Make the match arm type mismatch message point to the arm's span

Fixes #11319

src/librustc/middle/typeck/check/_match.rs

@@ -87,7 +87,7 @@ pub fn check_match(fcx: &FnCtxt,
         result_ty =
             infer::common_supertype(
                 fcx.infcx(),
-                infer::MatchExpression(expr.span),
+                infer::MatchExpressionArm(expr.span, arm.body.span),
                 true, // result_ty is "expected" here
                 result_ty,
                 bty);

src/librustc/middle/typeck/infer/error_reporting.rs

@@ -346,7 +346,7 @@ impl<'a> ErrorReporting for InferCtxt<'a> {
             infer::ExprAssignable(_) => "mismatched types",
             infer::RelateTraitRefs(_) => "mismatched traits",
             infer::RelateSelfType(_) => "mismatched types",
-            infer::MatchExpression(_) => "match arms have incompatible types",
+            infer::MatchExpressionArm(_, _) => "match arms have incompatible types",
             infer::IfExpression(_) => "if and else have incompatible types",
         };
 
@@ -356,6 +356,12 @@ impl<'a> ErrorReporting for InferCtxt<'a> {
                  message_root_str,
                  expected_found_str,
                  ty::type_err_to_str(self.tcx, terr)).as_slice());
+
+        match trace.origin {
+            infer::MatchExpressionArm(_, arm_span) =>
+                self.tcx.sess.span_note(arm_span, "match arm with an incompatible type"),
+            _ => ()
+        }
     }
 
     fn report_and_explain_type_error(&self,
@@ -1281,7 +1287,7 @@ impl<'a> ErrorReportingHelpers for InferCtxt<'a> {
                     infer::RelateSelfType(_) => {
                         format!("type matches impl")
                     }
-                    infer::MatchExpression(_) => {
+                    infer::MatchExpressionArm(_, _) => {
                         format!("match arms have compatible types")
                     }
                     infer::IfExpression(_) => {

src/librustc/middle/typeck/infer/mod.rs

@@ -116,8 +116,8 @@ pub enum TypeOrigin {
     // Relating trait refs when resolving vtables
     RelateSelfType(Span),
 
-    // Computing common supertype in a match expression
-    MatchExpression(Span),
+    // Computing common supertype in the arms of a match expression
+    MatchExpressionArm(Span, Span),
 
     // Computing common supertype in an if expression
     IfExpression(Span),
@@ -831,7 +831,7 @@ impl TypeOrigin {
             Misc(span) => span,
             RelateTraitRefs(span) => span,
             RelateSelfType(span) => span,
-            MatchExpression(span) => span,
+            MatchExpressionArm(match_span, _) => match_span,
             IfExpression(span) => span,
         }
     }
@@ -853,8 +853,8 @@ impl Repr for TypeOrigin {
             RelateSelfType(a) => {
                 format!("RelateSelfType({})", a.repr(tcx))
             }
-            MatchExpression(a) => {
-                format!("MatchExpression({})", a.repr(tcx))
+            MatchExpressionArm(a, b) => {
+                format!("MatchExpressionArm({}, {})", a.repr(tcx), b.repr(tcx))
             }
             IfExpression(a) => {
                 format!("IfExpression({})", a.repr(tcx))

src/test/compile-fail/issue-11319.rs

@@ -0,0 +1,19 @@
+// Copyright 2014 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+fn main() {
+    match Some(10) {
+    //~^ ERROR match arms have incompatible types: expected `bool` but found `()`
+        Some(5) => false,
+        Some(2) => true,
+        None    => (), //~ NOTE match arm with an incompatible type
+        _       => true
+    }
+}