Replace Vec by Option.

Author: cjgillot

compiler/rustc_ast_lowering/src/expr.rs

@@ -1186,9 +1186,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 }
             }
             None => self
-                .loop_scopes
-                .last()
-                .cloned()
+                .loop_scope
                 .map(|id| Ok(self.lower_node_id(id)))
                 .unwrap_or(Err(hir::LoopIdError::OutsideLoopScope)),
         };
@@ -1208,18 +1206,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     fn with_catch_scope<T>(&mut self, catch_id: NodeId, f: impl FnOnce(&mut Self) -> T) -> T {
-        let len = self.catch_scopes.len();
-        self.catch_scopes.push(catch_id);
-
+        let old_scope = self.catch_scope.replace(catch_id);
         let result = f(self);
-        assert_eq!(
-            len + 1,
-            self.catch_scopes.len(),
-            "catch scopes should be added and removed in stack order"
-        );
-
-        self.catch_scopes.pop().unwrap();
-
+        self.catch_scope = old_scope;
         result
     }
 
@@ -1228,17 +1217,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let was_in_loop_condition = self.is_in_loop_condition;
         self.is_in_loop_condition = false;
 
-        let len = self.loop_scopes.len();
-        self.loop_scopes.push(loop_id);
-
+        let old_scope = self.loop_scope.replace(loop_id);
         let result = f(self);
-        assert_eq!(
-            len + 1,
-            self.loop_scopes.len(),
-            "loop scopes should be added and removed in stack order"
-        );
-
-        self.loop_scopes.pop().unwrap();
+        self.loop_scope = old_scope;
 
         self.is_in_loop_condition = was_in_loop_condition;
 
@@ -1565,8 +1546,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 unstable_span,
             );
             let thin_attrs = ThinVec::from(attrs);
-            let catch_scope = self.catch_scopes.last().copied();
-            let ret_expr = if let Some(catch_node) = catch_scope {
+            let ret_expr = if let Some(catch_node) = self.catch_scope {
                 let target_id = Ok(self.lower_node_id(catch_node));
                 self.arena.alloc(self.expr(
                     try_span,

compiler/rustc_ast_lowering/src/lib.rs

@@ -64,7 +64,6 @@ use rustc_span::{Span, DUMMY_SP};
 
 use smallvec::SmallVec;
 use std::collections::BTreeMap;
-use std::mem;
 use tracing::{debug, trace};
 
 macro_rules! arena_vec {
@@ -117,8 +116,8 @@ struct LoweringContext<'a, 'hir: 'a> {
     /// outside of an `async fn`.
     current_item: Option<Span>,
 
-    catch_scopes: Vec<NodeId>,
-    loop_scopes: Vec<NodeId>,
+    catch_scope: Option<NodeId>,
+    loop_scope: Option<NodeId>,
     is_in_loop_condition: bool,
     is_in_trait_impl: bool,
     is_in_dyn_type: bool,
@@ -323,8 +322,8 @@ pub fn lower_crate<'a, 'hir>(
         bodies: BTreeMap::new(),
         modules: BTreeMap::new(),
         attrs: BTreeMap::default(),
-        catch_scopes: Vec::new(),
-        loop_scopes: Vec::new(),
+        catch_scope: None,
+        loop_scope: None,
         is_in_loop_condition: false,
         is_in_trait_impl: false,
         is_in_dyn_type: false,
@@ -911,11 +910,11 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let was_in_loop_condition = self.is_in_loop_condition;
         self.is_in_loop_condition = false;
 
-        let catch_scopes = mem::take(&mut self.catch_scopes);
-        let loop_scopes = mem::take(&mut self.loop_scopes);
+        let catch_scope = self.catch_scope.take();
+        let loop_scope = self.loop_scope.take();
         let ret = f(self);
-        self.catch_scopes = catch_scopes;
-        self.loop_scopes = loop_scopes;
+        self.catch_scope = catch_scope;
+        self.loop_scope = loop_scope;
 
         self.is_in_loop_condition = was_in_loop_condition;