fix!: Fix panic in model resolver when variable is used outside of symbol. (#2362)

Closes #2312.

BREAKING CHANGE: `UnknownVarError` in `hugr-model` is changed into an
enum with two cases.

Author: zrho

hugr-model/src/v0/ast/resolve.rs

@@ -388,3 +388,18 @@ fn try_alloc_slice<T, E>(
     }
     Ok(vec.into_bump_slice())
 }
+
+#[cfg(test)]
+mod test {
+    use crate::v0::ast;
+    use bumpalo::Bump;
+    use std::str::FromStr as _;
+
+    #[test]
+    fn vars_in_root_scope() {
+        let text = "(hugr 0) (mod) (meta ?x)";
+        let ast = ast::Package::from_str(text).unwrap();
+        let bump = Bump::new();
+        assert!(ast.resolve(&bump).is_err());
+    }
+}

hugr-model/src/v0/scope/vars.rs

@@ -78,28 +78,23 @@ impl<'a> VarTable<'a> {
     /// # Errors
     ///
     /// Returns an error if the variable is not defined in the current scope.
-    ///
-    /// # Panics
-    ///
-    /// Panics if there are no open scopes.
     pub fn resolve(&self, name: &'a str) -> Result<VarId, UnknownVarError<'a>> {
-        let scope = self.scopes.last().unwrap();
+        let scope = self.scopes.last().ok_or(UnknownVarError::Root(name))?;
         let set_index = self
             .vars
             .get_index_of(&(scope.node, name))
-            .ok_or(UnknownVarError(scope.node, name))?;
+            .ok_or(UnknownVarError::WithinNode(scope.node, name))?;
         let var_index = (set_index - scope.var_stack) as u16;
         Ok(VarId(scope.node, var_index))
     }
 
     /// Check if a variable is visible in the current scope.
-    ///
-    /// # Panics
-    ///
-    /// Panics if there are no open scopes.
     #[must_use]
     pub fn is_visible(&self, var: VarId) -> bool {
-        let scope = self.scopes.last().unwrap();
+        let Some(scope) = self.scopes.last() else {
+            return false;
+        };
+
         scope.node == var.0 && var.1 < scope.var_count
     }
 
@@ -149,5 +144,11 @@ pub struct DuplicateVarError<'a>(NodeId, &'a str);
 
 /// Error that occurs when a variable is not defined in the current scope.
 #[derive(Debug, Clone, Error)]
-#[error("can not resolve variable `{1}` in node {0}")]
-pub struct UnknownVarError<'a>(NodeId, &'a str);
+pub enum UnknownVarError<'a> {
+    /// Failed to resolve a variable when in scope of a node.
+    #[error("can not resolve variable `{1}` in node {0}")]
+    WithinNode(NodeId, &'a str),
+    /// Failed to resolve a variable when in the root scope.
+    #[error("can not resolve variable `{0}` in the root scope")]
+    Root(&'a str),
+}