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]

1 files changed, 1 insertions, 5 deletions

diff --git a/src/libsyntax/ext/source_util.rs b/src/libsyntax/ext/source_util.rs
index 62d98be8b85..eb0daac3ab8 100644
--- a/src/libsyntax/ext/source_util.rs
+++ b/src/libsyntax/ext/source_util.rs
@@ -195,11 +195,7 @@ fn res_rel_file(cx: &mut ExtCtxt, sp: codemap::Span, arg: &Path) -> PathBuf {
     // NB: relative paths are resolved relative to the compilation unit
     if !arg.is_absolute() {
         let mut cu = PathBuf::new(&cx.codemap().span_to_filename(sp));
-        if cu.parent().is_some() {
-            cu.pop();
-        } else {
-            cu = PathBuf::new("");
-        }
+        cu.pop();
         cu.push(arg);
         cu
     } else {