Rollup merge of #145279 - clarfonthey:const-convert-initial, r=tgross35

Constify conversion traits (part 1)

This is the first part of rust-lang/rust#144289 being split into smaller pieces. It adds/moves constness of several traits under the `const_convert` feature:

* `From`
* `Into`
* `TryFrom`
* `TryInto`
* `FromStr`
* `AsRef`
* `AsMut`
* `Borrow`
* `BorrowMut`
* `Deref`
* `DerefMut`

There are a few methods that are intrinsically tied to these traits which I've included in the feature. Particularly, those which are wrappers over `AsRef`:

* `ByteStr::new` (unstable under `bstr` feature)
* `OsStr::new`
* `Path::new`

Those which directly use `Into`:

* `Result::into_ok`
* `Result::into_err`

And those which use `Deref` and `DerefMut`:

* `Pin::as_ref`
* `Pin::as_mut`
* `Pin::as_deref_mut`
* `Option::as_deref`
* `Option::as_deref_mut`
* `Result::as_deref`
* `Result::as_deref_mut`

(note: the `Option` and `Result` methods were suggested by ``@npmccallum`` initially as rust-lang/rust#146101)

The parts which are missing from this PR are:

* Anything that involves heap-allocated types
* Making any method const than the ones listed above
* Anything that could rely on the above, *or* could rely on system-specific code for `OsStr` or `Path` (note: this mostly makes these methods useless since `str` doesn't implement `AsRef<OsStr>` yet, but it's better to track the method for now and add impls later, IMHO)

r? ``@tgross35`` (who mostly already reviewed this)

1 files changed, 2 insertions, 1 deletions

diff --git a/library/core/src/str/mod.rs b/library/core/src/str/mod.rs
index f1db385e212..04fdaa8143e 100644
--- a/library/core/src/str/mod.rs
+++ b/library/core/src/str/mod.rs
@@ -3078,7 +3078,8 @@ impl str {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl AsRef<[u8]> for str {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const AsRef<[u8]> for str {
     #[inline]
     fn as_ref(&self) -> &[u8] {
         self.as_bytes()