Stabilize std::path

This commit stabilizes essentially all of the new `std::path` API. The
API itself is changed in a couple of ways (which brings it in closer
alignment with the RFC):

* `.` components are now normalized away, unless they appear at the
  start of a path. This in turn effects the semantics of e.g. asking for
  the file name of `foo/` or `foo/.`, both of which yield `Some("foo")`
  now. This semantics is what the original RFC specified, and is also
  desirable given early experience rolling out the new API.

* The `parent` function now succeeds if, and only if, the path has at
  least one non-root/prefix component. This change affects `pop` as
  well.

* The `Prefix` component now involves a separate `PrefixComponent`
  struct, to better allow for keeping both parsed and unparsed prefix data.

In addition, the `old_path` module is now deprecated.

Closes #23264

[breaking-change]

2 files changed, 3 insertions, 1 deletions

diff --git a/src/librustc_back/fs.rs b/src/librustc_back/fs.rs
index c00c33d4c2f..20335bc8c09 100644
--- a/src/librustc_back/fs.rs
+++ b/src/librustc_back/fs.rs
@@ -11,6 +11,7 @@
 use std::io;
 use std::old_io::fs;
 use std::old_io;
+#[allow(deprecated)]
 use std::old_path;
 use std::os;
 use std::path::{Path, PathBuf};
diff --git a/src/librustc_back/lib.rs b/src/librustc_back/lib.rs
index 333c97b446b..77338dddefa 100644
--- a/src/librustc_back/lib.rs
+++ b/src/librustc_back/lib.rs
@@ -42,11 +42,12 @@
 #![feature(old_io)]
 #![feature(old_path)]
 #![feature(os)]
-#![feature(path)]
 #![feature(rustc_private)]
 #![feature(staged_api)]
 #![feature(rand)]
 #![feature(path_ext)]
+#![feature(std_misc)]
+#![feature(path_relative_from)]
 
 extern crate syntax;
 extern crate serialize;