1 files changed, 101 insertions, 0 deletions
diff --git a/crates/typst-eval/src/access.rs b/crates/typst-eval/src/access.rs
new file mode 100644
index 00000000..9bcac4d6
--- /dev/null
+++ b/crates/typst-eval/src/access.rs
@@ -0,0 +1,101 @@
+use ecow::eco_format;
+use typst_library::diag::{bail, At, Hint, SourceResult, Trace, Tracepoint};
+use typst_library::foundations::{Dict, Value};
+use typst_syntax::ast::{self, AstNode};
+
+use crate::{call_method_access, is_accessor_method, Eval, Vm};
+
+/// Access an expression mutably.
+pub(crate) trait Access {
+    /// Access the expression's evaluated value mutably.
+    fn access<'a>(self, vm: &'a mut Vm) -> SourceResult<&'a mut Value>;
+}
+
+impl Access for ast::Expr<'_> {
+    fn access<'a>(self, vm: &'a mut Vm) -> SourceResult<&'a mut Value> {
+        match self {
+            Self::Ident(v) => v.access(vm),
+            Self::Parenthesized(v) => v.access(vm),
+            Self::FieldAccess(v) => v.access(vm),
+            Self::FuncCall(v) => v.access(vm),
+            _ => {
+                let _ = self.eval(vm)?;
+                bail!(self.span(), "cannot mutate a temporary value");
+            }
+        }
+    }
+}
+
+impl Access for ast::Ident<'_> {
+    fn access<'a>(self, vm: &'a mut Vm) -> SourceResult<&'a mut Value> {
+        let span = self.span();
+        if vm.inspected == Some(span) {
+            if let Ok(value) = vm.scopes.get(&self).cloned() {
+                vm.trace(value);
+            }
+        }
+        let value = vm.scopes.get_mut(&self).at(span)?;
+        Ok(value)
+    }
+}
+
+impl Access for ast::Parenthesized<'_> {
+    fn access<'a>(self, vm: &'a mut Vm) -> SourceResult<&'a mut Value> {
+        self.expr().access(vm)
+    }
+}
+
+impl Access for ast::FieldAccess<'_> {
+    fn access<'a>(self, vm: &'a mut Vm) -> SourceResult<&'a mut Value> {
+        access_dict(vm, self)?.at_mut(self.field().get()).at(self.span())
+    }
+}
+
+impl Access for ast::FuncCall<'_> {
+    fn access<'a>(self, vm: &'a mut Vm) -> SourceResult<&'a mut Value> {
+        if let ast::Expr::FieldAccess(access) = self.callee() {
+            let method = access.field();
+            if is_accessor_method(&method) {
+                let span = self.span();
+                let world = vm.world();
+                let args = self.args().eval(vm)?.spanned(span);
+                let value = access.target().access(vm)?;
+                let result = call_method_access(value, &method, args, span);
+                let point = || Tracepoint::Call(Some(method.get().clone()));
+                return result.trace(world, point, span);
+            }
+        }
+
+        let _ = self.eval(vm)?;
+        bail!(self.span(), "cannot mutate a temporary value");
+    }
+}
+
+pub(crate) fn access_dict<'a>(
+    vm: &'a mut Vm,
+    access: ast::FieldAccess,
+) -> SourceResult<&'a mut Dict> {
+    match access.target().access(vm)? {
+        Value::Dict(dict) => Ok(dict),
+        value => {
+            let ty = value.ty();
+            let span = access.target().span();
+            if matches!(
+                value, // those types have their own field getters
+                Value::Symbol(_) | Value::Content(_) | Value::Module(_) | Value::Func(_)
+            ) {
+                bail!(span, "cannot mutate fields on {ty}");
+            } else if typst_library::foundations::fields_on(ty).is_empty() {
+                bail!(span, "{ty} does not have accessible fields");
+            } else {
+                // type supports static fields, which don't yet have
+                // setters
+                Err(eco_format!("fields on {ty} are not yet mutable"))
+                    .hint(eco_format!(
+                        "try creating a new {ty} with the updated field value instead"
+                    ))
+                    .at(span)
+            }
+        }
+    }
+}