use snapshots for inner scopes

compiler/can/src/def.rs

@@ -208,10 +208,10 @@ pub(crate) fn canonicalize_defs<'a>(
     env: &mut Env<'a>,
     mut output: Output,
     var_store: &mut VarStore,
-    mut scope: Scope,
+    scope: &mut Scope,
     loc_defs: &'a [&'a Loc<ast::Def<'a>>],
     pattern_type: PatternType,
-) -> (CanDefs, Scope, Output, MutMap<Symbol, Region>) {
+) -> (CanDefs, Output, MutMap<Symbol, Region>) {
     // Canonicalizing defs while detecting shadowing involves a multi-step process:
     //
     // 1. Go through each of the patterns.
@@ -236,7 +236,7 @@ pub(crate) fn canonicalize_defs<'a>(
     for loc_def in loc_defs {
         match loc_def.value.unroll_def() {
             Ok(type_def) => {
-                pending_type_defs.push(to_pending_type_def(env, type_def, &mut scope, pattern_type))
+                pending_type_defs.push(to_pending_type_def(env, type_def, scope, pattern_type))
             }
             Err(value_def) => value_defs.push(Loc::at(loc_def.region, value_def)),
         }
@@ -322,7 +322,7 @@ pub(crate) fn canonicalize_defs<'a>(
                 let symbol = name.value;
                 let can_ann = canonicalize_annotation(
                     env,
-                    &mut scope,
+                    scope,
                     &ann.value,
                     ann.region,
                     var_store,
@@ -446,7 +446,7 @@ pub(crate) fn canonicalize_defs<'a>(
         env,
         &mut output,
         var_store,
-        &mut scope,
+        scope,
         abilities,
         &abilities_in_scope,
         pattern_type,
@@ -465,7 +465,7 @@ pub(crate) fn canonicalize_defs<'a>(
             env,
             var_store,
             loc_def.value,
-            &mut scope,
+            scope,
             &mut new_output,
             pattern_type,
         ) {
@@ -512,7 +512,7 @@ pub(crate) fn canonicalize_defs<'a>(
             env,
             pending_def,
             output,
-            &mut scope,
+            scope,
             var_store,
             &mut aliases,
             &abilities_in_scope,
@@ -525,13 +525,6 @@ pub(crate) fn canonicalize_defs<'a>(
         def_ordering.insert_symbol_references(def_id as u32, &temp_output.references)
     }
 
-    // This returns both the defs info as well as the new scope.
-    //
-    // We have to return the new scope because we added defs to it
-    // (and those lookups shouldn't fail later, e.g. when canonicalizing
-    // the return expr), but we didn't want to mutate the original scope
-    // directly because we wanted to keep a clone of it around to diff
-    // when looking for unused idents.
     (
         CanDefs {
             defs,
@@ -539,7 +532,6 @@ pub(crate) fn canonicalize_defs<'a>(
             // The result needs a thread-safe `SendMap`
             aliases,
         },
-        scope,
         output,
         symbols_introduced,
     )
@@ -1278,11 +1270,11 @@ fn canonicalize_pending_body<'a>(
 pub fn can_defs_with_return<'a>(
     env: &mut Env<'a>,
     var_store: &mut VarStore,
-    scope: Scope,
+    scope: &mut Scope,
     loc_defs: &'a [&'a Loc<ast::Def<'a>>],
     loc_ret: &'a Loc<ast::Expr<'a>>,
 ) -> (Expr, Output) {
-    let (unsorted, mut scope, defs_output, symbols_introduced) = canonicalize_defs(
+    let (unsorted, defs_output, symbols_introduced) = canonicalize_defs(
         env,
         Output::default(),
         var_store,
@@ -1294,7 +1286,7 @@ pub fn can_defs_with_return<'a>(
     // The def as a whole is a tail call iff its return expression is a tail call.
     // Use its output as a starting point because its tail_call already has the right answer!
     let (ret_expr, mut output) =
-        canonicalize_expr(env, var_store, &mut scope, loc_ret.region, &loc_ret.value);
+        canonicalize_expr(env, var_store, scope, loc_ret.region, &loc_ret.value);
 
     output
         .introduced_variables

compiler/can/src/expr.rs

@@ -654,15 +654,10 @@ pub fn canonicalize_expr<'a>(
             (RuntimeError(problem), Output::default())
         }
         ast::Expr::Defs(loc_defs, loc_ret) => {
-            can_defs_with_return(
-                env,
-                var_store,
             // The body expression gets a new scope for canonicalization,
-                // so clone it.
+            scope.inner_scope(|inner_scope| {
-                scope.clone(),
+                can_defs_with_return(env, var_store, inner_scope, loc_defs, loc_ret)
-                loc_defs,
+            })
-                loc_ret,
-            )
         }
         ast::Expr::Backpassing(_, _, _) => {
             unreachable!("Backpassing should have been desugared by now")
@@ -685,14 +680,16 @@ pub fn canonicalize_expr<'a>(
             let mut can_branches = Vec::with_capacity(branches.len());
 
             for branch in branches.iter() {
-                let (can_when_branch, branch_references) = canonicalize_when_branch(
+                let (can_when_branch, branch_references) = scope.inner_scope(|inner_scope| {
+                    canonicalize_when_branch(
                         env,
                         var_store,
-                    scope.clone(),
+                        inner_scope,
                         region,
                         *branch,
                         &mut output,
-                );
+                    )
+                });
 
                 output.references.union_mut(&branch_references);
 
@@ -956,17 +953,31 @@ pub fn canonicalize_closure<'a>(
     loc_arg_patterns: &'a [Loc<ast::Pattern<'a>>],
     loc_body_expr: &'a Loc<ast::Expr<'a>>,
     opt_def_name: Option<Symbol>,
+) -> (ClosureData, Output) {
+    scope.inner_scope(|inner_scope| {
+        canonicalize_closure_inner_scope(
+            env,
+            var_store,
+            inner_scope,
+            loc_arg_patterns,
+            loc_body_expr,
+            opt_def_name,
+        )
+    })
+}
+
+fn canonicalize_closure_inner_scope<'a>(
+    env: &mut Env<'a>,
+    var_store: &mut VarStore,
+    scope: &mut Scope,
+    loc_arg_patterns: &'a [Loc<ast::Pattern<'a>>],
+    loc_body_expr: &'a Loc<ast::Expr<'a>>,
+    opt_def_name: Option<Symbol>,
 ) -> (ClosureData, Output) {
     // The globally unique symbol that will refer to this closure once it gets converted
     // into a top-level procedure for code gen.
     let symbol = opt_def_name.unwrap_or_else(|| env.gen_unique_symbol());
 
-    // The body expression gets a new scope for canonicalization.
-    // Shadow `scope` to make sure we don't accidentally use the original one for the
-    // rest of this block, but keep the original around for later diffing.
-    let original_scope = scope;
-    let mut scope = original_scope.clone();
-
     let mut can_args = Vec::with_capacity(loc_arg_patterns.len());
     let mut output = Output::default();
 
@@ -974,7 +985,7 @@ pub fn canonicalize_closure<'a>(
         let can_argument_pattern = canonicalize_pattern(
             env,
             var_store,
-            &mut scope,
+            scope,
             &mut output,
             FunctionArg,
             &loc_pattern.value,
@@ -990,7 +1001,7 @@ pub fn canonicalize_closure<'a>(
     let (loc_body_expr, new_output) = canonicalize_expr(
         env,
         var_store,
-        &mut scope,
+        scope,
         loc_body_expr.region,
         &loc_body_expr.value,
     );
@@ -1062,7 +1073,7 @@ pub fn canonicalize_closure<'a>(
 fn canonicalize_when_branch<'a>(
     env: &mut Env<'a>,
     var_store: &mut VarStore,
-    mut scope: Scope,
+    scope: &mut Scope,
     _region: Region,
     branch: &'a ast::WhenBranch<'a>,
     output: &mut Output,
@@ -1074,7 +1085,7 @@ fn canonicalize_when_branch<'a>(
         let can_pattern = canonicalize_pattern(
             env,
             var_store,
-            &mut scope,
+            scope,
             output,
             WhenBranch,
             &loc_pattern.value,
@@ -1087,7 +1098,7 @@ fn canonicalize_when_branch<'a>(
     let (value, mut branch_output) = canonicalize_expr(
         env,
         var_store,
-        &mut scope,
+        scope,
         branch.value.region,
         &branch.value.value,
     );
@@ -1096,7 +1107,7 @@ fn canonicalize_when_branch<'a>(
         None => None,
         Some(loc_expr) => {
             let (can_guard, guard_branch_output) =
-                canonicalize_expr(env, var_store, &mut scope, loc_expr.region, &loc_expr.value);
+                canonicalize_expr(env, var_store, scope, loc_expr.region, &loc_expr.value);
 
             branch_output.union(guard_branch_output);
             Some(can_guard)

compiler/can/src/module.rs

@@ -285,11 +285,11 @@ pub fn canonicalize_module_defs<'a>(
         }
     }
 
-    let (defs, mut scope, output, symbols_introduced) = canonicalize_defs(
+    let (defs, output, symbols_introduced) = canonicalize_defs(
         &mut env,
         Output::default(),
         var_store,
-        scope,
+        &mut scope,
         &desugared,
         PatternType::TopLevelDef,
     );

compiler/can/src/scope.rs

@@ -355,7 +355,7 @@ impl Scope {
         result
     }
 
-    pub fn snapshot(&self) -> ScopeSnapshot {
+    fn snapshot(&self) -> ScopeSnapshot {
         ScopeSnapshot {
             idents: self.idents.clone(),
             aliases: self.aliases.clone(),
@@ -364,7 +364,7 @@ impl Scope {
         }
     }
 
-    pub fn rollback(snapshot: ScopeSnapshot) -> Scope {
+    fn rollback(snapshot: ScopeSnapshot) -> Scope {
         Scope {
             idents: snapshot.idents,
             aliases: snapshot.aliases,