Change a few error messages to give code suggestions

PR #24242 added the ability to the compiler to directly give suggestions about
how to modify code to fix an error. The new errors look like this:

    foobar.rs:5:12: 5:25 error: expected a path on the left-hand side of `+`,
                                not `&'static Copy` [E0178]
    foobar.rs:5     let x: &'static Copy + 'static;
                           ^~~~~~~~~~~~~
    foobar.rs:5:12: 5:35 help: try adding parentheses (per RFC 438):
    foobar.rs:      let x: &'static (Copy + 'static);

    foobar.rs:2:13: 2:23 error: cast to unsized type: `&_` as `core::marker::Copy`
    foobar.rs:2     let x = &1 as Copy;
                            ^~~~~~~~~~
    foobar.rs:2:19: 2:23 help: try casting to a reference instead:
    foobar.rs:      let x = &1 as &Copy;

    foobar.rs:7:24: 7:25 error: expected expression, found `;`
    foobar.rs:7     let x = box (1 + 1);
                                       ^
    foobar.rs:7:13: 7:16 help: try using `box()` instead:
    foobar.rs:      let x = box() (1 + 1);

This also modifies compiletest to give the ability to directly test suggestions
given by error messages.

1 files changed, 8 insertions, 3 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index 0515d1ae945..ae396d7c4e6 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -972,6 +972,9 @@ impl<'a> Parser<'a> {
     pub fn span_help(&self, sp: Span, m: &str) {
         self.sess.span_diagnostic.span_help(sp, m)
     }
+    pub fn span_suggestion(&self, sp: Span, m: &str, n: String) {
+        self.sess.span_diagnostic.span_suggestion(sp, m, n)
+    }
     pub fn fileline_help(&self, sp: Span, m: &str) {
         self.sess.span_diagnostic.fileline_help(sp, m)
     }
@@ -2594,6 +2597,7 @@ impl<'a> Parser<'a> {
             }
 
             let lo = self.span.lo;
+            let box_hi = self.span.hi;
 
             try!(self.bump());
 
@@ -2610,9 +2614,10 @@ impl<'a> Parser<'a> {
                         self.span_err(span,
                                       &format!("expected expression, found `{}`",
                                               this_token_to_string));
-                        let box_span = mk_sp(lo, self.last_span.hi);
-                        self.span_help(box_span,
-                                       "perhaps you meant `box() (foo)` instead?");
+                        let box_span = mk_sp(lo, box_hi);
+                        self.span_suggestion(box_span,
+                                             "try using `box()` instead:",
+                                             "box()".to_string());
                         self.abort_if_errors();
                     }
                     let subexpression = try!(self.parse_prefix_expr());