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, 1 insertions, 2 deletions

diff --git a/library/coretests/tests/lib.rs b/library/coretests/tests/lib.rs
index bf0a3ae7870..ffb96b5a7cf 100644
--- a/library/coretests/tests/lib.rs
+++ b/library/coretests/tests/lib.rs
@@ -15,10 +15,9 @@
 #![feature(cfg_target_has_reliable_f16_f128)]
 #![feature(char_max_len)]
 #![feature(clone_to_uninit)]
-#![feature(const_deref)]
+#![feature(const_convert)]
 #![feature(const_destruct)]
 #![feature(const_eval_select)]
-#![feature(const_from)]
 #![feature(const_ops)]
 #![feature(const_option_ops)]
 #![feature(const_ref_cell)]