Array and dictionary indexing

src/diag.rs

@@ -58,21 +58,20 @@ impl Error {
         Box::new(vec![Self::new(source, span, message)])
     }
 
-    /// Partially build a vec-boxed error, returning a function that just needs
+    /// Create a closure that contains the positional information for an error
-    /// the message.
+    /// and just needs the message to yield a vec-boxed error.
     ///
     /// This is useful in to convert from [`StrResult`] to a [`TypResult`] using
     /// [`map_err`](Result::map_err).
-    pub fn partial(
+    pub fn at<S: Into<String>>(
         source: SourceId,
         span: impl Into<Span>,
-    ) -> impl FnOnce(String) -> Box<Vec<Self>> {
+    ) -> impl FnOnce(S) -> Box<Vec<Self>> {
         move |message| Self::boxed(source, span, message)
     }
 }
 
 /// Early-return with a vec-boxed [`Error`].
-#[macro_export]
 macro_rules! bail {
     ($source:expr, $span:expr, $message:expr $(,)?) => {
         return Err(Box::new(vec![$crate::diag::Error::new(
@@ -81,6 +80,6 @@ macro_rules! bail {
     };
 
     ($source:expr, $span:expr, $fmt:expr, $($arg:expr),+ $(,)?) => {
-        $crate::bail!($source, $span, format!($fmt, $($arg),+));
+        bail!($source, $span, format!($fmt, $($arg),+));
     };
 }

src/eval/array.rs

@@ -1,12 +1,14 @@
+use std::convert::TryFrom;
 use std::fmt::{self, Debug, Formatter};
 use std::iter::FromIterator;
 use std::ops::{Add, AddAssign};
 use std::rc::Rc;
 
 use super::Value;
+use crate::diag::StrResult;
 
 /// Create a new [`Array`] from values.
-#[macro_export]
+#[allow(unused_macros)]
 macro_rules! array {
     ($value:expr; $count:expr) => {
         $crate::eval::Array::from_vec(vec![$crate::eval::Value::from($value); $count])
@@ -47,25 +49,25 @@ impl Array {
     }
 
     /// The length of the array.
-    pub fn len(&self) -> usize {
+    pub fn len(&self) -> i64 {
-        self.vec.len()
+        self.vec.len() as i64
     }
 
     /// Borrow the value at the given index.
-    pub fn get(&self, index: usize) -> Option<&Value> {
+    pub fn get(&self, index: i64) -> StrResult<&Value> {
-        self.vec.get(index)
+        usize::try_from(index)
+            .ok()
+            .and_then(|i| self.vec.get(i))
+            .ok_or_else(|| out_of_bounds(index, self.len()))
     }
 
     /// Mutably borrow the value at the given index.
-    pub fn get_mut(&mut self, index: usize) -> Option<&mut Value> {
+    pub fn get_mut(&mut self, index: i64) -> StrResult<&mut Value> {
-        Rc::make_mut(&mut self.vec).get_mut(index)
+        let len = self.len();
-    }
+        usize::try_from(index)
-
+            .ok()
-    /// Set the value at the given index.
+            .and_then(move |i| Rc::make_mut(&mut self.vec).get_mut(i))
-    ///
+            .ok_or_else(|| out_of_bounds(index, len))
-    /// This panics the `index` is out of range.
-    pub fn set(&mut self, index: usize, value: Value) {
-        Rc::make_mut(&mut self.vec)[index] = value;
     }
 
     /// Push a value to the end of the array.
@@ -82,16 +84,26 @@ impl Array {
         }
     }
 
-    /// Repeat this array `n` times.
-    pub fn repeat(&self, n: usize) -> Self {
-        let len = self.len().checked_mul(n).expect("capacity overflow");
-        self.iter().cloned().cycle().take(len).collect()
-    }
-
     /// Iterate over references to the contained values.
     pub fn iter(&self) -> std::slice::Iter<Value> {
         self.vec.iter()
     }
+
+    /// Repeat this array `n` times.
+    pub fn repeat(&self, n: i64) -> StrResult<Self> {
+        let count = usize::try_from(n)
+            .ok()
+            .and_then(|n| self.vec.len().checked_mul(n))
+            .ok_or_else(|| format!("cannot repeat this array {} times", n))?;
+
+        Ok(self.iter().cloned().cycle().take(count).collect())
+    }
+}
+
+/// The out of bounds access error message.
+#[cold]
+fn out_of_bounds(index: i64, len: i64) -> String {
+    format!("array index out of bounds (index: {}, len: {})", index, len)
 }
 
 impl Default for Array {

src/eval/dict.rs

@@ -5,10 +5,12 @@ use std::ops::{Add, AddAssign};
 use std::rc::Rc;
 
 use super::Value;
+use crate::diag::StrResult;
+use crate::pretty::pretty;
 use crate::util::EcoString;
 
 /// Create a new [`Dict`] from key-value pairs.
-#[macro_export]
+#[allow(unused_macros)]
 macro_rules! dict {
     ($($key:expr => $value:expr),* $(,)?) => {{
         #[allow(unused_mut)]
@@ -41,18 +43,21 @@ impl Dict {
     }
 
     /// The number of pairs in the dictionary.
-    pub fn len(&self) -> usize {
+    pub fn len(&self) -> i64 {
-        self.map.len()
+        self.map.len() as i64
     }
 
     /// Borrow the value the given `key` maps to.
-    pub fn get(&self, key: &str) -> Option<&Value> {
+    pub fn get(&self, key: &str) -> StrResult<&Value> {
-        self.map.get(key)
+        self.map.get(key).ok_or_else(|| missing_key(key))
     }
 
     /// Mutably borrow the value the given `key` maps to.
-    pub fn get_mut(&mut self, key: &str) -> Option<&mut Value> {
+    ///
-        Rc::make_mut(&mut self.map).get_mut(key)
+    /// This inserts the key with [`None`](Value::None) as the value if not
+    /// present so far.
+    pub fn get_mut(&mut self, key: EcoString) -> &mut Value {
+        Rc::make_mut(&mut self.map).entry(key).or_default()
     }
 
     /// Insert a mapping from the given `key` to the given `value`.
@@ -75,6 +80,12 @@ impl Dict {
     }
 }
 
+/// The missing key access error message.
+#[cold]
+fn missing_key(key: &str) -> String {
+    format!("dictionary does not contain key: {}", pretty(key))
+}
+
 impl Default for Dict {
     fn default() -> Self {
         Self::new()

src/eval/function.rs

@@ -134,7 +134,7 @@ impl FuncArgs {
         let value = self.items.remove(index).value;
         let span = value.span;
 
-        T::cast(value).map(Some).map_err(Error::partial(self.source, span))
+        T::cast(value).map(Some).map_err(Error::at(self.source, span))
     }
 
     /// Return an "unexpected argument" error if there is any remaining
@@ -146,3 +146,40 @@ impl FuncArgs {
         Ok(())
     }
 }
+
+impl FuncArgs {
+    /// Reinterpret these arguments as actually being an array index.
+    pub fn into_index(self) -> TypResult<i64> {
+        self.into_castable("index")
+    }
+
+    /// Reinterpret these arguments as actually being a dictionary key.
+    pub fn into_key(self) -> TypResult<EcoString> {
+        self.into_castable("key")
+    }
+
+    /// Reinterpret these arguments as actually being a single castable thing.
+    fn into_castable<T>(self, what: &str) -> TypResult<T>
+    where
+        T: Cast<Value>,
+    {
+        let mut iter = self.items.into_iter();
+        let value = match iter.next() {
+            None => {
+                bail!(self.source, self.span, "missing {}", what);
+            }
+            Some(FuncArg { name: Some(_), span, .. }) => {
+                bail!(self.source, span, "named pair is not allowed here");
+            }
+            Some(FuncArg { name: None, value, .. }) => {
+                value.v.cast().map_err(Error::at(self.source, value.span))?
+            }
+        };
+
+        if let Some(arg) = iter.next() {
+            bail!(self.source, arg.span, "only one {} is allowed", what);
+        }
+
+        Ok(value)
+    }
+}

src/eval/mod.rs

@@ -20,6 +20,7 @@ pub use scope::*;
 pub use template::*;
 pub use value::*;
 
+use std::cell::RefMut;
 use std::collections::HashMap;
 use std::io;
 use std::mem;
@@ -34,7 +35,7 @@ use crate::parse::parse;
 use crate::source::{SourceId, SourceStore};
 use crate::syntax::visit::Visit;
 use crate::syntax::*;
-use crate::util::EcoString;
+use crate::util::{EcoString, RefMutExt};
 use crate::Context;
 
 /// Evaluate a parsed source file into a module.
@@ -60,15 +61,6 @@ pub struct Module {
     pub template: Template,
 }
 
-/// Evaluate an expression.
-pub trait Eval {
-    /// The output of evaluating the expression.
-    type Output;
-
-    /// Evaluate the expression to the output value.
-    fn eval(&self, ctx: &mut EvalContext) -> TypResult<Self::Output>;
-}
-
 /// The context for evaluation.
 pub struct EvalContext<'a> {
     /// The loader from which resources (files and images) are loaded.
@@ -168,45 +160,19 @@ impl<'a> EvalContext<'a> {
     }
 }
 
+/// Evaluate an expression.
+pub trait Eval {
+    /// The output of evaluating the expression.
+    type Output;
+
+    /// Evaluate the expression to the output value.
+    fn eval(&self, ctx: &mut EvalContext) -> TypResult<Self::Output>;
+}
+
 impl Eval for Rc<SyntaxTree> {
     type Output = Template;
 
     fn eval(&self, ctx: &mut EvalContext) -> TypResult<Self::Output> {
-        trait Walk {
-            fn walk(&self, ctx: &mut EvalContext) -> TypResult<()>;
-        }
-
-        impl Walk for SyntaxTree {
-            fn walk(&self, ctx: &mut EvalContext) -> TypResult<()> {
-                for node in self.iter() {
-                    node.walk(ctx)?;
-                }
-                Ok(())
-            }
-        }
-
-        impl Walk for SyntaxNode {
-            fn walk(&self, ctx: &mut EvalContext) -> TypResult<()> {
-                match self {
-                    Self::Text(_) => {}
-                    Self::Space => {}
-                    Self::Linebreak(_) => {}
-                    Self::Parbreak(_) => {}
-                    Self::Strong(_) => {}
-                    Self::Emph(_) => {}
-                    Self::Raw(_) => {}
-                    Self::Heading(n) => n.body.walk(ctx)?,
-                    Self::List(n) => n.body.walk(ctx)?,
-                    Self::Enum(n) => n.body.walk(ctx)?,
-                    Self::Expr(n) => {
-                        let value = n.eval(ctx)?;
-                        ctx.map.insert(n as *const _, value);
-                    }
-                }
-                Ok(())
-            }
-        }
-
         let map = {
             let prev = mem::take(&mut ctx.map);
             self.walk(ctx)?;
@@ -302,8 +268,8 @@ impl Eval for BlockExpr {
         let mut output = Value::None;
         for expr in &self.exprs {
             let value = expr.eval(ctx)?;
-            output = ops::join(output, value)
+            output =
-                .map_err(Error::partial(ctx.source, expr.span()))?;
+                ops::join(output, value).map_err(Error::at(ctx.source, expr.span()))?;
         }
 
         if self.scoping {
@@ -324,7 +290,7 @@ impl Eval for UnaryExpr {
             UnOp::Neg => ops::neg(value),
             UnOp::Not => ops::not(value),
         };
-        result.map_err(Error::partial(ctx.source, self.span))
+        result.map_err(Error::at(ctx.source, self.span))
     }
 }
 
@@ -371,7 +337,7 @@ impl BinaryExpr {
         }
 
         let rhs = self.rhs.eval(ctx)?;
-        op(lhs, rhs).map_err(Error::partial(ctx.source, self.span))
+        op(lhs, rhs).map_err(Error::at(ctx.source, self.span))
     }
 
     /// Apply an assignment operation.
@@ -379,27 +345,11 @@ impl BinaryExpr {
     where
         F: FnOnce(Value, Value) -> StrResult<Value>,
     {
-        let lspan = self.lhs.span();
+        let source = ctx.source;
-        let slot = if let Expr::Ident(id) = self.lhs.as_ref() {
-            match ctx.scopes.get(id) {
-                Some(slot) => Rc::clone(slot),
-                None => bail!(ctx.source, lspan, "unknown variable"),
-            }
-        } else {
-            bail!(ctx.source, lspan, "cannot assign to this expression",);
-        };
-
         let rhs = self.rhs.eval(ctx)?;
-        let mut mutable = match slot.try_borrow_mut() {
+        let mut target = self.lhs.access(ctx)?;
-            Ok(mutable) => mutable,
+        let lhs = mem::take(&mut *target);
-            Err(_) => {
+        *target = op(lhs, rhs).map_err(Error::at(source, self.span))?;
-                bail!(ctx.source, lspan, "cannot assign to a constant",);
-            }
-        };
-
-        let lhs = mem::take(&mut *mutable);
-        *mutable = op(lhs, rhs).map_err(Error::partial(ctx.source, self.span))?;
-
         Ok(Value::None)
     }
 }
@@ -408,32 +358,45 @@ impl Eval for CallExpr {
     type Output = Value;
 
     fn eval(&self, ctx: &mut EvalContext) -> TypResult<Self::Output> {
-        let callee = self
+        let callee = self.callee.eval(ctx)?;
-            .callee
-            .eval(ctx)?
-            .cast::<Function>()
-            .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.source == ctx.source && self.span.contains(error.span) {
-                    continue;
-                }
 
-                error.trace.push((
+        match callee {
-                    ctx.source,
+            Value::Array(array) => array
-                    self.span,
+                .get(args.into_index()?)
-                    Tracepoint::Call(callee.name().map(Into::into)),
+                .map(Value::clone)
-                ));
+                .map_err(Error::at(ctx.source, self.span)),
+
+            Value::Dict(dict) => dict
+                .get(&args.into_key()?)
+                .map(Value::clone)
+                .map_err(Error::at(ctx.source, self.span)),
+
+            Value::Func(func) => {
+                let returned = func(ctx, &mut args).map_err(|mut errors| {
+                    for error in errors.iter_mut() {
+                        // Skip errors directly related to arguments.
+                        if error.source != ctx.source || !self.span.contains(error.span) {
+                            error.trace.push((
+                                ctx.source,
+                                self.span,
+                                Tracepoint::Call(func.name().map(Into::into)),
+                            ));
+                        }
+                    }
+                    errors
+                })?;
+                args.finish()?;
+                Ok(returned)
             }
-            errors
-        })?;
 
-        args.finish()?;
+            v => bail!(
-
+                ctx.source,
-        Ok(returned)
+                self.callee.span(),
+                "expected function or collection, found {}",
+                v.type_name(),
+            ),
+        }
     }
 }
 
@@ -518,7 +481,7 @@ impl Eval for WithExpr {
             .callee
             .eval(ctx)?
             .cast::<Function>()
-            .map_err(Error::partial(ctx.source, self.callee.span()))?;
+            .map_err(Error::at(ctx.source, self.callee.span()))?;
 
         let applied = self.args.eval(ctx)?;
 
@@ -568,7 +531,7 @@ impl Eval for IfExpr {
             .condition
             .eval(ctx)?
             .cast::<bool>()
-            .map_err(Error::partial(ctx.source, self.condition.span()))?;
+            .map_err(Error::at(ctx.source, self.condition.span()))?;
 
         if condition {
             self.if_body.eval(ctx)
@@ -590,11 +553,11 @@ impl Eval for WhileExpr {
             .condition
             .eval(ctx)?
             .cast::<bool>()
-            .map_err(Error::partial(ctx.source, self.condition.span()))?
+            .map_err(Error::at(ctx.source, self.condition.span()))?
         {
             let value = self.body.eval(ctx)?;
             output = ops::join(output, value)
-                .map_err(Error::partial(ctx.source, self.body.span()))?;
+                .map_err(Error::at(ctx.source, self.body.span()))?;
         }
 
         Ok(output)
@@ -616,7 +579,7 @@ impl Eval for ForExpr {
 
                     let value = self.body.eval(ctx)?;
                     output = ops::join(output, value)
-                        .map_err(Error::partial(ctx.source, self.body.span()))?;
+                        .map_err(Error::at(ctx.source, self.body.span()))?;
                 }
 
                 ctx.scopes.exit();
@@ -662,7 +625,7 @@ impl Eval for ImportExpr {
             .path
             .eval(ctx)?
             .cast::<EcoString>()
-            .map_err(Error::partial(ctx.source, self.path.span()))?;
+            .map_err(Error::at(ctx.source, self.path.span()))?;
 
         let file = ctx.import(&path, self.path.span())?;
         let module = &ctx.modules[&file];
@@ -696,7 +659,7 @@ impl Eval for IncludeExpr {
             .path
             .eval(ctx)?
             .cast::<EcoString>()
-            .map_err(Error::partial(ctx.source, self.path.span()))?;
+            .map_err(Error::at(ctx.source, self.path.span()))?;
 
         let file = ctx.import(&path, self.path.span())?;
         let module = &ctx.modules[&file];
@@ -704,3 +667,97 @@ impl Eval for IncludeExpr {
         Ok(Value::Template(module.template.clone()))
     }
 }
+
+/// Walk a node in a template, filling the context's expression map.
+pub trait Walk {
+    /// Walk the node.
+    fn walk(&self, ctx: &mut EvalContext) -> TypResult<()>;
+}
+
+impl Walk for SyntaxTree {
+    fn walk(&self, ctx: &mut EvalContext) -> TypResult<()> {
+        for node in self.iter() {
+            node.walk(ctx)?;
+        }
+        Ok(())
+    }
+}
+
+impl Walk for SyntaxNode {
+    fn walk(&self, ctx: &mut EvalContext) -> TypResult<()> {
+        match self {
+            Self::Text(_) => {}
+            Self::Space => {}
+            Self::Linebreak(_) => {}
+            Self::Parbreak(_) => {}
+            Self::Strong(_) => {}
+            Self::Emph(_) => {}
+            Self::Raw(_) => {}
+            Self::Heading(n) => n.body.walk(ctx)?,
+            Self::List(n) => n.body.walk(ctx)?,
+            Self::Enum(n) => n.body.walk(ctx)?,
+            Self::Expr(n) => {
+                let value = n.eval(ctx)?;
+                ctx.map.insert(n as *const _, value);
+            }
+        }
+        Ok(())
+    }
+}
+
+/// Try to mutably access the value an expression points to.
+///
+/// This only works if the expression is a valid lvalue.
+pub trait Access {
+    /// Try to access the value.
+    fn access<'a>(&self, ctx: &'a mut EvalContext) -> TypResult<RefMut<'a, Value>>;
+}
+
+impl Access for Expr {
+    fn access<'a>(&self, ctx: &'a mut EvalContext) -> TypResult<RefMut<'a, Value>> {
+        match self {
+            Expr::Ident(ident) => ident.access(ctx),
+            Expr::Call(call) => call.access(ctx),
+            _ => bail!(
+                ctx.source,
+                self.span(),
+                "cannot access this expression mutably",
+            ),
+        }
+    }
+}
+
+impl Access for Ident {
+    fn access<'a>(&self, ctx: &'a mut EvalContext) -> TypResult<RefMut<'a, Value>> {
+        ctx.scopes
+            .get(self)
+            .ok_or("unknown variable")
+            .and_then(|slot| {
+                slot.try_borrow_mut().map_err(|_| "cannot mutate a constant")
+            })
+            .map_err(Error::at(ctx.source, self.span))
+    }
+}
+
+impl Access for CallExpr {
+    fn access<'a>(&self, ctx: &'a mut EvalContext) -> TypResult<RefMut<'a, Value>> {
+        let source = ctx.source;
+        let args = self.args.eval(ctx)?;
+        let guard = self.callee.access(ctx)?;
+
+        RefMut::try_map(guard, |value| match value {
+            Value::Array(array) => array
+                .get_mut(args.into_index()?)
+                .map_err(Error::at(source, self.span)),
+
+            Value::Dict(dict) => Ok(dict.get_mut(args.into_key()?)),
+
+            v => bail!(
+                source,
+                self.callee.span(),
+                "expected collection, found {}",
+                v.type_name(),
+            ),
+        })
+    }
+}

src/eval/ops.rs

@@ -150,12 +150,12 @@ pub fn mul(lhs: Value, rhs: Value) -> StrResult<Value> {
         (Fractional(a), Float(b)) => Fractional(a * b),
         (Int(a), Fractional(b)) => Fractional(a as f64 * b),
 
-        (Str(a), Int(b)) => Str(a.repeat(b.max(0) as usize)),
+        (Str(a), Int(b)) => Str(a.repeat(b)?),
-        (Int(a), Str(b)) => Str(b.repeat(a.max(0) as usize)),
+        (Int(a), Str(b)) => Str(b.repeat(a)?),
-        (Array(a), Int(b)) => Array(a.repeat(b.max(0) as usize)),
+        (Array(a), Int(b)) => Array(a.repeat(b)?),
-        (Int(a), Array(b)) => Array(b.repeat(a.max(0) as usize)),
+        (Int(a), Array(b)) => Array(b.repeat(a)?),
-        (Template(a), Int(b)) => Template(a.repeat(b.max(0) as usize)),
+        (Template(a), Int(b)) => Template(a.repeat(b)?),
-        (Int(a), Template(b)) => Template(b.repeat(a.max(0) as usize)),
+        (Int(a), Template(b)) => Template(b.repeat(a)?),
 
         (a, b) => mismatch!("cannot multiply {} with {}", a, b),
     })

src/eval/template.rs

@@ -1,9 +1,11 @@
 use std::collections::HashMap;
+use std::convert::TryFrom;
 use std::fmt::{self, Debug, Formatter};
 use std::ops::{Add, AddAssign, Deref};
 use std::rc::Rc;
 
 use super::Value;
+use crate::diag::StrResult;
 use crate::exec::ExecContext;
 use crate::syntax::{Expr, SyntaxTree};
 use crate::util::EcoString;
@@ -26,10 +28,15 @@ impl Template {
     }
 
     /// Repeat this template `n` times.
-    pub fn repeat(&self, n: usize) -> Self {
+    pub fn repeat(&self, n: i64) -> StrResult<Self> {
-        let len = self.nodes.len().checked_mul(n).expect("capacity overflow");
+        let count = usize::try_from(n)
-        let nodes = self.iter().cloned().cycle().take(len).collect();
+            .ok()
-        Self { nodes: Rc::new(nodes) }
+            .and_then(|n| self.nodes.len().checked_mul(n))
+            .ok_or_else(|| format!("cannot repeat this template {} times", n))?;
+
+        Ok(Self {
+            nodes: Rc::new(self.iter().cloned().cycle().take(count).collect()),
+        })
     }
 }
 
@@ -123,7 +130,7 @@ pub struct TemplateTree {
 /// The raw pointers point into the expressions contained in some
 /// [`SyntaxTree`]. Since the lifetime is erased, the tree could go out of scope
 /// while the hash map still lives. Although this could lead to lookup panics,
-/// it is not unsafe since the pointers are never dereferenced.
+/// it is safe since the pointers are never dereferenced.
 pub type ExprMap = HashMap<*const Expr, Value>;
 
 /// A reference-counted dynamic template node that can implement custom

src/font.rs

@@ -173,7 +173,7 @@ pub struct LineMetrics {
 impl Face {
     /// Parse a font face from a buffer and collection index.
     pub fn new(buffer: Rc<Vec<u8>>, index: u32) -> Option<Self> {
-        // SAFETY:
+        // Safety:
         // - The slices's location is stable in memory:
         //   - We don't move the underlying vector
         //   - Nobody else can move it since we have a strong ref to the `Rc`.

src/library/utility.rs

@@ -23,8 +23,8 @@ pub fn len(_: &mut EvalContext, args: &mut FuncArgs) -> TypResult<Value> {
     let Spanned { v, span } = args.expect("collection")?;
     Ok(match v {
         Value::Str(v) => Value::Int(v.len() as i64),
-        Value::Array(v) => Value::Int(v.len() as i64),
+        Value::Array(v) => Value::Int(v.len()),
-        Value::Dict(v) => Value::Int(v.len() as i64),
+        Value::Dict(v) => Value::Int(v.len()),
         _ => bail!(args.source, span, "expected string, array or dictionary"),
     })
 }

src/parse/scanner.rs

@@ -128,9 +128,10 @@ impl<'s> Scanner<'s> {
     /// The remaining source string after the current index.
     #[inline]
     pub fn rest(&self) -> &'s str {
-        // SAFETY: The index is always in bounds and on a codepoint boundary
+        // Safety: The index is always in bounds and on a codepoint boundary
-        // since it is:
+        // since it starts at zero and is is:
-        // - either increased by the length of a scanned character,
+        // - either increased by the length of a scanned character, advacing
+        //   from one codepoint boundary to the next,
         // - or checked upon jumping.
         unsafe { self.src.get_unchecked(self.index ..) }
     }
@@ -138,7 +139,7 @@ impl<'s> Scanner<'s> {
     /// The full source string up to the current index.
     #[inline]
     pub fn eaten(&self) -> &'s str {
-        // SAFETY: The index is always okay, for details see `rest()`.
+        // Safety: The index is always okay, for details see `rest()`.
         unsafe { self.src.get_unchecked(.. self.index) }
     }
 

src/parse/tokens.rs

@@ -595,9 +595,9 @@ mod tests {
             // Test with each applicable suffix.
             for &(block, mode, suffix, token) in SUFFIXES {
                 let src = $src;
-                #[allow(unused)]
+                #[allow(unused_variables)]
-                let mut blocks = BLOCKS;
+                let blocks = BLOCKS;
-                $(blocks = $blocks;)?
+                $(let blocks = $blocks;)?
                 assert!(!blocks.contains(|c| !BLOCKS.contains(c)));
                 if (mode.is_none() || mode == Some($mode)) && blocks.contains(block) {
                     t!(@$mode: format!("{}{}", src, suffix) => $($token,)* token);

src/pretty.rs

@@ -554,18 +554,6 @@ impl Pretty for FuncArg {
     }
 }
 
-impl Pretty for i64 {
-    fn pretty(&self, p: &mut Printer) {
-        write!(p, "{}", self).unwrap();
-    }
-}
-
-impl Pretty for f64 {
-    fn pretty(&self, p: &mut Printer) {
-        write!(p, "{}", self).unwrap();
-    }
-}
-
 impl Pretty for str {
     fn pretty(&self, p: &mut Printer) {
         p.push('"');
@@ -594,6 +582,8 @@ macro_rules! pretty_display {
 }
 
 pretty_display! {
+    i64,
+    f64,
     bool,
     Length,
     Angle,

src/syntax/visit.rs

@@ -68,7 +68,6 @@ macro_rules! impl_visitor {
         pub mod $module {
             use super::*;
             $(
-                #[allow(unused_variables)]
                 pub fn $name<'ast, V>($v: &mut V, $node: &'ast $($fmut)? $ty)
                 where
                     V: $visit<'ast> + ?Sized

src/util/eco.rs

@@ -2,11 +2,14 @@
 
 use std::borrow::Borrow;
 use std::cmp::Ordering;
+use std::convert::TryFrom;
 use std::fmt::{self, Debug, Display, Formatter, Write};
 use std::hash::{Hash, Hasher};
 use std::ops::{Add, AddAssign, Deref};
 use std::rc::Rc;
 
+use crate::diag::StrResult;
+
 /// An economical string with inline storage and clone-on-write value semantics.
 #[derive(Clone)]
 pub struct EcoString(Repr);
@@ -143,21 +146,25 @@ impl EcoString {
     }
 
     /// Repeat this string `n` times.
-    pub fn repeat(&self, n: usize) -> Self {
+    pub fn repeat(&self, n: i64) -> StrResult<Self> {
+        let (n, new) = usize::try_from(n)
+            .ok()
+            .and_then(|n| self.len().checked_mul(n).map(|new| (n, new)))
+            .ok_or_else(|| format!("cannot repeat this string {} times", n))?;
+
         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 });
+                return Ok(Self(Repr::Small { buf, len: new as u8 }));
             }
         }
 
-        self.as_str().repeat(n).into()
+        Ok(self.as_str().repeat(n).into())
     }
 }
 
@@ -216,6 +223,13 @@ impl Deref for EcoString {
 
     fn deref(&self) -> &str {
         match &self.0 {
+            // Safety:
+            // The buffer contents stem from correct UTF-8 sources:
+            // - Valid ASCII characters
+            // - Other string slices
+            // - Chars that were encoded with char::encode_utf8
+            // Furthermore, we still do the bounds-check on the len in case
+            // it gets corrupted somehow.
             Repr::Small { buf, len } => unsafe {
                 std::str::from_utf8_unchecked(&buf[.. usize::from(*len)])
             },
@@ -424,13 +438,17 @@ mod tests {
     #[test]
     fn test_str_repeat() {
         // Test with empty string.
-        assert_eq!(EcoString::new().repeat(0), "");
+        assert_eq!(EcoString::new().repeat(0).unwrap(), "");
-        assert_eq!(EcoString::new().repeat(100), "");
+        assert_eq!(EcoString::new().repeat(100).unwrap(), "");
 
         // Test non-spilling and spilling case.
         let v = EcoString::from("abc");
-        assert_eq!(v.repeat(0), "");
+        assert_eq!(v.repeat(0).unwrap(), "");
-        assert_eq!(v.repeat(3), "abcabcabc");
+        assert_eq!(v.repeat(3).unwrap(), "abcabcabc");
-        assert_eq!(v.repeat(5), "abcabcabcabcabc");
+        assert_eq!(v.repeat(5).unwrap(), "abcabcabcabcabc");
+        assert_eq!(
+            v.repeat(-1).unwrap_err(),
+            "cannot repeat this string -1 times",
+        );
     }
 }

src/util/mod.rs

@@ -4,6 +4,7 @@ mod eco;
 
 pub use eco::EcoString;
 
+use std::cell::RefMut;
 use std::cmp::Ordering;
 use std::ops::Range;
 use std::path::{Component, Path, PathBuf};
@@ -153,3 +154,23 @@ impl PathExt for Path {
         out
     }
 }
+
+/// Additional methods for [`RefMut`].
+pub trait RefMutExt<'a, T> {
+    fn try_map<U, F, E>(orig: Self, f: F) -> Result<RefMut<'a, U>, E>
+    where
+        F: FnOnce(&mut T) -> Result<&mut U, E>;
+}
+
+impl<'a, T> RefMutExt<'a, T> for RefMut<'a, T> {
+    fn try_map<U, F, E>(mut orig: Self, f: F) -> Result<RefMut<'a, U>, E>
+    where
+        F: FnOnce(&mut T) -> Result<&mut U, E>,
+    {
+        // Taken from here:
+        // https://github.com/rust-lang/rust/issues/27746#issuecomment-172899746
+        f(&mut orig)
+            .map(|new| new as *mut U)
+            .map(|raw| RefMut::map(orig, |_| unsafe { &mut *raw }))
+    }
+}

tests/typ/code/array.typ

@@ -1,6 +1,9 @@
 // Test arrays.
+// Ref: false
 
 ---
+// Ref: true
+
 // Empty.
 {()}
 
@@ -18,6 +21,39 @@
      , rgb("002")
      ,)}
 
+---
+// Test lvalue and rvalue access.
+{
+  let array = (1, 2)
+  array(1) += 5 + array(0)
+  test(array, (1, 8))
+}
+
+---
+// Test rvalue out of bounds.
+{
+  let array = (1, 2, 3)
+  // Error: 3-11 array index out of bounds (index: 5, len: 3)
+  array(5)
+}
+
+---
+// Test lvalue out of bounds.
+{
+  let array = (1, 2, 3)
+  // Error: 3-12 array index out of bounds (index: -1, len: 3)
+  array(-1) = 5
+}
+
+---
+// Test non-collection indexing.
+
+{
+  let x = 10pt
+  // Error: 3-4 expected collection, found length
+  x() = 1
+}
+
 ---
 // Error: 3 expected closing paren
 {(}

tests/typ/code/call.typ

@@ -68,25 +68,25 @@ C
 }
 
 ---
-// Error: 2-6 expected function, found boolean
+// Error: 2-6 expected function or collection, found boolean
 {true()}
 
 ---
 #let x = "x"
 
-// Error: 1-3 expected function, found string
+// Error: 1-3 expected function or collection, found string
 #x()
 
 ---
 #let f(x) = x
 
-// Error: 1-6 expected function, found integer
+// Error: 1-6 expected function or collection, found integer
 #f(1)(2)
 
 ---
 #let f(x) = x
 
-// Error: 1-6 expected function, found template
+// Error: 1-6 expected function or collection, found template
 #f[1](2)
 
 ---

tests/typ/code/dict.typ

@@ -1,12 +1,40 @@
 // Test dictionaries.
+// Ref: false
 
 ---
+// Ref: true
+
 // Empty
 {(:)}
 
 // Two pairs.
 {(a1: 1, a2: 2)}
 
+---
+// Test lvalue and rvalue access.
+{
+  let dict = (a: 1, b: 1)
+  dict("b") += 1
+  dict("c") = 3
+  test(dict, (a: 1, b: 2, c: 3))
+}
+
+---
+// Test rvalue missing key.
+{
+  let dict = (a: 1, b: 2)
+  // Error: 11-20 dictionary does not contain key: "c"
+  let x = dict("c")
+}
+
+---
+// Missing lvalue is automatically none-initialized.
+{
+  let dict = (:)
+  // Error: 3-17 cannot add none and integer
+  dict("b") += 1
+}
+
 ---
 // Simple expression after already being identified as a dictionary.
 // Error: 9-10 expected named pair, found expression

tests/typ/code/ops-invalid.typ

@@ -46,6 +46,20 @@
 // Error: 2-10 cannot divide integer by length
 {3 / 12pt}
 
+---
+// Error: 2-9 cannot repeat this string -1 times
+{-1 * ""}
+
+---
+{
+  let x = 2
+  for _ in 0..60 {
+    x *= 2
+  }
+  // Error: 4-18 cannot repeat this string 4611686018427387904 times
+  {x * "abcdefgh"}
+}
+
 ---
 // Error: 14-22 cannot add integer and string
 { let x = 1; x += "2" }
@@ -63,11 +77,11 @@
 { 1 .. "" }
 
 ---
-// Error: 3-6 cannot assign to this expression
+// Error: 3-6 cannot access this expression mutably
 { (x) = "" }
 
 ---
-// Error: 3-8 cannot assign to this expression
+// Error: 3-8 cannot access this expression mutably
 { 1 + 2 += 3 }
 
 ---
@@ -75,7 +89,7 @@
 { z = 1 }
 
 ---
-// Error: 3-7 cannot assign to a constant
+// Error: 3-7 cannot mutate a constant
 { rect = "hi" }
 
 ---

tests/typ/code/ops-prec.typ

@@ -13,7 +13,7 @@
 #test(not "b" == "b", false)
 
 // Assignment binds stronger than boolean operations.
-// Error: 2-7 cannot assign to this expression
+// Error: 2-7 cannot access this expression mutably
 {not x = "a"}
 
 ---

tests/typeset.rs

@@ -70,13 +70,11 @@ fn main() {
         let lhs = args.expect::<Value>("left-hand side")?;
         let rhs = args.expect::<Value>("right-hand side")?;
         if lhs != rhs {
-            typst::bail!(
+            return Err(Error::boxed(
                 args.source,
                 args.span,
-                "Assertion failed: {:?} != {:?}",
+                format!("Assertion failed: {:?} != {:?}", lhs, rhs),
-                lhs,
+            ));
-                rhs
-            );
         }
         Ok(Value::None)
     });