Sort reported 'similar impl candidates' alphabetically

Fixes rust-lang#53302

Author: BurntPizza

Diff for: src/librustc/traits/error_reporting.rs

@@ -442,7 +442,8 @@ impl<'a, 'gcx, 'tcx> InferCtxt<'a, 'gcx, 'tcx> {
             return;
         }
 
-        let len = impl_candidates.len();
+        impl_candidates.sort_by_cached_key(ToString::to_string);
+
         let end = if impl_candidates.len() <= 5 {
             impl_candidates.len()
         } else {

Diff for: src/test/ui/did_you_mean/issue-21659-show-relevant-trait-impls-2.stderr

@@ -8,7 +8,7 @@ LL |     f1.foo(1usize);
              <Bar as Foo<i16>>
              <Bar as Foo<i32>>
              <Bar as Foo<i8>>
-             <Bar as Foo<u8>>
+             <Bar as Foo<u16>>
            and 2 others
 
 error: aborting due to previous error

Diff for: src/test/ui/did_you_mean/issue-39802-show-5-trait-impls.stderr

@@ -40,10 +40,10 @@ LL |     Foo::<i32>::bar(&true); //~ ERROR is not satisfied
    |     ^^^^^^^^^^^^^^^ the trait `Foo<i32>` is not implemented for `bool`
    |
    = help: the following implementations were found:
+             <bool as Foo<bool>>
+             <bool as Foo<i8>>
              <bool as Foo<u16>>
              <bool as Foo<u32>>
-             <bool as Foo<u64>>
-             <bool as Foo<u8>>
            and 2 others
 note: required by `Foo::bar`
   --> $DIR/issue-39802-show-5-trait-impls.rs:12:5

Diff for: src/test/ui/report-similar-candidates-ordering.rs

@@ -0,0 +1,15 @@
+// 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.
+
+// issue-53302
+
+fn main() {
+    let _: i32 = "".into(); //~ ERROR [E0277]
+}

Diff for: src/test/ui/report-similar-candidates-ordering.stderr

@@ -0,0 +1,17 @@
+error[E0277]: the trait bound `i32: std::convert::From<&str>` is not satisfied
+  --> $DIR/report-similar-candidates-ordering.rs:14:21
+   |
+LL |     let _: i32 = "".into(); //~ ERROR [E0277]
+   |                     ^^^^ the trait `std::convert::From<&str>` is not implemented for `i32`
+   |
+   = help: the following implementations were found:
+             <i32 as std::convert::From<bool>>
+             <i32 as std::convert::From<i16>>
+             <i32 as std::convert::From<i8>>
+             <i32 as std::convert::From<u16>>
+             <i32 as std::convert::From<u8>>
+   = note: required because of the requirements on the impl of `std::convert::Into<i32>` for `&str`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0277`.