Add a way to get shorter spans until `char` for pointing at defs

```rust
error[E0072]: recursive type `X` has infinite size
  --> file.rs:10:1
   |
10 | struct X {
   | ^^^^^^^^ recursive type has infinite size
   |
   = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `X` representable
```

vs

```rust
error[E0072]: recursive type `X` has infinite size
  --> file.rs:10:1
   |
10 |   struct X {
   |  _^ starting here...
11 | |     x: X,
12 | | }
   | |_^ ...ending here: recursive type has infinite size
   |
   = help: insert indirection (e.g., a `Box`, `Rc`, or `&`) at some point to make `X` representable
```

1 files changed, 19 insertions, 0 deletions

diff --git a/src/libsyntax/codemap.rs b/src/libsyntax/codemap.rs
index 4d67390d442..da2d0a33d1a 100644
--- a/src/libsyntax/codemap.rs
+++ b/src/libsyntax/codemap.rs
@@ -441,6 +441,25 @@ impl CodeMap {
         }
     }
 
+    /// Given a `Span`, try to get a shorter span ending before the first occurrence of `c` `char`
+    pub fn span_until_char(&self, sp: Span, c: char) -> Span {
+        match self.span_to_snippet(sp) {
+            Ok(snippet) => {
+                let snippet = snippet.split(c).nth(0).unwrap_or("").trim_right();
+                if snippet.len() > 0 && !snippet.contains('\n') {
+                    Span { hi: BytePos(sp.lo.0 + snippet.len() as u32), ..sp }
+                } else {
+                    sp
+                }
+            }
+            _ => sp,
+        }
+    }
+
+    pub fn def_span(&self, sp: Span) -> Span {
+        self.span_until_char(sp, '{')
+    }
+
     pub fn get_filemap(&self, filename: &str) -> Option<Rc<FileMap>> {
         for fm in self.files.borrow().iter() {
             if filename == fm.name {