Add generic conversion traits

[aturon]

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 As<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#14433
Closes#22751

[breaking-change]

r? @alexcrichton
cc @gankro@erickt@cmr

[aturon]

PRELIMINARY PR: Do not merge until the RFC is approved.

[aturon]

cc @sfackler@wycats

[aturon]

Per IRC: @alexcrichton is very uncomfortable with these living in libcollections rather than libcore. This placement was to facilitate impls like impl<T: Clone> To<Vec<T>> for [T] which would otherwise be disallowed due to coherence.

[aturon]

cc @reem

[aturon]

Per IRC: @reem points out that the same coherence problems make it much more difficult to define "implicit conversions" via generics targetting user-land types: you cannot implement To<MyType<T>> for StdType<T> due to coherence.

Introducing an ad hoc conversion trait, on the other hand, makes it possible to do this in userland.

[aturon]

Updated: I've now moved the traits into libcore, and have a (rather sad) impl of To<Vec<u8>> for [u8] which coherence permits.

[bors]

☔ The latest upstream changes (presumably #23548) made this pull request unmergeable. Please resolve the merge conflicts.

[aturon]

I believe this is now ready to go, once the RFC itself has been merged.

[alexcrichton]

Could these traits also go into the libcore prelude? I think files like this can have use core::prelude::* at the top as well (slowly been migrating over time).

[alexcrichton]

cc #23567

[aturon]

Updated:

• Removed To for the time being.

• Renamed As to AsRef.

• Renamed all methods to match their trait names.

• All conversion functionality now in libcore; added to core::prelude

• Added From => Into implementation, which makes it possible to add conversions in both directions without running afoul of coherence. For example, we now have From<[T]> for Vec<T> where T: Clone, which yields the corresponding Into going in the other direction -- despite the fact that the two types live in different crates.

  I also believe this addresses a few concerns about things implementing From instead of Into

• Removed AsRef impls for slices from Option/Result, which was causing a bad interaction with their as_ref methods. This functionality is available by calling as_slice directly.

All in all, I feel much better with the form this PR is taking now. It feels cleaner, leaner, more consistent and more extensible. Thanks for the feedback making this possible!

@alexcrichton: re-r?

[alexcrichton]

Could this module import Path directly and use AsRef<Path>?

[aturon]

Ahha! It can now. I think when I first wrote it Path was still in the prelude :)

[alexcrichton]

I, too, am quite happy with how this is turning out! r=me with the as_slice methods on Option and Result remaining #[unstable] (as they were before)

[aturon]

pings addressed :)

[alexcrichton]

@bors: r+ 8389253

[aturon]

@bors: r=alexcrichton 97c8e43

[alexcrichton]

Rolled into #23654

[bors]

☔ The latest upstream changes (presumably #23654) made this pull request unmergeable. Please resolve the merge conflicts.