Eco string 🌱

21 files changed, 472 insertions, 55 deletions

diff --git a/src/eco.rs b/src/eco.rs
new file mode 100644
index 00000000..2d2ab2dc
--- /dev/null
+++ b/src/eco.rs
@@ -0,0 +1,389 @@
+//! An economical string.
+
+use std::borrow::Borrow;
+use std::cmp::Ordering;
+use std::fmt::{self, Debug, Display, Formatter, Write};
+use std::hash::{Hash, Hasher};
+use std::ops::{Add, AddAssign, Deref};
+use std::rc::Rc;
+
+/// A economical string with inline storage and clone-on-write value semantics.
+#[derive(Clone)]
+pub struct EcoString(Repr);
+
+/// The internal representation. Either:
+/// - inline when below a certain number of bytes,
+/// - or reference-counted on the heap with COW semantics.
+#[derive(Clone)]
+enum Repr {
+    Small { buf: [u8; LIMIT], len: u8 },
+    Large(Rc<String>),
+}
+
+/// The maximum number of bytes that can be stored inline.
+///
+/// The value is chosen such that `Repr` fits exactly into 16 bytes
+/// (which are needed anyway due to `Rc`s alignment).
+///
+/// Must be at least 4 to hold any char.
+const LIMIT: usize = 14;
+
+impl EcoString {
+    /// Create a new, empty string.
+    pub fn new() -> Self {
+        Self(Repr::Small { buf: [0; LIMIT], len: 0 })
+    }
+
+    /// Create a new, empty string with the given `capacity`.
+    pub fn with_capacity(capacity: usize) -> Self {
+        if capacity <= LIMIT {
+            Self::new()
+        } else {
+            Self(Repr::Large(Rc::new(String::with_capacity(capacity))))
+        }
+    }
+
+    /// Create an instance from an existing string-like type.
+    pub fn from_str<S>(s: S) -> Self
+    where
+        S: AsRef<str> + Into<String>,
+    {
+        let slice = s.as_ref();
+        let len = slice.len();
+        Self(if len <= LIMIT {
+            let mut buf = [0; LIMIT];
+            buf[.. len].copy_from_slice(slice.as_bytes());
+            Repr::Small { buf, len: len as u8 }
+        } else {
+            Repr::Large(Rc::new(s.into()))
+        })
+    }
+
+    /// Whether the string is empty.
+    pub fn is_empty(&self) -> bool {
+        self.len() == 0
+    }
+
+    /// The length of the string in bytes.
+    pub fn len(&self) -> usize {
+        match &self.0 {
+            Repr::Small { len, .. } => usize::from(*len),
+            Repr::Large(string) => string.len(),
+        }
+    }
+
+    /// A string slice containing the entire string.
+    pub fn as_str(&self) -> &str {
+        self
+    }
+
+    /// Appends the given character at the end.
+    pub fn push(&mut self, c: char) {
+        match &mut self.0 {
+            Repr::Small { buf, len } => {
+                let prev = usize::from(*len);
+                if c.len_utf8() == 1 && prev < LIMIT {
+                    buf[prev] = c as u8;
+                    *len += 1;
+                } else {
+                    self.push_str(c.encode_utf8(&mut [0; 4]));
+                }
+            }
+            Repr::Large(rc) => Rc::make_mut(rc).push(c),
+        }
+    }
+
+    /// Appends the given string slice at the end.
+    pub fn push_str(&mut self, string: &str) {
+        match &mut self.0 {
+            Repr::Small { buf, len } => {
+                let prev = usize::from(*len);
+                let new = prev + string.len();
+                if new <= LIMIT {
+                    buf[prev .. new].copy_from_slice(string.as_bytes());
+                    *len = new as u8;
+                } else {
+                    let mut spilled = String::with_capacity(new);
+                    spilled.push_str(self);
+                    spilled.push_str(string);
+                    self.0 = Repr::Large(Rc::new(spilled));
+                }
+            }
+            Repr::Large(rc) => Rc::make_mut(rc).push_str(string),
+        }
+    }
+
+    /// Removes the last character from the string.
+    pub fn pop(&mut self) -> Option<char> {
+        let c = self.as_str().chars().rev().next()?;
+        match &mut self.0 {
+            Repr::Small { len, .. } => {
+                *len -= c.len_utf8() as u8;
+            }
+            Repr::Large(rc) => {
+                Rc::make_mut(rc).pop();
+            }
+        }
+        Some(c)
+    }
+
+    /// Repeats this string `n` times.
+    pub fn repeat(&self, n: usize) -> Self {
+        if let Repr::Small { buf, len } = &self.0 {
+            let prev = usize::from(*len);
+            let new = prev.saturating_mul(n);
+            if new <= LIMIT {
+                let src = &buf[.. prev];
+                let mut buf = [0; LIMIT];
+                for i in 0 .. n {
+                    buf[prev * i .. prev * (i + 1)].copy_from_slice(src);
+                }
+                return Self(Repr::Small { buf, len: new as u8 });
+            }
+        }
+
+        self.as_str().repeat(n).into()
+    }
+}
+
+impl From<&Self> for EcoString {
+    fn from(s: &Self) -> Self {
+        s.clone()
+    }
+}
+
+impl From<char> for EcoString {
+    fn from(c: char) -> Self {
+        let mut buf = [0; LIMIT];
+        let len = c.encode_utf8(&mut buf).len();
+        Self(Repr::Small { buf, len: len as u8 })
+    }
+}
+
+impl From<&str> for EcoString {
+    fn from(s: &str) -> Self {
+        Self::from_str(s)
+    }
+}
+
+impl From<String> for EcoString {
+    fn from(s: String) -> Self {
+        Self::from_str(s)
+    }
+}
+
+impl From<&String> for EcoString {
+    fn from(s: &String) -> Self {
+        Self::from_str(s)
+    }
+}
+
+impl Deref for EcoString {
+    type Target = str;
+
+    fn deref(&self) -> &str {
+        match &self.0 {
+            Repr::Small { buf, len } => unsafe {
+                std::str::from_utf8_unchecked(&buf[.. usize::from(*len)])
+            },
+            Repr::Large(string) => string.as_str(),
+        }
+    }
+}
+
+impl AsRef<str> for EcoString {
+    fn as_ref(&self) -> &str {
+        self
+    }
+}
+
+impl Borrow<str> for EcoString {
+    fn borrow(&self) -> &str {
+        self
+    }
+}
+
+impl Default for EcoString {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+impl Debug for EcoString {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        Debug::fmt(self.as_str(), f)
+    }
+}
+
+impl Display for EcoString {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        Display::fmt(self.as_str(), f)
+    }
+}
+
+impl Eq for EcoString {}
+
+impl PartialEq for EcoString {
+    fn eq(&self, other: &Self) -> bool {
+        self.as_str().eq(other.as_str())
+    }
+}
+
+impl PartialEq<str> for EcoString {
+    fn eq(&self, other: &str) -> bool {
+        self.as_str().eq(other)
+    }
+}
+
+impl PartialEq<&str> for EcoString {
+    fn eq(&self, other: &&str) -> bool {
+        self.as_str().eq(*other)
+    }
+}
+
+impl PartialEq<String> for EcoString {
+    fn eq(&self, other: &String) -> bool {
+        self.as_str().eq(other.as_str())
+    }
+}
+
+impl Ord for EcoString {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.as_str().cmp(other.as_str())
+    }
+}
+
+impl PartialOrd for EcoString {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        self.as_str().partial_cmp(other.as_str())
+    }
+}
+
+impl Add<&str> for EcoString {
+    type Output = Self;
+
+    fn add(mut self, rhs: &str) -> Self::Output {
+        self.push_str(rhs);
+        self
+    }
+}
+
+impl AddAssign<&str> for EcoString {
+    fn add_assign(&mut self, rhs: &str) {
+        self.push_str(rhs);
+    }
+}
+
+impl Hash for EcoString {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.as_str().hash(state);
+    }
+}
+
+impl Write for EcoString {
+    fn write_str(&mut self, s: &str) -> fmt::Result {
+        self.push_str(s);
+        Ok(())
+    }
+
+    fn write_char(&mut self, c: char) -> fmt::Result {
+        self.push(c);
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    const ALPH: &str = "abcdefghijklmnopqrstuvwxyz";
+
+    #[test]
+    fn test_str_new() {
+        // Test inline strings.
+        assert_eq!(EcoString::new(), "");
+        assert_eq!(EcoString::from('a'), "a");
+        assert_eq!(EcoString::from('😀'), "😀");
+        assert_eq!(EcoString::from("abc"), "abc");
+
+        // Test around the inline limit.
+        assert_eq!(EcoString::from(&ALPH[.. LIMIT - 1]), ALPH[.. LIMIT - 1]);
+        assert_eq!(EcoString::from(&ALPH[.. LIMIT]), ALPH[.. LIMIT]);
+        assert_eq!(EcoString::from(&ALPH[.. LIMIT + 1]), ALPH[.. LIMIT + 1]);
+
+        // Test heap string.
+        assert_eq!(EcoString::from(ALPH), ALPH);
+    }
+
+    #[test]
+    fn test_str_push() {
+        let mut v = EcoString::new();
+        v.push('a');
+        v.push('b');
+        v.push_str("cd😀");
+        assert_eq!(v, "abcd😀");
+        assert_eq!(v.len(), 8);
+
+        // Test fully filling the inline storage.
+        v.push_str("efghij");
+        assert_eq!(v.len(), LIMIT);
+
+        // Test spilling with `push`.
+        let mut a = v.clone();
+        a.push('k');
+        assert_eq!(a, "abcd😀efghijk");
+        assert_eq!(a.len(), 15);
+
+        // Test spilling with `push_str`.
+        let mut b = v.clone();
+        b.push_str("klmn");
+        assert_eq!(b, "abcd😀efghijklmn");
+        assert_eq!(b.len(), 18);
+
+        // v should be unchanged.
+        assert_eq!(v.len(), LIMIT);
+    }
+
+    #[test]
+    fn test_str_pop() {
+        // Test with inline string.
+        let mut v = EcoString::from("Hello World!");
+        assert_eq!(v.pop(), Some('!'));
+        assert_eq!(v, "Hello World");
+
+        // Remove one-by-one.
+        for _ in 0 .. 10 {
+            v.pop();
+        }
+
+        assert_eq!(v, "H");
+        assert_eq!(v.pop(), Some('H'));
+        assert_eq!(v, "");
+        assert!(v.is_empty());
+
+        // Test with large string.
+        let mut v = EcoString::from(ALPH);
+        assert_eq!(v.pop(), Some('z'));
+        assert_eq!(v.len(), 25);
+    }
+
+    #[test]
+    fn test_str_index() {
+        // Test that we can use the index syntax.
+        let v = EcoString::from("abc");
+        assert_eq!(&v[.. 2], "ab");
+    }
+
+    #[test]
+    fn test_str_repeat() {
+        // Test with empty string.
+        assert_eq!(EcoString::new().repeat(0), "");
+        assert_eq!(EcoString::new().repeat(100), "");
+
+        // Test non-spilling and spilling case.
+        let v = EcoString::from("abc");
+        assert_eq!(v.repeat(0), "");
+        assert_eq!(v.repeat(3), "abcabcabc");
+        assert_eq!(v.repeat(5), "abcabcabcabcabc");
+    }
+}
diff --git a/src/eval/mod.rs b/src/eval/mod.rs
index 4992f70c..dc4ab7ee 100644
--- a/src/eval/mod.rs
+++ b/src/eval/mod.rs
@@ -17,6 +17,7 @@ use std::rc::Rc;
 
 use crate::cache::Cache;
 use crate::diag::{Diag, DiagSet, Pass};
+use crate::eco::EcoString;
 use crate::geom::{Angle, Fractional, Length, Relative};
 use crate::loading::{FileHash, Loader};
 use crate::parse::parse;
@@ -528,7 +529,7 @@ impl Eval for ClosureExpr {
             visitor.finish()
         };
 
-        let name = self.name.as_ref().map(|id| id.to_string());
+        let name = self.name.as_ref().map(|name| name.string.clone());
         Value::Func(FuncValue::new(name, move |ctx, args| {
             // Don't leak the scopes from the call site. Instead, we use the
             // scope of captured variables we collected earlier.
@@ -555,7 +556,7 @@ impl Eval for WithExpr {
         let callee = self.callee.eval(ctx);
         if let Some(func) = ctx.cast::<FuncValue>(callee, self.callee.span()) {
             let applied = self.args.eval(ctx);
-            let name = func.name().map(|s| s.to_string());
+            let name = func.name().cloned();
             Value::Func(FuncValue::new(name, move |ctx, args| {
                 // Remove named arguments that were overridden.
                 let kept: Vec<_> = applied
@@ -698,7 +699,7 @@ impl Eval for ImportExpr {
 
     fn eval(&self, ctx: &mut EvalContext) -> Self::Output {
         let path = self.path.eval(ctx);
-        if let Some(path) = ctx.cast::<String>(path, self.path.span()) {
+        if let Some(path) = ctx.cast::<EcoString>(path, self.path.span()) {
             if let Some(hash) = ctx.import(&path, self.path.span()) {
                 let mut module = &ctx.modules[&hash];
                 match &self.imports {
@@ -734,7 +735,7 @@ impl Eval for IncludeExpr {
 
     fn eval(&self, ctx: &mut EvalContext) -> Self::Output {
         let path = self.path.eval(ctx);
-        if let Some(path) = ctx.cast::<String>(path, self.path.span()) {
+        if let Some(path) = ctx.cast::<EcoString>(path, self.path.span()) {
             if let Some(hash) = ctx.import(&path, self.path.span()) {
                 return Value::Template(ctx.modules[&hash].template.clone());
             }
diff --git a/src/eval/scope.rs b/src/eval/scope.rs
index 3f7c4c62..05bbeda2 100644
--- a/src/eval/scope.rs
+++ b/src/eval/scope.rs
@@ -4,7 +4,7 @@ use std::fmt::{self, Debug, Display, Formatter};
 use std::iter;
 use std::rc::Rc;
 
-use super::{AnyValue, EvalContext, FuncArgs, FuncValue, Type, Value};
+use super::{AnyValue, EcoString, EvalContext, FuncArgs, FuncValue, Type, Value};
 
 /// A slot where a variable is stored.
 pub type Slot = Rc<RefCell<Value>>;
@@ -39,17 +39,17 @@ impl<'a> Scopes<'a> {
     }
 
     /// Define a constant variable with a value in the active scope.
-    pub fn def_const(&mut self, var: impl Into<String>, value: impl Into<Value>) {
+    pub fn def_const(&mut self, var: impl Into<EcoString>, value: impl Into<Value>) {
         self.top.def_const(var, value);
     }
 
     /// Define a mutable variable with a value in the active scope.
-    pub fn def_mut(&mut self, var: impl Into<String>, value: impl Into<Value>) {
+    pub fn def_mut(&mut self, var: impl Into<EcoString>, value: impl Into<Value>) {
         self.top.def_mut(var, value);
     }
 
     /// Define a variable with a slot in the active scope.
-    pub fn def_slot(&mut self, var: impl Into<String>, slot: Slot) {
+    pub fn def_slot(&mut self, var: impl Into<EcoString>, slot: Slot) {
         self.top.def_slot(var, slot);
     }
 
@@ -66,7 +66,7 @@ impl<'a> Scopes<'a> {
 #[derive(Default, Clone, PartialEq)]
 pub struct Scope {
     /// The mapping from names to slots.
-    values: HashMap<String, Slot>,
+    values: HashMap<EcoString, Slot>,
 }
 
 impl Scope {
@@ -76,7 +76,7 @@ impl Scope {
     }
 
     /// Define a constant variable with a value.
-    pub fn def_const(&mut self, var: impl Into<String>, value: impl Into<Value>) {
+    pub fn def_const(&mut self, var: impl Into<EcoString>, value: impl Into<Value>) {
         let cell = RefCell::new(value.into());
 
         // Make it impossible to write to this value again.
@@ -87,7 +87,7 @@ impl Scope {
     }
 
     /// Define a constant function.
-    pub fn def_func<F>(&mut self, name: impl Into<String>, f: F)
+    pub fn def_func<F>(&mut self, name: impl Into<EcoString>, f: F)
     where
         F: Fn(&mut EvalContext, &mut FuncArgs) -> Value + 'static,
     {
@@ -96,7 +96,7 @@ impl Scope {
     }
 
     /// Define a constant variable with a value of variant `Value::Any`.
-    pub fn def_any<T>(&mut self, var: impl Into<String>, any: T)
+    pub fn def_any<T>(&mut self, var: impl Into<EcoString>, any: T)
     where
         T: Type + Debug + Display + Clone + PartialEq + 'static,
     {
@@ -104,12 +104,12 @@ impl Scope {
     }
 
     /// Define a mutable variable with a value.
-    pub fn def_mut(&mut self, var: impl Into<String>, value: impl Into<Value>) {
+    pub fn def_mut(&mut self, var: impl Into<EcoString>, value: impl Into<Value>) {
         self.values.insert(var.into(), Rc::new(RefCell::new(value.into())));
     }
 
     /// Define a variable with a slot.
-    pub fn def_slot(&mut self, var: impl Into<String>, slot: Slot) {
+    pub fn def_slot(&mut self, var: impl Into<EcoString>, slot: Slot) {
         self.values.insert(var.into(), slot);
     }
 
diff --git a/src/eval/value.rs b/src/eval/value.rs
index 07552c84..2881399b 100644
--- a/src/eval/value.rs
+++ b/src/eval/value.rs
@@ -5,8 +5,7 @@ use std::fmt::{self, Debug, Display, Formatter};
 use std::ops::Deref;
 use std::rc::Rc;
 
-use super::ops;
-use super::EvalContext;
+use super::*;
 use crate::color::{Color, RgbaColor};
 use crate::exec::ExecContext;
 use crate::geom::{Angle, Fractional, Length, Linear, Relative};
@@ -38,7 +37,7 @@ pub enum Value {
     /// A color value: `#f79143ff`.
     Color(Color),
     /// A string: `"string"`.
-    Str(String),
+    Str(EcoString),
     /// An array value: `(1, "hi", 12cm)`.
     Array(ArrayValue),
     /// A dictionary value: `(color: #f79143, pattern: dashed)`.
@@ -76,7 +75,7 @@ impl Value {
             Self::Linear(_) => Linear::TYPE_NAME,
             Self::Fractional(_) => Fractional::TYPE_NAME,
             Self::Color(_) => Color::TYPE_NAME,
-            Self::Str(_) => String::TYPE_NAME,
+            Self::Str(_) => EcoString::TYPE_NAME,
             Self::Array(_) => ArrayValue::TYPE_NAME,
             Self::Dict(_) => DictValue::TYPE_NAME,
             Self::Template(_) => TemplateValue::TYPE_NAME,
@@ -151,7 +150,7 @@ impl Default for Value {
 pub type ArrayValue = Vec<Value>;
 
 /// A dictionary value: `(color: #f79143, pattern: dashed)`.
-pub type DictValue = BTreeMap<String, Value>;
+pub type DictValue = BTreeMap<EcoString, Value>;
 
 /// A template value: `[*Hi* there]`.
 pub type TemplateValue = Rc<Vec<TemplateNode>>;
@@ -171,7 +170,7 @@ pub enum TemplateNode {
         map: ExprMap,
     },
     /// A template that was converted from a string.
-    Str(String),
+    Str(EcoString),
     /// A function template that can implement custom behaviour.
     Func(TemplateFunc),
 }
@@ -224,14 +223,14 @@ impl Debug for TemplateFunc {
 pub struct FuncValue {
     /// The string is boxed to make the whole struct fit into 24 bytes, so that
     /// a [`Value`] fits into 32 bytes.
-    name: Option<Box<String>>,
+    name: Option<Box<EcoString>>,
     /// The closure that defines the function.
     f: Rc<dyn Fn(&mut EvalContext, &mut FuncArgs) -> Value>,
 }
 
 impl FuncValue {
     /// Create a new function value from a rust function or closure.
-    pub fn new<F>(name: Option<String>, f: F) -> Self
+    pub fn new<F>(name: Option<EcoString>, f: F) -> Self
     where
         F: Fn(&mut EvalContext, &mut FuncArgs) -> Value + 'static,
     {
@@ -239,8 +238,8 @@ impl FuncValue {
     }
 
     /// The name of the function.
-    pub fn name(&self) -> Option<&str> {
-        self.name.as_ref().map(|s| s.as_str())
+    pub fn name(&self) -> Option<&EcoString> {
+        self.name.as_ref().map(|s| &**s)
     }
 }
 
@@ -342,7 +341,7 @@ impl FuncArgs {
         let index = self
             .items
             .iter()
-            .position(|arg| arg.name.as_ref().map_or(false, |other| name == other))?;
+            .position(|arg| arg.name.as_ref().map_or(false, |other| other == name))?;
 
         let value = self.items.remove(index).value;
         let span = value.span;
@@ -381,7 +380,7 @@ pub struct FuncArg {
     /// The span of the whole argument.
     pub span: Span,
     /// The name of the argument (`None` for positional arguments).
-    pub name: Option<String>,
+    pub name: Option<EcoString>,
     /// The value of the argument.
     pub value: Spanned<Value>,
 }
@@ -608,7 +607,7 @@ primitive! {
 }
 primitive! { Fractional: "fractional", Value::Fractional }
 primitive! { Color: "color", Value::Color }
-primitive! { String: "string", Value::Str }
+primitive! { EcoString: "string", Value::Str }
 primitive! { ArrayValue: "array", Value::Array }
 primitive! { DictValue: "dictionary", Value::Dict }
 primitive! {
@@ -624,9 +623,15 @@ impl From<usize> for Value {
     }
 }
 
+impl From<String> for Value {
+    fn from(v: String) -> Self {
+        Self::Str(v.into())
+    }
+}
+
 impl From<&str> for Value {
     fn from(v: &str) -> Self {
-        Self::Str(v.to_string())
+        Self::Str(v.into())
     }
 }
 
diff --git a/src/exec/context.rs b/src/exec/context.rs
index 0f6d47f5..4764a808 100644
--- a/src/exec/context.rs
+++ b/src/exec/context.rs
@@ -3,6 +3,7 @@ use std::rc::Rc;
 
 use super::{Exec, ExecWithMap, FontFamily, State};
 use crate::diag::{Diag, DiagSet, Pass};
+use crate::eco::EcoString;
 use crate::eval::{ExprMap, TemplateValue};
 use crate::geom::{Align, Dir, Gen, GenAxis, Length, Linear, Sides, Size};
 use crate::layout::{
@@ -77,7 +78,7 @@ impl ExecContext {
     /// Push text into the active paragraph.
     ///
     /// The text is split into lines at newlines.
-    pub fn push_text(&mut self, text: impl Into<String>) {
+    pub fn push_text(&mut self, text: impl Into<EcoString>) {
         self.stack.par.push(self.make_text_node(text));
     }
 
@@ -143,7 +144,7 @@ impl ExecContext {
         Pass::new(self.tree, self.diags)
     }
 
-    fn make_text_node(&self, text: impl Into<String>) -> ParChild {
+    fn make_text_node(&self, text: impl Into<EcoString>) -> ParChild {
         ParChild::Text(
             text.into(),
             self.state.aligns.cross,
diff --git a/src/exec/mod.rs b/src/exec/mod.rs
index fc829676..2a145fbc 100644
--- a/src/exec/mod.rs
+++ b/src/exec/mod.rs
@@ -6,6 +6,7 @@ mod state;
 pub use context::*;
 pub use state::*;
 
+use std::fmt::Write;
 use std::rc::Rc;
 
 use crate::diag::Pass;
@@ -13,6 +14,7 @@ use crate::eval::{ExprMap, TemplateFunc, TemplateNode, TemplateValue, Value};
 use crate::geom::{Dir, Gen};
 use crate::layout::{LayoutTree, StackChild, StackNode};
 use crate::pretty::pretty;
+use crate::eco::EcoString;
 use crate::syntax::*;
 
 /// Execute a template to produce a layout tree.
@@ -102,18 +104,25 @@ impl ExecWithMap for HeadingNode {
 
 impl ExecWithMap for ListItem {
     fn exec_with_map(&self, ctx: &mut ExecContext, map: &ExprMap) {
-        exec_item(ctx, "•".to_string(), &self.body, map);
+        exec_item(ctx, '•'.into(), &self.body, map);
     }
 }
 
 impl ExecWithMap for EnumItem {
     fn exec_with_map(&self, ctx: &mut ExecContext, map: &ExprMap) {
-        let label = self.number.unwrap_or(1).to_string() + ".";
+        let mut label = EcoString::new();
+        write!(&mut label, "{}", self.number.unwrap_or(1)).unwrap();
+        label.push('.');
         exec_item(ctx, label, &self.body, map);
     }
 }
 
-fn exec_item(ctx: &mut ExecContext, label: String, body: &SyntaxTree, map: &ExprMap) {
+fn exec_item(
+    ctx: &mut ExecContext,
+    label: EcoString,
+    body: &SyntaxTree,
+    map: &ExprMap,
+) {
     let label = ctx.exec_stack(|ctx| ctx.push_text(label));
     let body = ctx.exec_tree_stack(body, map);
     let stack = StackNode {
diff --git a/src/layout/par.rs b/src/layout/par.rs
index 5d21e05e..bd744201 100644
--- a/src/layout/par.rs
+++ b/src/layout/par.rs
@@ -6,6 +6,7 @@ use xi_unicode::LineBreakIterator;
 
 use super::*;
 use crate::exec::FontState;
+use crate::eco::EcoString;
 use crate::util::{RangeExt, SliceExt};
 
 type Range = std::ops::Range<usize>;
@@ -29,7 +30,7 @@ pub enum ParChild {
     /// Spacing between other nodes.
     Spacing(Length),
     /// A run of text and how to align it in its line.
-    Text(String, Align, Rc<FontState>),
+    Text(EcoString, Align, Rc<FontState>),
     /// Any child node and how to align it in its line.
     Any(AnyNode, Align),
 }
diff --git a/src/lib.rs b/src/lib.rs
index f083163b..000d61d3 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -34,6 +34,7 @@ pub mod diag;
 pub mod eval;
 pub mod cache;
 pub mod color;
+pub mod eco;
 pub mod exec;
 pub mod export;
 pub mod font;
diff --git a/src/library/elements.rs b/src/library/elements.rs
index c57504fb..e669369a 100644
--- a/src/library/elements.rs
+++ b/src/library/elements.rs
@@ -9,7 +9,7 @@ use crate::layout::{
 
 /// `image`: An image.
 pub fn image(ctx: &mut EvalContext, args: &mut FuncArgs) -> Value {
-    let path = args.expect::<Spanned<String>>(ctx, "path to image file");
+    let path = args.expect::<Spanned<EcoString>>(ctx, "path to image file");
     let width = args.named(ctx, "width");
     let height = args.named(ctx, "height");
 
diff --git a/src/library/layout.rs b/src/library/layout.rs
index 4903077b..28ee27e1 100644
--- a/src/library/layout.rs
+++ b/src/library/layout.rs
@@ -5,7 +5,7 @@ use crate::paper::{Paper, PaperClass};
 /// `page`: Configure pages.
 pub fn page(ctx: &mut EvalContext, args: &mut FuncArgs) -> Value {
     let span = args.span;
-    let paper = args.eat::<Spanned<String>>(ctx).and_then(|name| {
+    let paper = args.eat::<Spanned<EcoString>>(ctx).and_then(|name| {
         Paper::from_name(&name.v).or_else(|| {
             ctx.diag(error!(name.span, "invalid paper name"));
             None
diff --git a/src/library/mod.rs b/src/library/mod.rs
index abc9ff5f..7c3e0a71 100644
--- a/src/library/mod.rs
+++ b/src/library/mod.rs
@@ -17,6 +17,7 @@ use std::fmt::{self, Display, Formatter};
 use std::rc::Rc;
 
 use crate::color::{Color, RgbaColor};
+use crate::eco::EcoString;
 use crate::eval::{EvalContext, FuncArgs, Scope, TemplateValue, Value};
 use crate::exec::{Exec, FontFamily};
 use crate::font::{FontStyle, FontWeight, VerticalFontMetric};
diff --git a/src/library/text.rs b/src/library/text.rs
index e1fff9f3..a0ffc56c 100644
--- a/src/library/text.rs
+++ b/src/library/text.rs
@@ -99,7 +99,7 @@ castable! {
     Value::Array(values) => Self(values
         .into_iter()
         .filter_map(|v| v.cast().ok())
-        .map(|string: String| string.to_lowercase())
+        .map(|string: EcoString| string.to_lowercase())
         .collect()
     ),
 }
@@ -185,7 +185,7 @@ pub fn par(ctx: &mut EvalContext, args: &mut FuncArgs) -> Value {
 
 /// `lang`: Configure the language.
 pub fn lang(ctx: &mut EvalContext, args: &mut FuncArgs) -> Value {
-    let iso = args.eat::<String>(ctx).map(|s| lang_dir(&s));
+    let iso = args.eat::<EcoString>(ctx).map(|s| lang_dir(&s));
     let dir = match args.named::<Spanned<Dir>>(ctx, "dir") {
         Some(dir) if dir.v.axis() == SpecAxis::Horizontal => Some(dir.v),
         Some(dir) => {
diff --git a/src/library/utility.rs b/src/library/utility.rs
index 272183aa..c1f20cc6 100644
--- a/src/library/utility.rs
+++ b/src/library/utility.rs
@@ -39,7 +39,7 @@ pub fn len(ctx: &mut EvalContext, args: &mut FuncArgs) -> Value {
 /// `rgb`: Create an RGB(A) color.
 pub fn rgb(ctx: &mut EvalContext, args: &mut FuncArgs) -> Value {
     Value::Color(Color::Rgba(
-        if let Some(string) = args.eat::<Spanned<String>>(ctx) {
+        if let Some(string) = args.eat::<Spanned<EcoString>>(ctx) {
             match RgbaColor::from_str(&string.v) {
                 Ok(color) => color,
                 Err(_) => {
diff --git a/src/parse/mod.rs b/src/parse/mod.rs
index c3d532a7..4c2fd129 100644
--- a/src/parse/mod.rs
+++ b/src/parse/mod.rs
@@ -15,6 +15,7 @@ pub use tokens::*;
 use std::rc::Rc;
 
 use crate::diag::Pass;
+use crate::eco::EcoString;
 use crate::syntax::*;
 
 /// Parse a string of source code.
@@ -151,10 +152,10 @@ fn node(p: &mut Parser, at_start: &mut bool) -> Option<Node> {
 }
 
 /// Handle a unicode escape sequence.
-fn unicode_escape(p: &mut Parser, token: UnicodeEscapeToken) -> String {
+fn unicode_escape(p: &mut Parser, token: UnicodeEscapeToken) -> EcoString {
     let span = p.peek_span();
     let text = if let Some(c) = resolve::resolve_hex(token.sequence) {
-        c.to_string()
+        c.into()
     } else {
         // Print out the escape sequence verbatim if it is invalid.
         p.diag(error!(span, "invalid unicode escape sequence"));
@@ -774,7 +775,7 @@ fn ident(p: &mut Parser) -> Option<Ident> {
     if let Some(Token::Ident(string)) = p.peek() {
         Some(Ident {
             span: p.eat_span(),
-            string: string.to_string(),
+            string: string.into(),
         })
     } else {
         p.expected("identifier");
diff --git a/src/parse/resolve.rs b/src/parse/resolve.rs
index b3fdef4a..c3676215 100644
--- a/src/parse/resolve.rs
+++ b/src/parse/resolve.rs
@@ -1,9 +1,10 @@
 use super::{is_newline, Scanner};
+use crate::eco::EcoString;
 use crate::syntax::{Ident, RawNode, Span};
 
 /// Resolve all escape sequences in a string.
-pub fn resolve_string(string: &str) -> String {
-    let mut out = String::with_capacity(string.len());
+pub fn resolve_string(string: &str) -> EcoString {
+    let mut out = EcoString::with_capacity(string.len());
     let mut s = Scanner::new(string);
 
     while let Some(c) = s.eat() {
@@ -52,12 +53,12 @@ pub fn resolve_raw(span: Span, text: &str, backticks: usize) -> RawNode {
         let (tag, inner) = split_at_lang_tag(text);
         let (text, block) = trim_and_split_raw(inner);
         let lang = Ident::new(tag, span.start .. span.start + tag.len());
-        RawNode { span, lang, text, block }
+        RawNode { span, lang, text: text.into(), block }
     } else {
         RawNode {
             span,
             lang: None,
-            text: split_lines(text).join("\n"),
+            text: split_lines(text).join("\n").into(),
             block: false,
         }
     }
diff --git a/src/pretty.rs b/src/pretty.rs
index 46b82eb7..83c207b5 100644
--- a/src/pretty.rs
+++ b/src/pretty.rs
@@ -617,7 +617,7 @@ mod tests {
         ($($k:ident: $v:expr),* $(,)?) => {{
             #[allow(unused_mut)]
             let mut m = BTreeMap::new();
-            $(m.insert(stringify!($k).to_string(), $v);)*
+            $(m.insert(stringify!($k).into(), $v);)*
             m
         }};
     }
diff --git a/src/syntax/expr.rs b/src/syntax/expr.rs
index 368fe41f..21df47c8 100644
--- a/src/syntax/expr.rs
+++ b/src/syntax/expr.rs
@@ -28,7 +28,7 @@ pub enum Expr {
     /// A fraction unit literal: `1fr`.
     Fractional(Span, f64),
     /// A string literal: `"hello!"`.
-    Str(Span, String),
+    Str(Span, EcoString),
     /// An identifier: `left`.
     Ident(Ident),
     /// An array expression: `(1, "hi", 12cm)`.
diff --git a/src/syntax/ident.rs b/src/syntax/ident.rs
index a46ad232..b1328bbe 100644
--- a/src/syntax/ident.rs
+++ b/src/syntax/ident.rs
@@ -3,6 +3,7 @@ use std::ops::Deref;
 use unicode_xid::UnicodeXID;
 
 use super::Span;
+use crate::eco::EcoString;
 
 /// An unicode identifier with a few extra permissible characters.
 ///
@@ -16,7 +17,7 @@ pub struct Ident {
     /// The source code location.
     pub span: Span,
     /// The identifier string.
-    pub string: String,
+    pub string: EcoString,
 }
 
 impl Ident {
@@ -26,7 +27,10 @@ impl Ident {
         span: impl Into<Span>,
     ) -> Option<Self> {
         if is_ident(string.as_ref()) {
-            Some(Self { span: span.into(), string: string.into() })
+            Some(Self {
+                span: span.into(),
+                string: EcoString::from_str(string),
+            })
         } else {
             None
         }
@@ -34,13 +38,13 @@ impl Ident {
 
     /// Return a reference to the underlying string.
     pub fn as_str(&self) -> &str {
-        self.string.as_str()
+        self
     }
 }
 
 impl AsRef<str> for Ident {
     fn as_ref(&self) -> &str {
-        self.as_str()
+        self
     }
 }
 
@@ -48,7 +52,7 @@ impl Deref for Ident {
     type Target = str;
 
     fn deref(&self) -> &Self::Target {
-        self.as_str()
+        self.string.as_str()
     }
 }
 
diff --git a/src/syntax/mod.rs b/src/syntax/mod.rs
index 6673fda6..1de5c1dd 100644
--- a/src/syntax/mod.rs
+++ b/src/syntax/mod.rs
@@ -13,6 +13,8 @@ pub use node::*;
 pub use span::*;
 pub use token::*;
 
+use crate::eco::EcoString;
+
 /// The abstract syntax tree.
 ///
 /// This type can represent a full parsed document.
diff --git a/src/syntax/node.rs b/src/syntax/node.rs
index 1fbdb3d8..bb9ff098 100644
--- a/src/syntax/node.rs
+++ b/src/syntax/node.rs
@@ -6,7 +6,7 @@ use super::*;
 #[derive(Debug, Clone, PartialEq)]
 pub enum Node {
     /// Plain text.
-    Text(String),
+    Text(EcoString),
     /// Whitespace containing less than two newlines.
     Space,
     /// A forced line break: `\`.
@@ -38,7 +38,7 @@ pub struct RawNode {
     pub lang: Option<Ident>,
     /// The raw text, determined as the raw string between the backticks trimmed
     /// according to the above rules.
-    pub text: String,
+    pub text: EcoString,
     /// Whether the element is block-level, that is, it has 3+ backticks
     /// and contains at least one newline.
     pub block: bool,
diff --git a/tests/typ/code/ops.typ b/tests/typ/code/ops.typ
index 58fd957f..d8e87308 100644
--- a/tests/typ/code/ops.typ
+++ b/tests/typ/code/ops.typ
@@ -27,6 +27,7 @@
 // Addition.
 #test(2 + 4, 6)
 #test("a" + "b", "ab")
+#test(13 * "a" + "bbbbbb", "aaaaaaaaaaaaabbbbbb")
 #test((1, 2) + (3, 4), (1, 2, 3, 4))
 #test((a: 1) + (b: 2, c: 3), (a: 1, b: 2, c: 3))