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.

src/test/compile-fail/cast-to-unsized-trait-object-suggestion.rs

@@ -0,0 +1,20 @@
+// Copyright 2015 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() {
+    &1 as Copy;
+    //~^ ERROR cast to unsized type
+    //~| HELP try casting to a reference instead:
+    //~| SUGGESTION &1 as &Copy;
+    Box::new(1) as Copy;
+    //~^ ERROR cast to unsized type
+    //~| HELP try casting to a `Box` instead:
+    //~| SUGGESTION Box::new(1) as Box<Copy>;
+}

src/test/compile-fail/issue-17441.rs

@@ -16,7 +16,7 @@ fn main() {
     // FIXME (#22405): Replace `Box::new` with `box` here when/if possible.
     let _bar = Box::new(1_usize) as std::fmt::Debug;
     //~^ ERROR cast to unsized type: `Box<usize>` as `core::fmt::Debug`
-    //~^^ HELP did you mean `Box<core::fmt::Debug>`?
+    //~^^ HELP try casting to a `Box` instead
 
     let _baz = 1_usize as std::fmt::Debug;
     //~^ ERROR cast to unsized type: `usize` as `core::fmt::Debug`

src/test/compile-fail/issue-6702.rs

@@ -15,5 +15,5 @@ struct Monster {
 
 fn main() {
     let _m = Monster(); //~ ERROR `Monster` is a structure name, but
-    //~^ HELP Did you mean to write: `Monster { /* fields */ }`?
+    //~^ HELP did you mean to write: `Monster { /* fields */ }`?
 }

src/test/compile-fail/trait-object-reference-without-parens-suggestion.rs

@@ -0,0 +1,20 @@
+// Copyright 2015 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() {
+    let _: &Copy + 'static;
+    //~^ ERROR expected a path
+    //~| HELP try adding parentheses
+    //~| SUGGESTION let _: &(Copy + 'static);
+    let _: &'static Copy + 'static;
+    //~^ ERROR expected a path
+    //~| HELP try adding parentheses
+    //~| SUGGESTION let _: &'static (Copy + 'static);
+}

src/test/parse-fail/parenthesized-box-expr-message.rs

@@ -9,6 +9,8 @@
 // except according to those terms.
 
 fn main() {
-    box(1 + 1) //~ HELP perhaps you meant `box() (foo)` instead?
+    box (1 + 1)
+    //~^ HELP try using `box()` instead:
+    //~| SUGGESTION box() (1 + 1)
     ; //~ ERROR expected expression, found `;`
 }