Correct binding scope when building MIR.

Author: cjgillot

compiler/rustc_mir_build/src/builder/matches/mod.rs

@@ -741,6 +741,8 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             pattern,
             &ProjectedUserTypesNode::None,
             &mut |this, name, mode, var, span, ty, user_tys| {
+                let saved_scope = this.source_scope;
+                this.set_correct_source_scope_for_arg(var.0, saved_scope, span);
                 let vis_scope = *visibility_scope
                     .get_or_insert_with(|| this.new_source_scope(scope_span, LintLevel::Inherited));
                 let source_info = SourceInfo { span, scope: this.source_scope };
@@ -758,6 +760,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
                     opt_match_place.map(|(x, y)| (x.cloned(), y)),
                     pattern.span,
                 );
+                this.source_scope = saved_scope;
             },
         );
         if let Some(guard_expr) = guard {