Add generic conversion traits

This commit:

* Introduces `std::convert`, providing an implementation of
RFC 529.

* Deprecates the `AsPath`, `AsOsStr`, and `IntoBytes` traits, all
in favor of the corresponding generic conversion traits.

  Consequently, various IO APIs now take `AsRef<Path>` rather than
`AsPath`, and so on. Since the types provided by `std` implement both
traits, this should cause relatively little breakage.

* Deprecates many `from_foo` constructors in favor of `from`.

* Changes `PathBuf::new` to take no argument (creating an empty buffer,
  as per convention). The previous behavior is now available as
  `PathBuf::from`.

* De-stabilizes `IntoCow`. It's not clear whether we need this separate trait.

Closes #22751
Closes #14433

[breaking-change]

2 files changed, 5 insertions, 6 deletions

diff --git a/src/libstd/sys/unix/fs2.rs b/src/libstd/sys/unix/fs2.rs
index ea74aab3331..202e5ddaec4 100644
--- a/src/libstd/sys/unix/fs2.rs
+++ b/src/libstd/sys/unix/fs2.rs
@@ -338,8 +338,7 @@ pub fn readlink(p: &Path) -> io::Result<PathBuf> {
         }));
         buf.set_len(n as usize);
     }
-    let s: OsString = OsStringExt::from_vec(buf);
-    Ok(PathBuf::new(&s))
+    Ok(PathBuf::from(OsString::from_vec(buf)))
 }
 
 pub fn symlink(src: &Path, dst: &Path) -> io::Result<()> {
diff --git a/src/libstd/sys/unix/os.rs b/src/libstd/sys/unix/os.rs
index a5a2f71acb7..6c191689255 100644
--- a/src/libstd/sys/unix/os.rs
+++ b/src/libstd/sys/unix/os.rs
@@ -36,7 +36,7 @@ const BUF_BYTES: usize = 2048;
 const TMPBUF_SZ: usize = 128;
 
 fn bytes2path(b: &[u8]) -> PathBuf {
-    PathBuf::new(<OsStr as OsStrExt>::from_bytes(b))
+    PathBuf::from(<OsStr as OsStrExt>::from_bytes(b))
 }
 
 fn os2path(os: OsString) -> PathBuf {
@@ -253,7 +253,7 @@ pub fn current_exe() -> io::Result<PathBuf> {
         let err = _NSGetExecutablePath(v.as_mut_ptr() as *mut i8, &mut sz);
         if err != 0 { return Err(io::Error::last_os_error()); }
         v.set_len(sz as uint - 1); // chop off trailing NUL
-        Ok(PathBuf::new(OsString::from_vec(v)))
+        Ok(PathBuf::from(OsString::from_vec(v)))
     }
 }
 
@@ -466,9 +466,9 @@ pub fn page_size() -> usize {
 pub fn temp_dir() -> PathBuf {
     getenv("TMPDIR".as_os_str()).map(os2path).unwrap_or_else(|| {
         if cfg!(target_os = "android") {
-            PathBuf::new("/data/local/tmp")
+            PathBuf::from("/data/local/tmp")
         } else {
-            PathBuf::new("/tmp")
+            PathBuf::from("/tmp")
         }
     })
 }