store the span of the match expression, and use it for diagnostics

Author: folkertdev

Diff for: compiler/rustc_middle/src/thir.rs

@@ -377,10 +377,11 @@ pub enum ExprKind<'tcx> {
     },
     /// A `#[loop_match] loop { state = 'blk: { match state { ... } } }` expression.
     LoopMatch {
+        /// The state variable that is updated, and also the scrutinee of the match
         state: ExprId,
-
         region_scope: region::Scope,
         arms: Box<[ArmId]>,
+        match_span: Span,
     },
     /// Special expression representing the `let` part of an `if let` or similar construct
     /// (including `if let` guards in match arms, and let-chains formed by `&&`).

Diff for: compiler/rustc_mir_build/src/builder/expr/into.rs

@@ -245,7 +245,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     None
                 })
             }
-            ExprKind::LoopMatch { state, region_scope, ref arms } => {
+            ExprKind::LoopMatch { state, region_scope, match_span, ref arms } => {
                 // FIXME add diagram
 
                 let loop_block = this.cfg.start_new_block();
@@ -268,7 +268,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     let scrutinee_place_builder =
                         unpack!(body_block = this.as_place_builder(body_block, state));
                     let scrutinee_span = this.thir.exprs[state].span;
-                    let match_start_span = scrutinee_span; // span.shrink_to_lo().to(scrutinee_span); FIXME
+                    let match_start_span = match_span.shrink_to_lo().to(scrutinee_span);
 
                     let mut patterns = Vec::with_capacity(arms.len());
                     for &arm_id in arms.iter() {
@@ -311,8 +311,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                                                 scrutinee_span,
                                                 arms,
                                                 built_tree,
-                                                // FIXME this should be the span of just the match
-                                                this.source_info(expr_span),
+                                                this.source_info(match_span),
                                             )
                                         },
                                     ))

Diff for: compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -938,6 +938,7 @@ impl<'tcx> ThirBuildCx<'tcx> {
                         },
 
                         arms: arms.iter().map(|a| self.convert_arm(a)).collect(),
+                        match_span: block_body_expr.span,
                     }
                 } else {
                     let block_ty = self.typeck_results.node_type(body.hir_id);

Diff for: compiler/rustc_mir_build/src/thir/print.rs

@@ -319,11 +319,12 @@ impl<'a, 'tcx> ThirPrinter<'a, 'tcx> {
                 self.print_expr(*body, depth_lvl + 2);
                 print_indented!(self, ")", depth_lvl);
             }
-            LoopMatch { state, region_scope, arms } => {
+            LoopMatch { state, region_scope, match_span, arms } => {
                 print_indented!(self, "LoopMatch (", depth_lvl);
                 print_indented!(self, "state:", depth_lvl + 1);
                 self.print_expr(*state, depth_lvl + 2);
                 print_indented!(self, format!("region_scope: {:?}", region_scope), depth_lvl + 1);
+                print_indented!(self, format!("match_span: {:?}", match_span), depth_lvl + 1);
 
                 print_indented!(self, "arms: [", depth_lvl + 1);
                 for arm_id in arms.iter() {