Put a BlockTailInfo in BlockFrame::TailExpr.

Because it has the same fields, and avoids the need to deconstruct the
latter to construct the former.

compiler/rustc_middle/src/mir/mod.rs

@@ -937,7 +937,7 @@ mod binding_form_impl {
 /// involved in borrow_check errors, e.g., explanations of where the
 /// temporaries come from, when their destructors are run, and/or how
 /// one might revise the code to satisfy the borrow checker's rules.
-#[derive(Clone, Debug, TyEncodable, TyDecodable, HashStable)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq, TyEncodable, TyDecodable, HashStable)]
 pub struct BlockTailInfo {
     /// If `true`, then the value resulting from evaluating this tail
     /// expression is ignored by the block's expression context.

compiler/rustc_mir_build/src/builder/block.rs

@@ -331,8 +331,9 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             let expr = &this.thir[expr_id];
             let tail_result_is_ignored =
                 destination_ty.is_unit() || this.block_context.currently_ignores_tail_results();
-            this.block_context
-                .push(BlockFrame::TailExpr { tail_result_is_ignored, span: expr.span });
+            this.block_context.push(BlockFrame::TailExpr {
+                info: BlockTailInfo { tail_result_is_ignored, span: expr.span },
+            });
 
             block = this.expr_into_dest(destination, block, expr_id).into_block();
             let popped = this.block_context.pop();

compiler/rustc_mir_build/src/builder/expr/stmt.rs

@@ -164,8 +164,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                         }
                     }
                     this.block_context.push(BlockFrame::TailExpr {
-                        tail_result_is_ignored: true,
-                        span: expr.span,
+                        info: BlockTailInfo { tail_result_is_ignored: true, span: expr.span },
                     });
                     Some(expr.span)
                 } else {

compiler/rustc_mir_build/src/builder/mod.rs

@@ -112,16 +112,7 @@ enum BlockFrame {
     /// Evaluation is currently within the tail expression of a block.
     ///
     /// Example: `{ STMT_1; STMT_2; EXPR }`
-    TailExpr {
-        /// If true, then the surrounding context of the block ignores
-        /// the result of evaluating the block's tail expression.
-        ///
-        /// Example: `let _ = { STMT_1; EXPR };`
-        tail_result_is_ignored: bool,
-
-        /// `Span` of the tail expression.
-        span: Span,
-    },
+    TailExpr { info: BlockTailInfo },
 
     /// Generic mark meaning that the block occurred as a subexpression
     /// where the result might be used.
@@ -277,9 +268,7 @@ impl BlockContext {
             match bf {
                 BlockFrame::SubExpr => continue,
                 BlockFrame::Statement { .. } => break,
-                &BlockFrame::TailExpr { tail_result_is_ignored, span } => {
-                    return Some(BlockTailInfo { tail_result_is_ignored, span });
-                }
+                &BlockFrame::TailExpr { info } => return Some(info),
             }
         }
 
@@ -302,9 +291,9 @@ impl BlockContext {
 
             // otherwise: use accumulated is_ignored state.
             Some(
-                BlockFrame::TailExpr { tail_result_is_ignored: ignored, .. }
-                | BlockFrame::Statement { ignores_expr_result: ignored },
-            ) => *ignored,
+                BlockFrame::TailExpr { info: BlockTailInfo { tail_result_is_ignored: ign, .. } }
+                | BlockFrame::Statement { ignores_expr_result: ign },
+            ) => *ign,
         }
     }
 }