ctx.scopes);
-            let prev_file = mem::replace(&mut ctx.file, file);
+            let prev_file = mem::replace(&mut ctx.source, file);
             ctx.scopes.top = captured.clone();
 
             for param in params.iter() {
@@ -499,7 +503,7 @@ impl Eval for ClosureExpr {
 
             let result = body.eval(ctx);
             ctx.scopes = prev_scopes;
-            ctx.file = prev_file;
+            ctx.source = prev_file;
             result
         });
 
@@ -515,7 +519,7 @@ impl Eval for WithExpr {
             .callee
             .eval(ctx)?
             .cast::<Function>()
-            .map_err(Error::partial(ctx.file, self.callee.span()))?;
+            .map_err(Error::partial(ctx.source, self.callee.span()))?;
 
         let applied = self.args.eval(ctx)?;
 
@@ -565,7 +569,7 @@ impl Eval for IfExpr {
             .condition
             .eval(ctx)?
             .cast::<bool>()
-            .map_err(Error::partial(ctx.file, self.condition.span()))?;
+            .map_err(Error::partial(ctx.source, self.condition.span()))?;
 
         if condition {
             self.if_body.eval(ctx)
@@ -587,11 +591,11 @@ impl Eval for WhileExpr {
             .condition
             .eval(ctx)?
             .cast::<bool>()
-            .map_err(Error::partial(ctx.file, self.condition.span()))?
+            .map_err(Error::partial(ctx.source, self.condition.span()))?
         {
             let value = self.body.eval(ctx)?;
             output = ops::join(output, value)
-                .map_err(Error::partial(ctx.file, self.body.span()))?;
+                .map_err(Error::partial(ctx.source, self.body.span()))?;
         }
 
         Ok(output)
@@ -613,7 +617,7 @@ impl Eval for ForExpr {
 
                     let value = self.body.eval(ctx)?;
                     output = ops::join(output, value)
-                        .map_err(Error::partial(ctx.file, self.body.span()))?;
+                        .map_err(Error::partial(ctx.source, self.body.span()))?;
                 }
 
                 ctx.scopes.exit();
@@ -639,10 +643,10 @@ impl Eval for ForExpr {
                 iter!(for (k => key, v => value) in dict.into_iter())
             }
             (ForPattern::KeyValue(_, _), Value::Str(_)) => {
-                bail!(ctx.file, self.pattern.span(), "mismatched pattern");
+                bail!(ctx.source, self.pattern.span(), "mismatched pattern");
             }
             (_, iter) => bail!(
-                ctx.file,
+                ctx.source,
                 self.iter.span(),
                 "cannot loop over {}",
                 iter.type_name(),
@@ -659,7 +663,7 @@ impl Eval for ImportExpr {
             .path
             .eval(ctx)?
             .cast::<EcoString>()
-            .map_err(Error::partial(ctx.file, self.path.span()))?;
+            .map_err(Error::partial(ctx.source, self.path.span()))?;
 
         let file = ctx.import(&path, self.path.span())?;
         let module = &ctx.modules[&file];
@@ -675,7 +679,7 @@ impl Eval for ImportExpr {
                     if let Some(slot) = module.scope.get(&ident) {
                         ctx.scopes.def_mut(ident.as_str(), slot.borrow().clone());
                     } else {
-                        bail!(ctx.file, ident.span, "unresolved import");
+                        bail!(ctx.source, ident.span, "unresolved import");
                     }
                 }
             }
@@ -693,7 +697,7 @@ impl Eval for IncludeExpr {
             .path
             .eval(ctx)?
             .cast::<EcoString>()
-            .map_err(Error::partial(ctx.file, self.path.span()))?;
+            .map_err(Error::partial(ctx.source, self.path.span()))?;
 
         let file = ctx.import(&path, self.path.span())?;
         let module = &ctx.modules[&file];
diff --git a/src/export/pdf.rs b/src/export/pdf.rs
index bfd36421..d4b3ac25 100644
--- a/src/export/pdf.rs
+++ b/src/export/pdf.rs
@@ -14,17 +14,17 @@ use pdf_writer::{
 use ttf_parser::{name_id, GlyphId};
 
 use crate::color::Color;
-use crate::font::{Em, FaceId, FontCache};
+use crate::font::{Em, FaceId, FontStore};
 use crate::geom::{self, Length, Size};
-use crate::image::{Image, ImageCache, ImageId};
+use crate::image::{Image, ImageId, ImageStore};
 use crate::layout::{Element, Frame, Geometry, Paint};
 use crate::Context;
 
 /// Export a collection of frames into a PDF document.
 ///
 /// This creates one page per frame. In addition to the frames, you need to pass
-/// in the cache used during compilation such that things like fonts and images
-/// can be included in the PDF.
+/// in the context used during compilation such that things like fonts and
+/// images can be included in the PDF.
 ///
 /// Returns the raw bytes making up the PDF document.
 pub fn pdf(ctx: &Context, frames: &[Rc<Frame>]) -> Vec<u8> {
@@ -33,19 +33,16 @@ pub fn pdf(ctx: &Context, frames: &[Rc<Frame>]) -> Vec<u8> {
 
 struct PdfExporter<'a> {
     writer: PdfWriter,
+    refs: Refs,
     frames: &'a [Rc<Frame>],
-    fonts: &'a FontCache,
+    fonts: &'a FontStore,
+    images: &'a ImageStore,
     font_map: Remapper<FaceId>,
-    images: &'a ImageCache,
     image_map: Remapper<ImageId>,
-    refs: Refs,
 }
 
 impl<'a> PdfExporter<'a> {
     fn new(ctx: &'a Context, frames: &'a [Rc<Frame>]) -> Self {
-        let mut writer = PdfWriter::new(1, 7);
-        writer.set_indent(2);
-
         let mut font_map = Remapper::new();
         let mut image_map = Remapper::new();
         let mut alpha_masks = 0;
@@ -66,14 +63,15 @@ impl<'a> PdfExporter<'a> {
             }
         }
 
-        let refs = Refs::new(frames.len(), font_map.len(), image_map.len(), alpha_masks);
+        let mut writer = PdfWriter::new(1, 7);
+        writer.set_indent(2);
 
         Self {
             writer,
+            refs: Refs::new(frames.len(), font_map.len(), image_map.len(), alpha_masks),
             frames,
             fonts: &ctx.fonts,
             images: &ctx.images,
-            refs,
             font_map,
             image_map,
         }
diff --git a/src/font.rs b/src/font.rs
index a609e934..e756f84e 100644
--- a/src/font.rs
+++ b/src/font.rs
@@ -3,13 +3,151 @@
 use std::collections::{hash_map::Entry, HashMap};
 use std::fmt::{self, Debug, Display, Formatter};
 use std::ops::Add;
+use std::path::PathBuf;
 use std::rc::Rc;
 
 use decorum::N64;
 use serde::{Deserialize, Serialize};
 
 use crate::geom::Length;
-use crate::loading::{FileId, Loader};
+use crate::loading::{FileHash, Loader};
+
+/// A unique identifier for a loaded font face.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[derive(Serialize, Deserialize)]
+pub struct FaceId(u32);
+
+impl FaceId {
+    /// Create a face id from the raw underlying value.
+    ///
+    /// This should only be called with values returned by
+    /// [`into_raw`](Self::into_raw).
+    pub const fn from_raw(v: u32) -> Self {
+        Self(v)
+    }
+
+    /// Convert into the raw underlying value.
+    pub const fn into_raw(self) -> u32 {
+        self.0
+    }
+}
+
+/// Storage for loaded and parsed font faces.
+pub struct FontStore {
+    loader: Rc<dyn Loader>,
+    faces: Vec<Option<Face>>,
+    families: HashMap<String, Vec<FaceId>>,
+    buffers: HashMap<FileHash, Rc<Vec<u8>>>,
+    on_load: Option<Box<dyn Fn(FaceId, &Face)>>,
+}
+
+impl FontStore {
+    /// Create a new, empty font store.
+    pub fn new(loader: Rc<dyn Loader>) -> Self {
+        let mut faces = vec![];
+        let mut families = HashMap::<String, Vec<FaceId>>::new();
+
+        for (i, info) in loader.faces().iter().enumerate() {
+            let id = FaceId(i as u32);
+            faces.push(None);
+            families
+                .entry(info.family.to_lowercase())
+                .and_modify(|vec| vec.push(id))
+                .or_insert_with(|| vec![id]);
+        }
+
+        Self {
+            loader,
+            faces,
+            families,
+            buffers: HashMap::new(),
+            on_load: None,
+        }
+    }
+
+    /// Register a callback which is invoked each time a font face is loaded.
+    pub fn on_load<F>(&mut self, f: F)
+    where
+        F: Fn(FaceId, &Face) + 'static,
+    {
+        self.on_load = Some(Box::new(f));
+    }
+
+    /// Query for and load the font face from the given `family` that most
+    /// closely matches the given `variant`.
+    pub fn select(&mut self, family: &str, variant: FontVariant) -> Option<FaceId> {
+        // Check whether a family with this name exists.
+        let ids = self.families.get(family)?;
+        let infos = self.loader.faces();
+
+        let mut best = None;
+        let mut best_key = None;
+
+        // Find the best matching variant of this font.
+        for &id in ids {
+            let current = infos[id.0 as usize].variant;
+
+            // This is a perfect match, no need to search further.
+            if current == variant {
+                best = Some(id);
+                break;
+            }
+
+            // If this is not a perfect match, we compute a key that we want to
+            // minimize among all variants. This key prioritizes style, then
+            // stretch distance and then weight distance.
+            let key = (
+                current.style != variant.style,
+                current.stretch.distance(variant.stretch),
+                current.weight.distance(variant.weight),
+            );
+
+            if best_key.map_or(true, |b| key < b) {
+                best = Some(id);
+                best_key = Some(key);
+            }
+        }
+
+        let id = best?;
+
+        // Load the face if it's not already loaded.
+        let idx = id.0 as usize;
+        let slot = &mut self.faces[idx];
+        if slot.is_none() {
+            let FaceInfo { ref path, index, .. } = infos[idx];
+
+            // Check the buffer cache since multiple faces may
+            // refer to the same data (font collection).
+            let hash = self.loader.resolve(path).ok()?;
+            let buffer = match self.buffers.entry(hash) {
+                Entry::Occupied(entry) => entry.into_mut(),
+                Entry::Vacant(entry) => {
+                    let buffer = self.loader.load(path).ok()?;
+                    entry.insert(Rc::new(buffer))
+                }
+            };
+
+            let face = Face::new(Rc::clone(buffer), index)?;
+            if let Some(callback) = &self.on_load {
+                callback(id, &face);
+            }
+
+            *slot = Some(face);
+        }
+
+        Some(id)
+    }
+
+    /// Get a reference to a loaded face.
+    ///
+    /// This panics if no face with this id was loaded. This function should
+    /// only be called with ids returned by this store's
+    /// [`select()`](Self::select) method.
+    #[track_caller]
+    pub fn get(&self, id: FaceId) -> &Face {
+        self.faces[id.0 as usize].as_ref().expect("font face was not loaded")
+    }
+}
 
 /// A font face.
 pub struct Face {
@@ -53,18 +191,20 @@ impl Face {
         let cap_height = ttf.capital_height().filter(|&h| h > 0).map_or(ascender, to_em);
         let x_height = ttf.x_height().filter(|&h| h > 0).map_or(ascender, to_em);
         let descender = to_em(ttf.typographic_descender().unwrap_or(ttf.descender()));
-
         let strikeout = ttf.strikeout_metrics();
         let underline = ttf.underline_metrics();
-        let default = Em::new(0.06);
 
         let strikethrough = LineMetrics {
-            strength: strikeout.or(underline).map_or(default, |s| to_em(s.thickness)),
+            strength: strikeout
+                .or(underline)
+                .map_or(Em::new(0.06), |s| to_em(s.thickness)),
             position: strikeout.map_or(Em::new(0.25), |s| to_em(s.position)),
         };
 
         let underline = LineMetrics {
-            strength: underline.or(strikeout).map_or(default, |s| to_em(s.thickness)),
+            strength: underline
+                .or(strikeout)
+                .map_or(Em::new(0.06), |s| to_em(s.thickness)),
             position: underline.map_or(Em::new(-0.2), |s| to_em(s.position)),
         };
 
@@ -127,39 +267,6 @@ impl Face {
     }
 }
 
-/// Identifies a vertical metric of a font.
-#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
-pub enum VerticalFontMetric {
-    /// The distance from the baseline to the typographic ascender.
-    ///
-    /// Corresponds to the typographic ascender from the `OS/2` table if present
-    /// and falls back to the ascender from the `hhea` table otherwise.
-    Ascender,
-    /// The approximate height of uppercase letters.
-    CapHeight,
-    /// The approximate height of non-ascending lowercase letters.
-    XHeight,
-    /// The baseline on which the letters rest.
-    Baseline,
-    /// The distance from the baseline to the typographic descender.
-    ///
-    /// Corresponds to the typographic descender from the `OS/2` table if
-    /// present and falls back to the descender from the `hhea` table otherwise.
-    Descender,
-}
-
-impl Display for VerticalFontMetric {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        f.pad(match self {
-            Self::Ascender => "ascender",
-            Self::CapHeight => "cap-height",
-            Self::XHeight => "x-height",
-            Self::Baseline => "baseline",
-            Self::Descender => "descender",
-        })
-    }
-}
-
 /// A length in em units.
 ///
 /// `1em` is the same as the font size.
@@ -201,137 +308,36 @@ impl Add for Em {
     }
 }
 
-/// Caches parsed font faces.
-pub struct FontCache {
-    loader: Rc<dyn Loader>,
-    faces: Vec<Option<Face>>,
-    families: HashMap<String, Vec<FaceId>>,
-    buffers: HashMap<FileId, Rc<Vec<u8>>>,
-    on_load: Option<Box<dyn Fn(FaceId, &Face)>>,
-}
-
-impl FontCache {
-    /// Create a new, empty font cache.
-    pub fn new(loader: Rc<dyn Loader>) -> Self {
-        let mut faces = vec![];
-        let mut families = HashMap::<String, Vec<FaceId>>::new();
-
-        for (i, info) in loader.faces().iter().enumerate() {
-            let id = FaceId(i as u64);
-            faces.push(None);
-            families
-                .entry(info.family.to_lowercase())
-                .and_modify(|vec| vec.push(id))
-                .or_insert_with(|| vec![id]);
-        }
-
-        Self {
-            loader,
-            faces,
-            families,
-            buffers: HashMap::new(),
-            on_load: None,
-        }
-    }
-
-    /// Query for and load the font face from the given `family` that most
-    /// closely matches the given `variant`.
-    pub fn select(&mut self, family: &str, variant: FontVariant) -> Option<FaceId> {
-        // Check whether a family with this name exists.
-        let ids = self.families.get(family)?;
-        let infos = self.loader.faces();
-
-        let mut best = None;
-        let mut best_key = None;
-
-        // Find the best matching variant of this font.
-        for &id in ids {
-            let current = infos[id.0 as usize].variant;
-
-            // This is a perfect match, no need to search further.
-            if current == variant {
-                best = Some(id);
-                break;
-            }
-
-            // If this is not a perfect match, we compute a key that we want to
-            // minimize among all variants. This key prioritizes style, then
-            // stretch distance and then weight distance.
-            let key = (
-                current.style != variant.style,
-                current.stretch.distance(variant.stretch),
-                current.weight.distance(variant.weight),
-            );
-
-            if best_key.map_or(true, |b| key < b) {
-                best = Some(id);
-                best_key = Some(key);
-            }
-        }
-
-        // Load the face if it's not already loaded.
-        let id = best?;
-        let idx = id.0 as usize;
-        let slot = &mut self.faces[idx];
-        if slot.is_none() {
-            let FaceInfo { file, index, .. } = infos[idx];
-
-            // Check the buffer cache since multiple faces may
-            // refer to the same data (font collection).
-            let buffer = match self.buffers.entry(file) {
-                Entry::Occupied(entry) => entry.into_mut(),
-                Entry::Vacant(entry) => {
-                    let buffer = self.loader.load_file(file).ok()?;
-                    entry.insert(Rc::new(buffer))
-                }
-            };
-
-            let face = Face::new(Rc::clone(buffer), index)?;
-            if let Some(callback) = &self.on_load {
-                callback(id, &face);
-            }
-
-            *slot = Some(face);
-        }
-
-        best
-    }
-
-    /// Get a reference to a loaded face.
-    ///
-    /// This panics if no face with this id was loaded. This function should
-    /// only be called with ids returned by [`select()`](Self::select).
-    #[track_caller]
-    pub fn get(&self, id: FaceId) -> &Face {
-        self.faces[id.0 as usize].as_ref().expect("font face was not loaded")
-    }
-
-    /// Register a callback which is invoked each time a font face is loaded.
-    pub fn on_load<F>(&mut self, f: F)
-    where
-        F: Fn(FaceId, &Face) + 'static,
-    {
-        self.on_load = Some(Box::new(f));
-    }
-}
-
-/// A unique identifier for a loaded font face.
+/// Identifies a vertical metric of a font.
 #[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
-#[derive(Serialize, Deserialize)]
-pub struct FaceId(u64);
-
-impl FaceId {
-    /// Create a face id from the raw underlying value.
+pub enum VerticalFontMetric {
+    /// The distance from the baseline to the typographic ascender.
     ///
-    /// This should only be called with values returned by
-    /// [`into_raw`](Self::into_raw).
-    pub const fn from_raw(v: u64) -> Self {
-        Self(v)
-    }
+    /// Corresponds to the typographic ascender from the `OS/2` table if present
+    /// and falls back to the ascender from the `hhea` table otherwise.
+    Ascender,
+    /// The approximate height of uppercase letters.
+    CapHeight,
+    /// The approximate height of non-ascending lowercase letters.
+    XHeight,
+    /// The baseline on which the letters rest.
+    Baseline,
+    /// The distance from the baseline to the typographic descender.
+    ///
+    /// Corresponds to the typographic descender from the `OS/2` table if
+    /// present and falls back to the descender from the `hhea` table otherwise.
+    Descender,
+}
 
-    /// Convert into the raw underlying value.
-    pub const fn into_raw(self) -> u64 {
-        self.0
+impl Display for VerticalFontMetric {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.pad(match self {
+            Self::Ascender => "ascender",
+            Self::CapHeight => "cap-height",
+            Self::XHeight => "x-height",
+            Self::Baseline => "baseline",
+            Self::Descender => "descender",
+        })
     }
 }
 
@@ -358,8 +364,8 @@ impl Display for FontFamily {
 /// Properties of a single font face.
 #[derive(Debug, Clone, Eq, PartialEq, Serialize, Deserialize)]
 pub struct FaceInfo {
-    /// The font file.
-    pub file: FileId,
+    /// The path to the font file.
+    pub path: PathBuf,
     /// The collection index in the font file.
     pub index: u32,
     /// The typographic font family this face is part of.
diff --git a/src/image.rs b/src/image.rs
index f041fac1..f98c7b1b 100644
--- a/src/image.rs
+++ b/src/image.rs
@@ -2,122 +2,137 @@
 
 use std::collections::{hash_map::Entry, HashMap};
 use std::fmt::{self, Debug, Formatter};
-use std::io::Cursor;
+use std::io;
+use std::path::Path;
 use std::rc::Rc;
 
 use image::io::Reader as ImageReader;
 use image::{DynamicImage, GenericImageView, ImageFormat};
 use serde::{Deserialize, Serialize};
 
-use crate::loading::{FileId, Loader};
+use crate::loading::{FileHash, Loader};
 
-/// A loaded image.
-pub struct Image {
-    /// The original format the image was encoded in.
-    pub format: ImageFormat,
-    /// The decoded image.
-    pub buf: DynamicImage,
-}
+/// A unique identifier for a loaded image.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[derive(Serialize, Deserialize)]
+pub struct ImageId(u32);
 
-impl Image {
-    /// Parse an image from raw data in a supported format (PNG or JPEG).
+impl ImageId {
+    /// Create an image id from the raw underlying value.
     ///
-    /// The image format is determined automatically.
-    pub fn parse(data: &[u8]) -> Option<Self> {
-        let cursor = Cursor::new(data);
-        let reader = ImageReader::new(cursor).with_guessed_format().ok()?;
-        let format = reader.format()?;
-        let buf = reader.decode().ok()?;
-        Some(Self { format, buf })
-    }
-
-    /// The width of the image.
-    pub fn width(&self) -> u32 {
-        self.buf.width()
-    }
-
-    /// The height of the image.
-    pub fn height(&self) -> u32 {
-        self.buf.height()
+    /// This should only be called with values returned by
+    /// [`into_raw`](Self::into_raw).
+    pub const fn from_raw(v: u32) -> Self {
+        Self(v)
     }
-}
 
-impl Debug for Image {
-    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
-        f.debug_struct("Image")
-            .field("format", &self.format)
-            .field("color", &self.buf.color())
-            .field("width", &self.width())
-            .field("height", &self.height())
-            .finish()
+    /// Convert into the raw underlying value.
+    pub const fn into_raw(self) -> u32 {
+        self.0
     }
 }
 
-/// Caches decoded images.
-pub struct ImageCache {
+/// Storage for loaded and decoded images.
+pub struct ImageStore {
     loader: Rc<dyn Loader>,
-    images: HashMap<ImageId, Image>,
+    files: HashMap<FileHash, ImageId>,
+    images: Vec<Image>,
     on_load: Option<Box<dyn Fn(ImageId, &Image)>>,
 }
 
-impl ImageCache {
-    /// Create a new, empty image cache.
+impl ImageStore {
+    /// Create a new, empty image store.
     pub fn new(loader: Rc<dyn Loader>) -> Self {
         Self {
             loader,
-            images: HashMap::new(),
+            files: HashMap::new(),
+            images: vec![],
             on_load: None,
         }
     }
 
+    /// Register a callback which is invoked each time an image is loaded.
+    pub fn on_load<F>(&mut self, f: F)
+    where
+        F: Fn(ImageId, &Image) + 'static,
+    {
+        self.on_load = Some(Box::new(f));
+    }
+
     /// Load and decode an image file from a path.
-    pub fn load(&mut self, file: FileId) -> Option<ImageId> {
-        let id = ImageId(file.into_raw());
-        if let Entry::Vacant(entry) = self.images.entry(id) {
-            let buffer = self.loader.load_file(file).ok()?;
-            let image = Image::parse(&buffer)?;
-            if let Some(callback) = &self.on_load {
-                callback(id, &image);
+    pub fn load(&mut self, path: &Path) -> io::Result<ImageId> {
+        let hash = self.loader.resolve(path)?;
+        Ok(*match self.files.entry(hash) {
+            Entry::Occupied(entry) => entry.into_mut(),
+            Entry::Vacant(entry) => {
+                let buffer = self.loader.load(path)?;
+                let image = Image::parse(&buffer)?;
+                let id = ImageId(self.images.len() as u32);
+                if let Some(callback) = &self.on_load {
+                    callback(id, &image);
+                }
+                self.images.push(image);
+                entry.insert(id)
             }
-            entry.insert(image);
-        }
-        Some(id)
+        })
     }
 
     /// Get a reference to a loaded image.
     ///
     /// This panics if no image with this id was loaded. This function should
-    /// only be called with ids returned by [`load()`](Self::load).
+    /// only be called with ids returned by this store's [`load()`](Self::load)
+    /// method.
     #[track_caller]
     pub fn get(&self, id: ImageId) -> &Image {
-        &self.images[&id]
-    }
-
-    /// Register a callback which is invoked each time an image is loaded.
-    pub fn on_load<F>(&mut self, f: F)
-    where
-        F: Fn(ImageId, &Image) + 'static,
-    {
-        self.on_load = Some(Box::new(f));
+        &self.images[id.0 as usize]
     }
 }
 
-/// A unique identifier for a loaded image.
-#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
-#[derive(Serialize, Deserialize)]
-pub struct ImageId(u64);
+/// A loaded image.
+pub struct Image {
+    /// The original format the image was encoded in.
+    pub format: ImageFormat,
+    /// The decoded image.
+    pub buf: DynamicImage,
+}
 
-impl ImageId {
-    /// Create an image id from the raw underlying value.
+impl Image {
+    /// Parse an image from raw data in a supported format (PNG or JPEG).
     ///
-    /// This should only be called with values returned by
-    /// [`into_raw`](Self::into_raw).
-    pub const fn from_raw(v: u64) -> Self {
-        Self(v)
+    /// The image format is determined automatically.
+    pub fn parse(data: &[u8]) -> io::Result<Self> {
+        let cursor = io::Cursor::new(data);
+        let reader = ImageReader::new(cursor).with_guessed_format()?;
+
+        let format = reader.format().ok_or_else(|| {
+            io::Error::new(io::ErrorKind::InvalidData, "unknown image format")
+        })?;
+
+        let buf = reader
+            .decode()
+            .map_err(|err| io::Error::new(io::ErrorKind::InvalidData, err))?;
+
+        Ok(Self { format, buf })
     }
 
-    /// Convert into the raw underlying value.
-    pub const fn into_raw(self) -> u64 {
-        self.0
+    /// The width of the image.
+    pub fn width(&self) -> u32 {
+        self.buf.width()
+    }
+
+    /// The height of the image.
+    pub fn height(&self) -> u32 {
+        self.buf.height()
+    }
+}
+
+impl Debug for Image {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        f.debug_struct("Image")
+            .field("format", &self.format)
+            .field("color", &self.buf.color())
+            .field("width", &self.width())
+            .field("height", &self.height())
+            .finish()
     }
 }
diff --git a/src/layout/incremental.rs b/src/layout/incremental.rs
index 32353d6f..baf2991a 100644
--- a/src/layout/incremental.rs
+++ b/src/layout/incremental.rs
@@ -1,5 +1,5 @@
 #[cfg(feature = "layout-cache")]
-use std::collections::{hash_map::Entry, HashMap};
+use std::collections::HashMap;
 use std::ops::Deref;
 
 use super::*;
@@ -68,13 +68,10 @@ impl LayoutCache {
         frames: Vec<Constrained<Rc<Frame>>>,
         level: usize,
     ) {
-        let entry = FramesEntry::new(frames, level);
-        match self.frames.entry(hash) {
-            Entry::Occupied(occupied) => occupied.into_mut().push(entry),
-            Entry::Vacant(vacant) => {
-                vacant.insert(vec![entry]);
-            }
-        }
+        self.frames
+            .entry(hash)
+            .or_default()
+            .push(FramesEntry::new(frames, level));
     }
 
     /// Clear the cache.
diff --git a/src/layout/mod.rs b/src/layout/mod.rs
index 56e0687a..246db714 100644
--- a/src/layout/mod.rs
+++ b/src/layout/mod.rs
@@ -29,9 +29,9 @@ use std::hash::Hash;
 use std::hash::Hasher;
 use std::rc::Rc;
 
-use crate::font::FontCache;
+use crate::font::FontStore;
 use crate::geom::*;
-use crate::image::ImageCache;
+use crate::image::ImageStore;
 use crate::util::OptionExt;
 use crate::Context;
 
@@ -53,11 +53,11 @@ pub trait Layout {
 
 /// The context for layouting.
 pub struct LayoutContext<'a> {
-    /// The cache for parsed font faces.
-    pub fonts: &'a mut FontCache,
-    /// The cache for decoded imges.
-    pub images: &'a mut ImageCache,
-    /// The cache for layouting artifacts.
+    /// Stores parsed font faces.
+    pub fonts: &'a mut FontStore,
+    /// Stores decoded images.
+    pub images: &'a mut ImageStore,
+    /// Caches layouting artifacts.
     #[cfg(feature = "layout-cache")]
     pub layouts: &'a mut LayoutCache,
     /// How deeply nested the current layout tree position is.
diff --git a/src/lib.rs b/src/lib.rs
index 0f556989..7447dad7 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -53,26 +53,26 @@ use std::rc::Rc;
 use crate::diag::TypResult;
 use crate::eval::{ModuleCache, Scope};
 use crate::exec::State;
-use crate::font::FontCache;
-use crate::image::ImageCache;
+use crate::font::FontStore;
+use crate::image::ImageStore;
 use crate::layout::Frame;
 #[cfg(feature = "layout-cache")]
 use crate::layout::LayoutCache;
 use crate::loading::Loader;
-use crate::source::{SourceFile, SourceMap};
+use crate::source::{SourceId, SourceStore};
 
 /// The core context which holds the loader, configuration and cached artifacts.
 pub struct Context {
     /// The loader the context was created with.
     pub loader: Rc<dyn Loader>,
     /// Stores loaded source files.
-    pub sources: SourceMap,
+    pub sources: SourceStore,
+    /// Stores parsed font faces.
+    pub fonts: FontStore,
+    /// Stores decoded images.
+    pub images: ImageStore,
     /// Caches evaluated modules.
     pub modules: ModuleCache,
-    /// Caches parsed font faces.
-    pub fonts: FontCache,
-    /// Caches decoded images.
-    pub images: ImageCache,
     /// Caches layouting artifacts.
     #[cfg(feature = "layout-cache")]
     pub layouts: LayoutCache,
@@ -93,24 +93,25 @@ impl Context {
         ContextBuilder::default()
     }
 
-    /// Garbage-collect caches.
-    pub fn turnaround(&mut self) {
-        #[cfg(feature = "layout-cache")]
-        self.layouts.turnaround();
-    }
-
     /// Typeset a source file into a collection of layouted frames.
     ///
     /// Returns either a vector of frames representing individual pages or
     /// diagnostics in the form of a vector of error message with file and span
     /// information.
-    pub fn typeset(&mut self, source: &SourceFile) -> TypResult<Vec<Rc<Frame>>> {
+    pub fn typeset(&mut self, id: SourceId) -> TypResult<Vec<Rc<Frame>>> {
+        let source = self.sources.get(id);
         let ast = parse::parse(source)?;
-        let module = eval::eval(self, source.file(), Rc::new(ast))?;
+        let module = eval::eval(self, id, Rc::new(ast))?;
         let tree = exec::exec(self, &module.template);
         let frames = layout::layout(self, &tree);
         Ok(frames)
     }
+
+    /// Garbage-collect caches.
+    pub fn turnaround(&mut self) {
+        #[cfg(feature = "layout-cache")]
+        self.layouts.turnaround();
+    }
 }
 
 /// A builder for a [`Context`].
@@ -140,10 +141,10 @@ impl ContextBuilder {
     /// fonts, images, source files and other resources.
     pub fn build(self, loader: Rc<dyn Loader>) -> Context {
         Context {
-            loader: Rc::clone(&loader),
-            sources: SourceMap::new(),
-            fonts: FontCache::new(Rc::clone(&loader)),
-            images: ImageCache::new(loader),
+            sources: SourceStore::new(Rc::clone(&loader)),
+            fonts: FontStore::new(Rc::clone(&loader)),
+            images: ImageStore::new(Rc::clone(&loader)),
+            loader,
             modules: ModuleCache::new(),
             #[cfg(feature = "layout-cache")]
             layouts: LayoutCache::new(),
diff --git a/src/library/elements.rs b/src/library/elements.rs
index 3d318d36..e021c0c6 100644
--- a/src/library/elements.rs
+++ b/src/library/elements.rs
@@ -1,8 +1,10 @@
 use std::f64::consts::SQRT_2;
+use std::io;
 
 use decorum::N64;
 
 use super::*;
+use crate::diag::Error;
 use crate::layout::{
     BackgroundNode, BackgroundShape, FixedNode, ImageNode, PadNode, Paint,
 };
@@ -13,13 +15,17 @@ pub fn image(ctx: &mut EvalContext, args: &mut FuncArgs) -> TypResult<Value> {
     let width = args.named("width")?;
     let height = args.named("height")?;
 
-    let file = ctx.resolve(&path.v, path.span)?;
-    let node = match ctx.images.load(file) {
-        Some(id) => ImageNode { id, width, height },
-        None => bail!(args.file, path.span, "failed to load image"),
-    };
-
-    Ok(Value::template(move |ctx| ctx.push_into_par(node)))
+    let full = ctx.relpath(path.v.as_str());
+    let id = ctx.images.load(&full).map_err(|err| {
+        Error::boxed(args.source, path.span, match err.kind() {
+            io::ErrorKind::NotFound => "file not found".into(),
+            _ => format!("failed to load image ({})", err),
+        })
+    })?;
+
+    Ok(Value::template(move |ctx| {
+        ctx.push_into_par(ImageNode { id, width, height })
+    }))
 }
 
 /// `rect`: A rectangle with optional content.
diff --git a/src/library/layout.rs b/src/library/layout.rs
index 727bbcc3..0d778206 100644
--- a/src/library/layout.rs
+++ b/src/library/layout.rs
@@ -6,7 +6,7 @@ use crate::paper::{Paper, PaperClass};
 pub fn page(_: &mut EvalContext, args: &mut FuncArgs) -> TypResult<Value> {
     let paper = match args.eat::<Spanned<EcoString>>() {
         Some(name) => match Paper::from_name(&name.v) {
-            None => bail!(args.file, name.span, "invalid paper name"),
+            None => bail!(args.source, name.span, "invalid paper name"),
             paper => paper,
         },
         None => None,
diff --git a/src/library/text.rs b/src/library/text.rs
index 5973a7e2..cd97691c 100644
--- a/src/library/text.rs
+++ b/src/library/text.rs
@@ -132,7 +132,7 @@ pub fn lang(_: &mut EvalContext, args: &mut FuncArgs) -> TypResult<Value> {
         if dir.v.axis() == SpecAxis::Horizontal {
             Some(dir.v)
         } else {
-            bail!(args.file, dir.span, "must be horizontal");
+            bail!(args.source, dir.span, "must be horizontal");
         }
     } else {
         iso.as_deref().map(lang_dir)
diff --git a/src/library/utility.rs b/src/library/utility.rs
index fb39fce3..3c157ea1 100644
--- a/src/library/utility.rs
+++ b/src/library/utility.rs
@@ -25,7 +25,7 @@ pub fn len(_: &mut EvalContext, args: &mut FuncArgs) -> TypResult<Value> {
         Value::Str(v) => Value::Int(v.len() as i64),
         Value::Array(v) => Value::Int(v.len() as i64),
         Value::Dict(v) => Value::Int(v.len() as i64),
-        _ => bail!(args.file, span, "expected string, array or dictionary"),
+        _ => bail!(args.source, span, "expected string, array or dictionary"),
     })
 }
 
@@ -35,7 +35,7 @@ pub fn rgb(_: &mut EvalContext, args: &mut FuncArgs) -> TypResult<Value> {
         if let Some(string) = args.eat::<Spanned<EcoString>>() {
             match RgbaColor::from_str(&string.v) {
                 Ok(color) => color,
-                Err(_) => bail!(args.file, string.span, "invalid color"),
+                Err(_) => bail!(args.source, string.span, "invalid color"),
             }
         } else {
             let r = args.expect("red component")?;
@@ -60,7 +60,7 @@ pub fn max(_: &mut EvalContext, args: &mut FuncArgs) -> TypResult<Value> {
 
 /// Find the minimum or maximum of a sequence of values.
 fn minmax(args: &mut FuncArgs, goal: Ordering) -> TypResult<Value> {
-    let &mut FuncArgs { file, span, .. } = args;
+    let &mut FuncArgs { source, span, .. } = args;
 
     let mut extremum = args.expect::<Value>("value")?;
     for value in args.all::<Value>() {
@@ -71,7 +71,7 @@ fn minmax(args: &mut FuncArgs, goal: Ordering) -> TypResult<Value> {
                 }
             }
             None => bail!(
-                file,
+                source,
                 span,
                 "cannot compare {} with {}",
                 extremum.type_name(),
diff --git a/src/loading/fs.rs b/src/loading/fs.rs
index c3ca332e..9289519c 100644
--- a/src/loading/fs.rs
+++ b/src/loading/fs.rs
@@ -1,8 +1,6 @@
-use std::cell::{Ref, RefCell};
-use std::collections::HashMap;
 use std::fs::{self, File};
 use std::io;
-use std::path::{Path, PathBuf};
+use std::path::Path;
 use std::rc::Rc;
 
 use memmap2::Mmap;
@@ -10,9 +8,8 @@ use same_file::Handle;
 use ttf_parser::{name_id, Face};
 use walkdir::WalkDir;
 
-use super::{FileId, Loader};
+use super::{FileHash, Loader};
 use crate::font::{FaceInfo, FontStretch, FontStyle, FontVariant, FontWeight};
-use crate::util::PathExt;
 
 /// Loads fonts and images from the local file system.
 ///
@@ -20,13 +17,12 @@ use crate::util::PathExt;
 #[derive(Debug, Default, Clone)]
 pub struct FsLoader {
     faces: Vec<FaceInfo>,
-    paths: RefCell<HashMap<FileId, PathBuf>>,
 }
 
 impl FsLoader {
     /// Create a new loader without any fonts.
     pub fn new() -> Self {
-        Self { faces: vec![], paths: RefCell::default() }
+        Self { faces: vec![] }
     }
 
     /// Builder-style variant of `search_system`.
@@ -52,51 +48,6 @@ impl FsLoader {
         self.search_system_impl();
     }
 
-    /// Search for all fonts at a path.
-    ///
-    /// If the path is a directory, all contained fonts will be searched for
-    /// recursively.
-    pub fn search_path(&mut self, dir: impl AsRef<Path>) {
-        let walk = WalkDir::new(dir)
-            .follow_links(true)
-            .sort_by(|a, b| a.file_name().cmp(b.file_name()))
-            .into_iter()
-            .filter_map(|e| e.ok());
-
-        for entry in walk {
-            let path = entry.path();
-            if let Some(ext) = path.extension().and_then(|s| s.to_str()) {
-                match ext {
-                    #[rustfmt::skip]
-                    "ttf" | "otf" | "TTF" | "OTF" |
-                    "ttc" | "otc" | "TTC" | "OTC" => {
-                        self.search_file(path).ok();
-                    }
-                    _ => {}
-                }
-            }
-        }
-    }
-
-    /// Resolve a file id for a path.
-    pub fn resolve(&self, path: &Path) -> io::Result<FileId> {
-        let file = File::open(path)?;
-        let meta = file.metadata()?;
-        if meta.is_file() {
-            let handle = Handle::from_file(file)?;
-            let id = FileId(fxhash::hash64(&handle));
-            self.paths.borrow_mut().insert(id, path.normalize());
-            Ok(id)
-        } else {
-            Err(io::Error::new(io::ErrorKind::Other, "not a file"))
-        }
-    }
-
-    /// Return the path of a resolved file.
-    pub fn path(&self, id: FileId) -> Ref<Path> {
-        Ref::map(self.paths.borrow(), |paths| paths[&id].as_path())
-    }
-
     #[cfg(all(unix, not(target_os = "macos")))]
     fn search_system_impl(&mut self) {
         self.search_path("/usr/share/fonts");
@@ -134,6 +85,32 @@ impl FsLoader {
         }
     }
 
+    /// Search for all fonts at a path.
+    ///
+    /// If the path is a directory, all contained fonts will be searched for
+    /// recursively.
+    pub fn search_path(&mut self, dir: impl AsRef<Path>) {
+        let walk = WalkDir::new(dir)
+            .follow_links(true)
+            .sort_by(|a, b| a.file_name().cmp(b.file_name()))
+            .into_iter()
+            .filter_map(|e| e.ok());
+
+        for entry in walk {
+            let path = entry.path();
+            if let Some(ext) = path.extension().and_then(|s| s.to_str()) {
+                match ext {
+                    #[rustfmt::skip]
+                    "ttf" | "otf" | "TTF" | "OTF" |
+                    "ttc" | "otc" | "TTC" | "OTC" => {
+                        self.search_file(path).ok();
+                    }
+                    _ => {}
+                }
+            }
+        }
+    }
+
     /// Index the font faces in the file at the given path.
     ///
     /// The file may form a font collection and contain multiple font faces,
@@ -180,8 +157,12 @@ impl FsLoader {
             stretch: FontStretch::from_number(face.width().to_number()),
         };
 
-        let file = self.resolve(path)?;
-        self.faces.push(FaceInfo { file, index, family, variant });
+        self.faces.push(FaceInfo {
+            path: path.to_owned(),
+            index,
+            family,
+            variant,
+        });
 
         Ok(())
     }
@@ -192,16 +173,19 @@ impl Loader for FsLoader {
         &self.faces
     }
 
-    fn resolve_from(&self, base: FileId, path: &Path) -> io::Result<FileId> {
-        let full = self.paths.borrow()[&base]
-            .parent()
-            .expect("base is a file")
-            .join(path);
-        self.resolve(&full)
+    fn resolve(&self, path: &Path) -> io::Result<FileHash> {
+        let file = File::open(path)?;
+        let meta = file.metadata()?;
+        if meta.is_file() {
+            let handle = Handle::from_file(file)?;
+            Ok(FileHash(fxhash::hash64(&handle)))
+        } else {
+            Err(io::Error::new(io::ErrorKind::Other, "not a file"))
+        }
     }
 
-    fn load_file(&self, id: FileId) -> io::Result<Vec<u8>> {
-        fs::read(&self.paths.borrow()[&id])
+    fn load(&self, path: &Path) -> io::Result<Vec<u8>> {
+        fs::read(path)
     }
 }
 
@@ -211,8 +195,8 @@ mod tests {
 
     #[test]
     fn test_index_font_dir() {
-        let map = FsLoader::new().with_path("fonts").paths.into_inner();
-        let mut paths: Vec<_> = map.into_iter().map(|p| p.1).collect();
+        let faces = FsLoader::new().with_path("fonts").faces;
+        let mut paths: Vec<_> = faces.into_iter().map(|info| info.path).collect();
         paths.sort();
 
         assert_eq!(paths, [
diff --git a/src/loading/mod.rs b/src/loading/mod.rs
index 65eb25c6..7d697310 100644
--- a/src/loading/mod.rs
+++ b/src/loading/mod.rs
@@ -13,41 +13,24 @@ use serde::{Deserialize, Serialize};
 
 use crate::font::FaceInfo;
 
+/// A hash that identifies a file.
+///
+/// Such a hash can be [resolved](Loader::resolve) from a path.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[derive(Serialize, Deserialize)]
+pub struct FileHash(pub u64);
+
 /// Loads resources from a local or remote source.
 pub trait Loader {
     /// Descriptions of all font faces this loader serves.
     fn faces(&self) -> &[FaceInfo];
 
-    /// Resolve a `path` relative to a `base` file.
-    ///
-    /// This should return the same id for all paths pointing to the same file
-    /// and `None` if the file does not exist.
-    fn resolve_from(&self, base: FileId, path: &Path) -> io::Result<FileId>;
-
-    /// Load a file by id.
-    ///
-    /// This must only be called with an `id` returned by a call to this
-    /// loader's `resolve_from` method.
-    fn load_file(&self, id: FileId) -> io::Result<Vec<u8>>;
-}
-
-/// A file id that can be [resolved](Loader::resolve_from) from a path.
-///
-/// Should be the same for all paths pointing to the same file.
-#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
-#[derive(Serialize, Deserialize)]
-pub struct FileId(u64);
+    /// Resolve a hash that is the same for this and all other paths pointing to
+    /// the same file.
+    fn resolve(&self, path: &Path) -> io::Result<FileHash>;
 
-impl FileId {
-    /// Create a file id from a raw value.
-    pub const fn from_raw(v: u64) -> Self {
-        Self(v)
-    }
-
-    /// Convert into the raw underlying value.
-    pub const fn into_raw(self) -> u64 {
-        self.0
-    }
+    /// Load a file from a path.
+    fn load(&self, path: &Path) -> io::Result<Vec<u8>>;
 }
 
 /// A loader which serves nothing.
@@ -58,11 +41,11 @@ impl Loader for BlankLoader {
         &[]
     }
 
-    fn resolve_from(&self, _: FileId, _: &Path) -> io::Result<FileId> {
+    fn resolve(&self, _: &Path) -> io::Result<FileHash> {
         Err(io::ErrorKind::NotFound.into())
     }
 
-    fn load_file(&self, _: FileId) -> io::Result<Vec<u8>> {
-        panic!("resolve_from never returns an id")
+    fn load(&self, _: &Path) -> io::Result<Vec<u8>> {
+        Err(io::ErrorKind::NotFound.into())
     }
 }
diff --git a/src/main.rs b/src/main.rs
index 51a6d833..f3a97d51 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,19 +1,16 @@
 use std::fs;
 use std::io::{self, Write};
-use std::ops::Range;
-use std::path::{Path, PathBuf};
+use std::path::Path;
 use std::process;
 
-use anyhow::{anyhow, bail, Context};
+use anyhow::Context as _;
 use codespan_reporting::diagnostic::{Diagnostic, Label};
-use codespan_reporting::files::{self, Files};
 use codespan_reporting::term::{self, termcolor, Config, Styles};
 use same_file::is_same_file;
 use termcolor::{ColorChoice, StandardStream, WriteColor};
 
 use typst::diag::{Error, Tracepoint};
-use typst::loading::{FileId, FsLoader};
-use typst::source::{SourceFile, SourceMap};
+use typst::source::SourceStore;
 
 fn main() {
     if let Err(error) = try_main() {
@@ -32,19 +29,17 @@ fn try_main() -> anyhow::Result<()> {
 
     // Determine source and destination path.
     let src_path = Path::new(&args[1]);
-    let dest_path = if let Some(arg) = args.get(2) {
-        PathBuf::from(arg)
-    } else {
-        let name = src_path
-            .file_name()
-            .ok_or_else(|| anyhow!("source path is not a file"))?;
-
-        Path::new(name).with_extension("pdf")
+    let dest_path = match args.get(2) {
+        Some(path) => path.into(),
+        None => {
+            let name = src_path.file_name().context("source path is not a file")?;
+            Path::new(name).with_extension("pdf")
+        }
     };
 
     // Ensure that the source file is not overwritten.
     if is_same_file(src_path, &dest_path).unwrap_or(false) {
-        bail!("source and destination files are the same");
+        anyhow::bail!("source and destination files are the same");
     }
 
     // Create a loader for fonts and files.
@@ -53,14 +48,15 @@ fn try_main() -> anyhow::Result<()> {
         .with_system()
         .wrap();
 
-    // Resolve the file id of the source file and read the file.
-    let file = loader.resolve(src_path).context("source file not found")?;
-    let string = fs::read_to_string(&src_path).context("failed to read source file")?;
-    let source = SourceFile::new(file, string);
+    // Create the context which holds loaded source files, fonts, images and
+    // cached artifacts.
+    let mut ctx = typst::Context::new(loader);
+
+    // Load the source file.
+    let id = ctx.sources.load(&src_path).context("source file not found")?;
 
     // Typeset.
-    let mut ctx = typst::Context::new(loader.clone());
-    match ctx.typeset(&source) {
+    match ctx.typeset(id) {
         // Export the PDF.
         Ok(document) => {
             let buffer = typst::export::pdf(&ctx, &document);
@@ -69,8 +65,7 @@ fn try_main() -> anyhow::Result<()> {
 
         // Print diagnostics.
         Err(errors) => {
-            ctx.sources.insert(source);
-            print_diagnostics(&loader, &ctx.sources, *errors)
+            print_diagnostics(&ctx.sources, *errors)
                 .context("failed to print diagnostics")?;
         }
     }
@@ -110,21 +105,19 @@ fn print_error(error: anyhow::Error) -> io::Result<()> {
 
 /// Print diagnostics messages to the terminal.
 fn print_diagnostics(
-    loader: &FsLoader,
-    sources: &SourceMap,
+    sources: &SourceStore,
     errors: Vec<Error>,
-) -> Result<(), files::Error> {
+) -> Result<(), codespan_reporting::files::Error> {
     let mut writer = StandardStream::stderr(ColorChoice::Always);
     let config = Config { tab_width: 2, ..Default::default() };
-    let files = FilesImpl(loader, sources);
 
     for error in errors {
         // The main diagnostic.
         let main = Diagnostic::error()
             .with_message(error.message)
-            .with_labels(vec![Label::primary(error.file, error.span.to_range())]);
+            .with_labels(vec![Label::primary(error.source, error.span.to_range())]);
 
-        term::emit(&mut writer, &config, &files, &main)?;
+        term::emit(&mut writer, &config, sources, &main)?;
 
         // Stacktrace-like helper diagnostics.
         for (file, span, point) in error.trace {
@@ -140,61 +133,9 @@ fn print_diagnostics(
                 .with_message(message)
                 .with_labels(vec![Label::primary(file, span.to_range())]);
 
-            term::emit(&mut writer, &config, &files, &help)?;
+            term::emit(&mut writer, &config, sources, &help)?;
         }
     }
 
     Ok(())
 }
-
-/// Required for error message formatting with codespan-reporting.
-struct FilesImpl<'a>(&'a FsLoader, &'a SourceMap);
-
-impl FilesImpl<'_> {
-    fn source(&self, id: FileId) -> Result<&SourceFile, files::Error> {
-        self.1.get(id).ok_or(files::Error::FileMissing)
-    }
-}
-
-impl<'a> Files<'a> for FilesImpl<'a> {
-    type FileId = FileId;
-    type Name = String;
-    type Source = &'a str;
-
-    fn name(&'a self, id: FileId) -> Result<Self::Name, files::Error> {
-        Ok(self.0.path(id).display().to_string())
-    }
-
-    fn source(&'a self, id: FileId) -> Result<Self::Source, files::Error> {
-        Ok(self.source(id)?.src())
-    }
-
-    fn line_index(
-        &'a self,
-        id: FileId,
-        byte_index: usize,
-    ) -> Result<usize, files::Error> {
-        let source = self.source(id)?;
-        source.pos_to_line(byte_index.into()).ok_or_else(|| {
-            let (given, max) = (byte_index, source.len_bytes());
-            if given <= max {
-                files::Error::InvalidCharBoundary { given }
-            } else {
-                files::Error::IndexTooLarge { given, max }
-            }
-        })
-    }
-
-    fn line_range(
-        &'a self,
-        id: FileId,
-        line_index: usize,
-    ) -> Result<Range<usize>, files::Error> {
-        let source = self.source(id)?;
-        let span = source.line_to_span(line_index).ok_or(files::Error::LineTooLarge {
-            given: line_index,
-            max: source.len_lines(),
-        })?;
-        Ok(span.to_range())
-    }
-}
diff --git a/src/parse/parser.rs b/src/parse/parser.rs
index 6b478780..326fc280 100644
--- a/src/parse/parser.rs
+++ b/src/parse/parser.rs
@@ -82,7 +82,7 @@ impl<'s> Parser<'s> {
 
     /// Add an error with location and message.
     pub fn error(&mut self, span: impl Into<Span>, message: impl Into<String>) {
-        self.errors.push(Error::new(self.source.file(), span, message));
+        self.errors.push(Error::new(self.source.id(), span, message));
     }
 
     /// Eat the next token and add an error that it is not the expected `thing`.
diff --git a/src/pretty.rs b/src/pretty.rs
index 2f3a6ef9..ceee61f8 100644
--- a/src/pretty.rs
+++ b/src/pretty.rs
@@ -608,7 +608,6 @@ pretty_display! {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::loading::FileId;
     use crate::parse::parse;
     use crate::source::SourceFile;
 
@@ -619,7 +618,7 @@ mod tests {
 
     #[track_caller]
     fn test_parse(src: &str, exp: &str) {
-        let source = SourceFile::new(FileId::from_raw(0), src.into());
+        let source = SourceFile::detached(src);
         let ast = parse(&source).unwrap();
         let found = pretty(&ast);
         if exp != found {
diff --git a/src/source.rs b/src/source.rs
index abd3c246..20ba137f 100644
--- a/src/source.rs
+++ b/src/source.rs
@@ -1,55 +1,126 @@
 //! Source files.
 
-use std::collections::{hash_map::Entry, HashMap};
+use std::collections::HashMap;
+use std::io;
+use std::path::{Path, PathBuf};
+use std::rc::Rc;
 
-use crate::loading::FileId;
+#[cfg(feature = "codespan-reporting")]
+use codespan_reporting::files::{self, Files};
+use serde::{Deserialize, Serialize};
+
+use crate::loading::{FileHash, Loader};
 use crate::parse::{is_newline, Scanner};
 use crate::syntax::{Pos, Span};
+use crate::util::PathExt;
+
+/// A unique identifier for a loaded source file.
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[derive(Serialize, Deserialize)]
+pub struct SourceId(u32);
+
+impl SourceId {
+    /// Create a source id from the raw underlying value.
+    ///
+    /// This should only be called with values returned by
+    /// [`into_raw`](Self::into_raw).
+    pub const fn from_raw(v: u32) -> Self {
+        Self(v)
+    }
 
-/// A store for loaded source files.
-#[derive(Default)]
-pub struct SourceMap {
-    sources: HashMap<FileId, SourceFile>,
+    /// Convert into the raw underlying value.
+    pub const fn into_raw(self) -> u32 {
+        self.0
+    }
+}
+
+/// Storage for loaded source files.
+pub struct SourceStore {
+    loader: Rc<dyn Loader>,
+    files: HashMap<FileHash, SourceId>,
+    sources: Vec<SourceFile>,
 }
 
-impl SourceMap {
-    /// Create a new, empty source map
-    pub fn new() -> Self {
-        Self::default()
+impl SourceStore {
+    /// Create a new, empty source store.
+    pub fn new(loader: Rc<dyn Loader>) -> Self {
+        Self {
+            loader,
+            files: HashMap::new(),
+            sources: vec![],
+        }
     }
 
-    /// Get a source file by id.
-    pub fn get(&self, file: FileId) -> Option<&SourceFile> {
-        self.sources.get(&file)
+    /// Load a source file from a path using the `loader`.
+    pub fn load(&mut self, path: &Path) -> io::Result<SourceId> {
+        let hash = self.loader.resolve(path)?;
+        if let Some(&id) = self.files.get(&hash) {
+            return Ok(id);
+        }
+
+        let data = self.loader.load(path)?;
+        let src = String::from_utf8(data).map_err(|_| {
+            io::Error::new(io::ErrorKind::InvalidData, "file is not valid utf-8")
+        })?;
+
+        Ok(self.insert(Some(hash), path, src))
     }
 
-    /// Insert a sources.
-    pub fn insert(&mut self, source: SourceFile) -> &SourceFile {
-        match self.sources.entry(source.file) {
-            Entry::Occupied(mut entry) => {
-                entry.insert(source);
-                entry.into_mut()
+    /// Directly provide a source file.
+    ///
+    /// The `path` does not need to be [resolvable](Loader::resolve) through the
+    /// `loader`. If it is though, imports that resolve to the same file hash
+    /// will use the inserted file instead of going through [`Loader::load`].
+    ///
+    /// If the path is resolvable and points to an existing source file, it is
+    /// overwritten.
+    pub fn provide(&mut self, path: &Path, src: String) -> SourceId {
+        if let Ok(hash) = self.loader.resolve(path) {
+            if let Some(&id) = self.files.get(&hash) {
+                // Already loaded, so we replace it.
+                self.sources[id.0 as usize] = SourceFile::new(id, path, src);
+                id
+            } else {
+                // Not loaded yet.
+                self.insert(Some(hash), path, src)
             }
-            Entry::Vacant(entry) => entry.insert(source),
+        } else {
+            // Not known to the loader.
+            self.insert(None, path, src)
+        }
+    }
+
+    /// Insert a new source file.
+    fn insert(&mut self, hash: Option<FileHash>, path: &Path, src: String) -> SourceId {
+        let id = SourceId(self.sources.len() as u32);
+        if let Some(hash) = hash {
+            self.files.insert(hash, id);
         }
+        self.sources.push(SourceFile::new(id, path, src));
+        id
     }
 
-    /// Remove all sources.
-    pub fn clear(&mut self) {
-        self.sources.clear();
+    /// Get a reference to a loaded source file.
+    ///
+    /// This panics if no source file with this id was loaded. This function
+    /// should only be called with ids returned by this store's
+    /// [`load()`](Self::load) and [`provide()`](Self::provide) methods.
+    #[track_caller]
+    pub fn get(&self, id: SourceId) -> &SourceFile {
+        &self.sources[id.0 as usize]
     }
 }
 
 /// A single source file.
 pub struct SourceFile {
-    file: FileId,
+    id: SourceId,
+    path: PathBuf,
     src: String,
     line_starts: Vec<Pos>,
 }
 
 impl SourceFile {
-    /// Create a new source file from  string.
-    pub fn new(file: FileId, src: String) -> Self {
+    fn new(id: SourceId, path: &Path, src: String) -> Self {
         let mut line_starts = vec![Pos::ZERO];
         let mut s = Scanner::new(&src);
 
@@ -62,12 +133,27 @@ impl SourceFile {
             }
         }
 
-        Self { file, src, line_starts }
+        Self {
+            id,
+            path: path.normalize(),
+            src,
+            line_starts,
+        }
+    }
+
+    /// Create a source file without a real id and path, usually for testing.
+    pub fn detached(src: impl Into<String>) -> Self {
+        Self::new(SourceId(0), Path::new(""), src.into())
+    }
+
+    /// The id of the source file.
+    pub fn id(&self) -> SourceId {
+        self.id
     }
 
-    /// The file id.
-    pub fn file(&self) -> FileId {
-        self.file
+    /// The path to the source file.
+    pub fn path(&self) -> &Path {
+        &self.path
     }
 
     /// The whole source as a string slice.
@@ -150,22 +236,73 @@ fn width(c: char) -> usize {
     if c == '\t' { 2 } else { 1 }
 }
 
+impl AsRef<str> for SourceFile {
+    fn as_ref(&self) -> &str {
+        &self.src
+    }
+}
+
+#[cfg(feature = "codespan-reporting")]
+impl<'a> Files<'a> for SourceStore {
+    type FileId = SourceId;
+    type Name = std::path::Display<'a>;
+    type Source = &'a SourceFile;
+
+    fn name(&'a self, id: SourceId) -> Result<Self::Name, files::Error> {
+        Ok(self.get(id).path().display())
+    }
+
+    fn source(&'a self, id: SourceId) -> Result<Self::Source, files::Error> {
+        Ok(self.get(id))
+    }
+
+    fn line_index(
+        &'a self,
+        id: SourceId,
+        byte_index: usize,
+    ) -> Result<usize, files::Error> {
+        let source = self.get(id);
+        source.pos_to_line(byte_index.into()).ok_or_else(|| {
+            let (given, max) = (byte_index, source.len_bytes());
+            if given <= max {
+                files::Error::InvalidCharBoundary { given }
+            } else {
+                files::Error::IndexTooLarge { given, max }
+            }
+        })
+    }
+
+    fn line_range(
+        &'a self,
+        id: SourceId,
+        line_index: usize,
+    ) -> Result<std::ops::Range<usize>, files::Error> {
+        let source = self.get(id);
+        match source.line_to_span(line_index) {
+            Some(span) => Ok(span.to_range()),
+            None => Err(files::Error::LineTooLarge {
+                given: line_index,
+                max: source.len_lines(),
+            }),
+        }
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
 
-    const ID: FileId = FileId::from_raw(0);
     const TEST: &str = "äbcde\nf💛g\r\nhi\rjkl";
 
     #[test]
     fn test_source_file_new() {
-        let source = SourceFile::new(ID, TEST.into());
+        let source = SourceFile::detached(TEST);
         assert_eq!(source.line_starts, vec![Pos(0), Pos(7), Pos(15), Pos(18)]);
     }
 
     #[test]
     fn test_source_file_pos_to_line() {
-        let source = SourceFile::new(ID, TEST.into());
+        let source = SourceFile::detached(TEST);
         assert_eq!(source.pos_to_line(Pos(0)), Some(0));
         assert_eq!(source.pos_to_line(Pos(2)), Some(0));
         assert_eq!(source.pos_to_line(Pos(6)), Some(0));
@@ -186,7 +323,7 @@ mod tests {
             assert_eq!(result, byte_pos);
         }
 
-        let source = SourceFile::new(ID, TEST.into());
+        let source = SourceFile::detached(TEST);
         roundtrip(&source, Pos(0));
         roundtrip(&source, Pos(7));
         roundtrip(&source, Pos(12));