auto merge of #19252 : japaric/rust/cow, r=aturon

- Add `IntoCow` trait, and put it in the prelude
- Add `is_owned`/`is_borrowed` methods to `Cow`
- Add `CowString`/`CowVec` type aliases (to `Cow<'_, String, str>`/`Cow<'_, Vec, [T]>` respectively)
- `Cow` implements: `Show`, `Hash`, `[Partial]{Eq,Ord}`
- `impl BorrowFrom<Cow<'a, T, B>> for B`

[breaking-change]s:

- `IntoMaybeOwned` has been removed from the prelude
- libcollections: `SendStr` is now an alias to `CowString<'static>` (it was aliased to `MaybeOwned<'static>`)
- libgraphviz:
  - `LabelText` variants now wrap `CowString` instead of `MaybeOwned`
  - `Nodes` and `Edges` are now type aliases to `CowVec` (they were aliased to `MaybeOwnedVec`)
- libstd/path: `Display::as_maybe_owned` has been renamed to `Display::as_cow` and now returns a `CowString`
- These functions now accept/return `Cow` instead of `MaybeOwned[Vector]`:
  - libregex: `Replacer::reg_replace`
  - libcollections: `str::from_utf8_lossy`
  - libgraphviz: `Id::new`, `Id::name`, `LabelText::pre_escaped_content`
  - libstd: `TaskBuilder::named`

r? @aturon 

1 files changed, 2 insertions, 2 deletions

diff --git a/src/libsyntax/parse/parser.rs b/src/libsyntax/parse/parser.rs
index a9306c71240..c731a0005f8 100644
--- a/src/libsyntax/parse/parser.rs
+++ b/src/libsyntax/parse/parser.rs
@@ -4966,10 +4966,10 @@ impl<'a> Parser<'a> {
                 let mut err = String::from_str("circular modules: ");
                 let len = included_mod_stack.len();
                 for p in included_mod_stack.slice(i, len).iter() {
-                    err.push_str(p.display().as_maybe_owned().as_slice());
+                    err.push_str(p.display().as_cow().as_slice());
                     err.push_str(" -> ");
                 }
-                err.push_str(path.display().as_maybe_owned().as_slice());
+                err.push_str(path.display().as_cow().as_slice());
                 self.span_fatal(id_sp, err.as_slice());
             }
             None => ()