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]

1 files changed, 3 insertions, 0 deletions

diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index b055796ba54..1488c7969f6 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -126,8 +126,10 @@
 #![feature(hash)]
 #![feature(int_uint)]
 #![feature(unique)]
+#![feature(convert)]
 #![feature(allow_internal_unstable)]
 #![feature(str_char)]
+#![feature(into_cow)]
 #![cfg_attr(test, feature(test, rustc_private))]
 
 // Don't link to std. We are std.
@@ -169,6 +171,7 @@ pub use core::any;
 pub use core::cell;
 pub use core::clone;
 #[cfg(not(test))] pub use core::cmp;
+pub use core::convert;
 pub use core::default;
 #[allow(deprecated)]
 pub use core::finally;