diff --git a/cli/src/main.rs b/cli/src/main.rs
index 8030a82d6..f7fd5903f 100644
--- a/cli/src/main.rs
+++ b/cli/src/main.rs
@@ -235,10 +235,12 @@ fn typeset(command: TypesetCommand) -> StrResult<()> {
 /// Typeset a single time.
 fn typeset_once(world: &mut SystemWorld, command: &TypesetCommand) -> StrResult<()> {
     status(command, Status::Compiling).unwrap();
-
     world.reset();
+
     let main = world.resolve(&command.input).map_err(|err| err.to_string())?;
-    match typst::typeset(world, main) {
+    let source = world.source(main);
+
+    match typst::typeset(world, source) {
         // Export the PDF.
         Ok(frames) => {
             let buffer = typst::export::pdf(&frames);
diff --git a/library/src/base/mod.rs b/library/src/base/mod.rs
index f1fdd2b90..837a2f0e8 100644
--- a/library/src/base/mod.rs
+++ b/library/src/base/mod.rs
@@ -11,7 +11,7 @@ pub use self::data::*;
 pub use self::string::*;
 
 use comemo::Track;
-use typst::model::{Eval, Route, Scopes, Vm};
+use typst::model::{self, Route, Vm};
 use typst::syntax::Source;
 
 use crate::prelude::*;
@@ -33,22 +33,8 @@ pub fn assert(_: &mut Vm, args: &mut Args) -> SourceResult<Value> {
 /// Evaluate a string as Typst markup.
 pub fn eval(vm: &mut Vm, args: &mut Args) -> SourceResult<Value> {
     let Spanned { v: text, span } = args.expect::<Spanned<String>>("source")?;
-
-    // Parse the source and set a synthetic span for all nodes.
     let source = Source::synthesized(text, span);
-    let ast = source.ast()?;
-
-    // Evaluate the source.
-    let std = &vm.world.config().scope;
-    let scopes = Scopes::new(Some(std));
     let route = Route::default();
-    let mut sub = Vm::new(vm.world, route.track(), None, scopes);
-    let result = ast.eval(&mut sub);
-
-    // Handle control flow.
-    if let Some(flow) = sub.flow {
-        bail!(flow.forbidden());
-    }
-
-    Ok(Value::Content(result?))
+    let module = model::eval(vm.world, route.track(), &source)?;
+    Ok(Value::Content(module.content))
 }
diff --git a/library/src/lib.rs b/library/src/lib.rs
index 7a292c6dd..e5e60968b 100644
--- a/library/src/lib.rs
+++ b/library/src/lib.rs
@@ -154,7 +154,7 @@ pub fn styles() -> StyleMap {
 /// Construct the standard lang item mapping.
 pub fn items() -> LangItems {
     LangItems {
-        root: |content, world, styles| content.layout_root(world, styles),
+        layout: |content, world, styles| content.layout_root(world, styles),
         em: |styles| styles.get(text::TextNode::SIZE),
         dir: |styles| styles.get(text::TextNode::DIR),
         space: || text::SpaceNode.pack(),
diff --git a/src/diag.rs b/src/diag.rs
index 613789f8b..99ef12914 100644
--- a/src/diag.rs
+++ b/src/diag.rs
@@ -49,6 +49,9 @@ pub use crate::__error as error;
 pub type SourceResult<T> = Result<T, Box<Vec<SourceError>>>;
 
 /// An error in a source file.
+///
+/// This contained spans will only be detached if any of the input source files
+/// were detached.
 #[derive(Debug, Clone, Eq, PartialEq, Hash)]
 pub struct SourceError {
     /// The span of the erroneous node in the source code.
diff --git a/src/lib.rs b/src/lib.rs
index bb3a1c027..319c13aba 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -60,14 +60,19 @@ use crate::util::Buffer;
 /// information.
 pub fn typeset(
     world: &(dyn World + 'static),
-    main: SourceId,
+    source: &Source,
 ) -> SourceResult<Vec<Frame>> {
+    // Set up the language items.
     let config = world.config();
     crate::model::set_lang_items(config.items);
+
+    // Evaluate the source file into a module.
     let route = Route::default();
-    let module = model::eval(world.track(), route.track(), main)?;
+    let module = model::eval(world.track(), route.track(), source)?;
+
+    // Layout the module's contents.
     let styles = StyleChain::with_root(&config.styles);
-    item!(root)(&module.content, world.track(), styles)
+    item!(layout)(&module.content, world.track(), styles)
 }
 
 /// The environment in which typesetting occurs.
diff --git a/src/model/eval.rs b/src/model/eval.rs
index 7c0cffb59..7f2cea639 100644
--- a/src/model/eval.rs
+++ b/src/model/eval.rs
@@ -13,7 +13,7 @@ use super::{
 use crate::diag::{bail, error, At, SourceResult, StrResult, Trace, Tracepoint};
 use crate::geom::{Abs, Angle, Em, Fr, Ratio};
 use crate::syntax::ast::AstNode;
-use crate::syntax::{ast, SourceId, Span, Spanned, Unit};
+use crate::syntax::{ast, Source, SourceId, Span, Spanned, Unit};
 use crate::util::{format_eco, EcoString};
 use crate::World;
 
@@ -26,9 +26,10 @@ use crate::World;
 pub fn eval(
     world: Tracked<dyn World>,
     route: Tracked<Route>,
-    id: SourceId,
+    source: &Source,
 ) -> SourceResult<Module> {
     // Prevent cyclic evaluation.
+    let id = source.id();
     if route.contains(id) {
         let path = world.source(id).path().display();
         panic!("Tried to cyclicly evaluate {}", path);
@@ -36,11 +37,10 @@ pub fn eval(
 
     // Evaluate the module.
     let route = unsafe { Route::insert(route, id) };
-    let ast = world.source(id).ast()?;
     let std = &world.config().scope;
     let scopes = Scopes::new(Some(std));
-    let mut vm = Vm::new(world, route.track(), Some(id), scopes);
-    let result = ast.eval(&mut vm);
+    let mut vm = Vm::new(world, route.track(), id, scopes);
+    let result = source.ast()?.eval(&mut vm);
 
     // Handle control flow.
     if let Some(flow) = vm.flow {
@@ -59,9 +59,9 @@ pub struct Route {
 }
 
 impl Route {
-    /// Create a new, empty route.
-    pub fn new(id: Option<SourceId>) -> Self {
-        Self { id, parent: None }
+    /// Create a new route with just one entry.
+    pub fn new(id: SourceId) -> Self {
+        Self { id: Some(id), parent: None }
     }
 
     /// Insert a new id into the route.
@@ -94,7 +94,7 @@ pub struct Module {
 }
 
 /// Evaluate an expression.
-pub trait Eval {
+pub(super) trait Eval {
     /// The output of evaluating the expression.
     type Output;
 
@@ -1038,10 +1038,9 @@ fn import(vm: &mut Vm, path: &str, span: Span) -> SourceResult<Module> {
     }
 
     // Evaluate the file.
-    let module =
-        eval(vm.world, vm.route, id).trace(vm.world, || Tracepoint::Import, span)?;
-
-    Ok(module)
+    let source = vm.world.source(id);
+    let point = || Tracepoint::Import;
+    eval(vm.world, vm.route, source).trace(vm.world, point, span)
 }
 
 impl Eval for ast::LoopBreak {
diff --git a/src/model/func.rs b/src/model/func.rs
index f313dcdaf..d84a81708 100644
--- a/src/model/func.rs
+++ b/src/model/func.rs
@@ -97,7 +97,9 @@ impl Func {
         args: Args,
     ) -> SourceResult<Value> {
         let route = Route::default();
-        let mut vm = Vm::new(world, route.track(), None, Scopes::new(None));
+        let id = SourceId::detached();
+        let scopes = Scopes::new(None);
+        let mut vm = Vm::new(world, route.track(), id, scopes);
         self.call(&mut vm, args)
     }
 
@@ -178,7 +180,7 @@ impl Hash for Native {
 #[derive(Hash)]
 pub struct Closure {
     /// The source file where the closure was defined.
-    pub location: Option<SourceId>,
+    pub location: SourceId,
     /// The name of the closure.
     pub name: Option<EcoString>,
     /// Captured values from outer scopes.
@@ -219,7 +221,7 @@ impl Closure {
         }
 
         // Determine the route inside the closure.
-        let detached = vm.location.is_none();
+        let detached = vm.location.is_detached();
         let fresh = Route::new(self.location);
         let route = if detached { fresh.track() } else { vm.route };
 
diff --git a/src/model/items.rs b/src/model/items.rs
index 771756fe8..50ea8b602 100644
--- a/src/model/items.rs
+++ b/src/model/items.rs
@@ -42,7 +42,7 @@ macro_rules! item {
 #[derive(Copy, Clone)]
 pub struct LangItems {
     /// The root layout function.
-    pub root: fn(
+    pub layout: fn(
         content: &Content,
         world: Tracked<dyn World>,
         styles: StyleChain,
@@ -104,7 +104,7 @@ impl Debug for LangItems {
 
 impl Hash for LangItems {
     fn hash<H: Hasher>(&self, state: &mut H) {
-        (self.root as usize).hash(state);
+        (self.layout as usize).hash(state);
         (self.em as usize).hash(state);
         (self.dir as usize).hash(state);
         self.space.hash(state);
diff --git a/src/model/vm.rs b/src/model/vm.rs
index db0bf77cb..28885f294 100644
--- a/src/model/vm.rs
+++ b/src/model/vm.rs
@@ -15,7 +15,7 @@ pub struct Vm<'a> {
     /// The route of source ids the VM took to reach its current location.
     pub route: Tracked<'a, Route>,
     /// The current location.
-    pub location: Option<SourceId>,
+    pub location: SourceId,
     /// The stack of scopes.
     pub scopes: Scopes<'a>,
     /// A control flow event that is currently happening.
@@ -29,7 +29,7 @@ impl<'a> Vm<'a> {
     pub fn new(
         world: Tracked<'a, dyn World>,
         route: Tracked<'a, Route>,
-        location: Option<SourceId>,
+        location: SourceId,
         scopes: Scopes<'a>,
     ) -> Self {
         Self {
@@ -45,12 +45,12 @@ impl<'a> Vm<'a> {
     /// Resolve a user-entered path to be relative to the compilation
     /// environment's root.
     pub fn locate(&self, path: &str) -> StrResult<PathBuf> {
-        if let Some(id) = self.location {
+        if !self.location.is_detached() {
             if let Some(path) = path.strip_prefix('/') {
                 return Ok(self.world.config().root.join(path).normalize());
             }
 
-            if let Some(dir) = self.world.source(id).path().parent() {
+            if let Some(dir) = self.world.source(self.location).path().parent() {
                 return Ok(dir.join(path).normalize());
             }
         }
diff --git a/src/syntax/source.rs b/src/syntax/source.rs
index 08b526859..6144d5241 100644
--- a/src/syntax/source.rs
+++ b/src/syntax/source.rs
@@ -21,26 +21,21 @@ use crate::util::{PathExt, StrExt};
 pub struct Source {
     id: SourceId,
     path: PathBuf,
-    text: Prehashed<String>,
     lines: Vec<Line>,
+    text: Prehashed<String>,
     root: Prehashed<SyntaxNode>,
 }
 
 impl Source {
     /// Create a new source file.
     pub fn new(id: SourceId, path: &Path, text: String) -> Self {
-        let lines = std::iter::once(Line { byte_idx: 0, utf16_idx: 0 })
-            .chain(lines(0, 0, &text))
-            .collect();
-
         let mut root = parse(&text);
         root.numberize(id, Span::FULL).unwrap();
-
         Self {
             id,
             path: path.normalize(),
+            lines: lines(&text),
             text: Prehashed::new(text),
-            lines,
             root: Prehashed::new(root),
         }
     }
@@ -51,13 +46,16 @@ impl Source {
     }
 
     /// Create a source file with the same synthetic span for all nodes.
-    pub fn synthesized(text: impl Into<String>, span: Span) -> Self {
-        let mut file = Self::detached(text);
-        let mut root = file.root.into_inner();
+    pub fn synthesized(text: String, span: Span) -> Self {
+        let mut root = parse(&text);
         root.synthesize(span);
-        file.root = Prehashed::new(root);
-        file.id = span.source();
-        file
+        Self {
+            id: SourceId::detached(),
+            path: PathBuf::new(),
+            lines: lines(&text),
+            text: Prehashed::new(text),
+            root: Prehashed::new(root),
+        }
     }
 
     /// The root node of the file's untyped syntax tree.
@@ -98,10 +96,9 @@ impl Source {
     /// Fully replace the source text.
     pub fn replace(&mut self, text: String) {
         self.text = Prehashed::new(text);
-        self.lines = vec![Line { byte_idx: 0, utf16_idx: 0 }];
-        self.lines.extend(lines(0, 0, &self.text));
+        self.lines = lines(&self.text);
         let mut root = parse(&self.text);
-        root.numberize(self.id(), Span::FULL).unwrap();
+        root.numberize(self.id, Span::FULL).unwrap();
         self.root = Prehashed::new(root);
     }
 
@@ -128,7 +125,7 @@ impl Source {
 
         // Recalculate the line starts after the edit.
         self.lines
-            .extend(lines(start_byte, start_utf16, &self.text[start_byte..]));
+            .extend(lines_from(start_byte, start_utf16, &self.text[start_byte..]));
 
         // Incrementally reparse the replaced range.
         let mut root = std::mem::take(&mut self.root).into_inner();
@@ -262,11 +259,16 @@ impl Hash for Source {
 pub struct SourceId(u16);
 
 impl SourceId {
-    /// Create a new source id for a file that is not part of a store.
+    /// Create a new source id for a file that is not part of the world.
     pub const fn detached() -> Self {
         Self(u16::MAX)
     }
 
+    /// Whether the source id is the detached.
+    pub const fn is_detached(self) -> bool {
+        self.0 == Self::detached().0
+    }
+
     /// Create a source id from a number.
     pub const fn from_u16(v: u16) -> Self {
         Self(v)
@@ -287,8 +289,15 @@ struct Line {
     utf16_idx: usize,
 }
 
-/// Iterate over the lines in the string.
-fn lines(
+/// Create a line vector.
+fn lines(text: &str) -> Vec<Line> {
+    std::iter::once(Line { byte_idx: 0, utf16_idx: 0 })
+        .chain(lines_from(0, 0, &text))
+        .collect()
+}
+
+/// Compute a line iterator from an offset.
+fn lines_from(
     byte_offset: usize,
     utf16_offset: usize,
     text: &str,
diff --git a/src/syntax/span.rs b/src/syntax/span.rs
index 7fbb73052..22ada3593 100644
--- a/src/syntax/span.rs
+++ b/src/syntax/span.rs
@@ -27,15 +27,13 @@ use super::SourceId;
 pub struct Span(NonZeroU64);
 
 impl Span {
-    // Data layout:
-    // | 16 bits source id | 48 bits number |
-
-    // Number of bits for and minimum and maximum numbers assignable to spans.
-    const BITS: usize = 48;
-    const DETACHED: u64 = 1;
-
     /// The full range of numbers available for span numbering.
     pub const FULL: Range<u64> = 2..(1 << Self::BITS);
+    const DETACHED: u64 = 1;
+
+    // Data layout:
+    // | 16 bits source id | 48 bits number |
+    const BITS: usize = 48;
 
     /// Create a new span from a source id and a unique number.
     ///
@@ -46,13 +44,26 @@ impl Span {
             "span number outside valid range"
         );
 
-        let bits = ((id.into_u16() as u64) << Self::BITS) | number;
-        Self(to_non_zero(bits))
+        Self::pack(id, number)
     }
 
     /// A span that does not point into any source file.
     pub const fn detached() -> Self {
-        Self(to_non_zero(Self::DETACHED))
+        Self::pack(SourceId::detached(), Self::DETACHED)
+    }
+
+    /// Pack the components into a span.
+    const fn pack(id: SourceId, number: u64) -> Span {
+        let bits = ((id.into_u16() as u64) << Self::BITS) | number;
+        match NonZeroU64::new(bits) {
+            Some(v) => Self(v),
+            None => panic!("span encoding is zero"),
+        }
+    }
+
+    /// Whether the span is detached.
+    pub const fn is_detached(self) -> bool {
+        self.source().is_detached()
     }
 
     /// The id of the source file the span points into.
@@ -66,14 +77,6 @@ impl Span {
     }
 }
 
-/// Convert to a non zero u64.
-const fn to_non_zero(v: u64) -> NonZeroU64 {
-    match NonZeroU64::new(v) {
-        Some(v) => v,
-        None => panic!("span encoding is zero"),
-    }
-}
-
 /// A value with a span locating it in the source code.
 #[derive(Copy, Clone, Eq, PartialEq, Hash)]
 pub struct Spanned<T> {
diff --git a/tests/src/benches.rs b/tests/src/benches.rs
index 5c9b54336..29117a909 100644
--- a/tests/src/benches.rs
+++ b/tests/src/benches.rs
@@ -74,21 +74,18 @@ fn bench_highlight(iai: &mut Iai) {
 
 fn bench_eval(iai: &mut Iai) {
     let world = BenchWorld::new();
-    let id = world.source.id();
     let route = typst::model::Route::default();
-    iai.run(|| typst::model::eval(world.track(), route.track(), id).unwrap());
+    iai.run(|| typst::model::eval(world.track(), route.track(), &world.source).unwrap());
 }
 
 fn bench_typeset(iai: &mut Iai) {
     let world = BenchWorld::new();
-    let id = world.source.id();
-    iai.run(|| typst::typeset(&world, id));
+    iai.run(|| typst::typeset(&world, &world.source));
 }
 
 fn bench_render(iai: &mut Iai) {
     let world = BenchWorld::new();
-    let id = world.source.id();
-    let frames = typst::typeset(&world, id).unwrap();
+    let frames = typst::typeset(&world, &world.source).unwrap();
     iai.run(|| typst::export::render(&frames[0], 1.0))
 }
 
@@ -110,8 +107,7 @@ impl BenchWorld {
 
         let font = Font::new(FONT.into(), 0).unwrap();
         let book = FontBook::from_fonts([&font]);
-        let id = SourceId::from_u16(0);
-        let source = Source::new(id, Path::new("bench.typ"), TEXT.into());
+        let source = Source::detached(TEXT);
 
         Self {
             config: Prehashed::new(config),
@@ -148,6 +144,6 @@ impl World for BenchWorld {
     }
 
     fn source(&self, _: SourceId) -> &Source {
-        &self.source
+        unimplemented!()
     }
 }
diff --git a/tests/src/tests.rs b/tests/src/tests.rs
index 52cc54db1..89c83857e 100644
--- a/tests/src/tests.rs
+++ b/tests/src/tests.rs
@@ -418,13 +418,13 @@ fn test_part(
     ok &= test_reparse(world.source(id).text(), i, rng);
 
     if world.print.model {
-        let tracked = (world as &dyn World).track();
+        let world = (world as &dyn World).track();
         let route = typst::model::Route::default();
-        let module = typst::model::eval(tracked, route.track(), id).unwrap();
+        let module = typst::model::eval(world, route.track(), source).unwrap();
         println!("Model:\n{:#?}\n", module.content);
     }
 
-    let (mut frames, errors) = match typst::typeset(world, id) {
+    let (mut frames, errors) = match typst::typeset(world, source) {
         Ok(frames) => (frames, vec![]),
         Err(errors) => (vec![], *errors),
     };