Minimize weird spans involving macro context

Sometimes the parser attempts to synthesize spans from within a macro
context with the span for the captured argument, leading to non-sensical
spans with very bad output. Given that an incorrect span is worse than
a partially incomplete span, when detecting this situation return only
one of the spans without mergin them.

3 files changed, 38 insertions, 2 deletions

diff --git a/src/test/ui/macros/span-covering-argument-1.rs b/src/test/ui/macros/span-covering-argument-1.rs
new file mode 100644
index 00000000000..bfc137fc7b2
--- /dev/null
+++ b/src/test/ui/macros/span-covering-argument-1.rs
@@ -0,0 +1,23 @@
+// Copyright 2018 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.
+
+macro_rules! bad {
+    ($s:ident whatever) => {
+        {
+            let $s = 0;
+            *&mut $s = 0;
+            //~^ ERROR cannot borrow immutable local variable `foo` as mutable [E0596]
+        }
+    }
+}
+
+fn main() {
+    bad!(foo whatever);
+}
diff --git a/src/test/ui/macros/span-covering-argument-1.stderr b/src/test/ui/macros/span-covering-argument-1.stderr
new file mode 100644
index 00000000000..677d2f10fd6
--- /dev/null
+++ b/src/test/ui/macros/span-covering-argument-1.stderr
@@ -0,0 +1,13 @@
+error[E0596]: cannot borrow immutable local variable `foo` as mutable
+  --> $DIR/span-covering-argument-1.rs:15:19
+   |
+14 |             let $s = 0;
+   |                 -- consider changing this to `mut $s`
+15 |             *&mut $s = 0;
+   |                   ^^ cannot borrow mutably
+...
+22 |     bad!(foo whatever);
+   |     ------------------- in this macro invocation
+
+error: aborting due to previous error
+
diff --git a/src/test/ui/span/macro-span-replacement.stderr b/src/test/ui/span/macro-span-replacement.stderr
index 2a0e71e192c..728cd12e2c6 100644
--- a/src/test/ui/span/macro-span-replacement.stderr
+++ b/src/test/ui/span/macro-span-replacement.stderr
@@ -1,8 +1,8 @@
 warning: struct is never used: `S`
-  --> $DIR/macro-span-replacement.rs:17:9
+  --> $DIR/macro-span-replacement.rs:17:14
    |
 17 |         $b $a; //~ WARN struct is never used
-   |         ^^^^^^
+   |              ^
 ...
 22 |     m!(S struct);
    |     ------------- in this macro invocation