Don't Suggest Labeling Inline const Blocks

Previously, both anonymous constant blocks (E.g. The labeled block
inside `['_'; 'block: { break 'block 1 + 2; }]`) and inline const
blocks (E.g. `const { ... }`) were considered to be the same
kind of blocks. This caused the compiler to incorrectly suggest
labeling both the blocks when only anonymous constant blocks can be
labeled.

This PR adds an other enum Variant to `Context` so that both the
blocks can be handled appropriately.

Also, adds some doc comments and removes unnecessary `&mut` in a
couple of places.

Author: veera-sivarajan

compiler/rustc_passes/src/loops.rs

@@ -19,17 +19,25 @@ use crate::errors::{
     OutsideLoopSuggestion, UnlabeledCfInWhileCondition, UnlabeledInLabeledBlock,
 };
 
+/// The context in which a block is encountered.
 #[derive(Clone, Copy, Debug, PartialEq)]
 enum Context {
     Normal,
     Fn,
     Loop(hir::LoopSource),
     Closure(Span),
-    Coroutine { coroutine_span: Span, kind: hir::CoroutineDesugaring, source: hir::CoroutineSource },
+    Coroutine {
+        coroutine_span: Span,
+        kind: hir::CoroutineDesugaring,
+        source: hir::CoroutineSource,
+    },
     UnlabeledBlock(Span),
     UnlabeledIfBlock(Span),
     LabeledBlock,
-    Constant,
+    /// E.g. The labeled block inside `['_'; 'block: { break 'block 1 + 2; }]`.
+    AnonConst,
+    /// E.g. `const { ... }`.
+    ConstBlock,
 }
 
 #[derive(Clone)]
@@ -90,11 +98,11 @@ impl<'a, 'hir> Visitor<'hir> for CheckLoopVisitor<'a, 'hir> {
     }
 
     fn visit_anon_const(&mut self, c: &'hir hir::AnonConst) {
-        self.with_context(Constant, |v| intravisit::walk_anon_const(v, c));
+        self.with_context(AnonConst, |v| intravisit::walk_anon_const(v, c));
     }
 
     fn visit_inline_const(&mut self, c: &'hir hir::ConstBlock) {
-        self.with_context(Constant, |v| intravisit::walk_inline_const(v, c));
+        self.with_context(ConstBlock, |v| intravisit::walk_inline_const(v, c));
     }
 
     fn visit_fn(
@@ -128,7 +136,7 @@ impl<'a, 'hir> Visitor<'hir> for CheckLoopVisitor<'a, 'hir> {
                         && matches!(
                             ck_loop.cx_stack.last(),
                             Some(&Normal)
-                                | Some(&Constant)
+                                | Some(&AnonConst)
                                 | Some(&UnlabeledBlock(_))
                                 | Some(&UnlabeledIfBlock(_))
                         )
@@ -175,13 +183,15 @@ impl<'a, 'hir> Visitor<'hir> for CheckLoopVisitor<'a, 'hir> {
             hir::ExprKind::Block(ref b, Some(_label)) => {
                 self.with_context(LabeledBlock, |v| v.visit_block(b));
             }
-            hir::ExprKind::Block(ref b, None) if matches!(self.cx_stack.last(), Some(&Fn)) => {
+            hir::ExprKind::Block(ref b, None)
+                if matches!(self.cx_stack.last(), Some(&Fn) | Some(&ConstBlock)) =>
+            {
                 self.with_context(Normal, |v| v.visit_block(b));
             }
             hir::ExprKind::Block(ref b, None)
                 if matches!(
                     self.cx_stack.last(),
-                    Some(&Normal) | Some(&Constant) | Some(&UnlabeledBlock(_))
+                    Some(&Normal) | Some(&AnonConst) | Some(&UnlabeledBlock(_))
                 ) =>
             {
                 self.with_context(UnlabeledBlock(b.span.shrink_to_lo()), |v| v.visit_block(b));
@@ -353,7 +363,7 @@ impl<'a, 'hir> CheckLoopVisitor<'a, 'hir> {
             UnlabeledIfBlock(_) if br_cx_kind == BreakContextKind::Break => {
                 self.require_break_cx(br_cx_kind, span, break_span, cx_pos - 1);
             }
-            Normal | Constant | Fn | UnlabeledBlock(_) | UnlabeledIfBlock(_) => {
+            Normal | AnonConst | Fn | UnlabeledBlock(_) | UnlabeledIfBlock(_) | ConstBlock => {
                 self.sess.dcx().emit_err(OutsideLoop {
                     spans: vec![span],
                     name: &br_cx_kind.to_string(),
@@ -365,7 +375,7 @@ impl<'a, 'hir> CheckLoopVisitor<'a, 'hir> {
     }
 
     fn require_label_in_labeled_block(
-        &mut self,
+        &self,
         span: Span,
         label: &Destination,
         cf_type: &str,
@@ -380,7 +390,7 @@ impl<'a, 'hir> CheckLoopVisitor<'a, 'hir> {
         false
     }
 
-    fn report_outside_loop_error(&mut self) {
+    fn report_outside_loop_error(&self) {
         for (s, block) in &self.block_breaks {
             self.sess.dcx().emit_err(OutsideLoop {
                 spans: block.spans.clone(),