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)

3 files changed, 19 insertions, 9 deletions

diff --git a/library/std/src/ffi/os_str.rs b/library/std/src/ffi/os_str.rs
index 1214490caad..a39565d2159 100644
--- a/library/std/src/ffi/os_str.rs
+++ b/library/std/src/ffi/os_str.rs
@@ -828,7 +828,8 @@ impl OsStr {
     /// ```
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn new<S: AsRef<OsStr> + ?Sized>(s: &S) -> &OsStr {
+    #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+    pub const fn new<S: [const] AsRef<OsStr> + ?Sized>(s: &S) -> &OsStr {
         s.as_ref()
     }
 
@@ -876,14 +877,16 @@ impl OsStr {
     }
 
     #[inline]
-    fn from_inner(inner: &Slice) -> &OsStr {
+    #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+    const fn from_inner(inner: &Slice) -> &OsStr {
         // SAFETY: OsStr is just a wrapper of Slice,
         // therefore converting &Slice to &OsStr is safe.
         unsafe { &*(inner as *const Slice as *const OsStr) }
     }
 
     #[inline]
-    fn from_inner_mut(inner: &mut Slice) -> &mut OsStr {
+    #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+    const fn from_inner_mut(inner: &mut Slice) -> &mut OsStr {
         // SAFETY: OsStr is just a wrapper of Slice,
         // therefore converting &mut Slice to &mut OsStr is safe.
         // Any method that mutates OsStr must be careful not to
@@ -1681,7 +1684,8 @@ impl ToOwned for OsStr {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl AsRef<OsStr> for OsStr {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const AsRef<OsStr> for OsStr {
     #[inline]
     fn as_ref(&self) -> &OsStr {
         self
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index 7bdc6ff7882..a4b63462053 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -332,6 +332,7 @@
 #![feature(char_internals)]
 #![feature(clone_to_uninit)]
 #![feature(const_cmp)]
+#![feature(const_convert)]
 #![feature(const_ops)]
 #![feature(const_option_ops)]
 #![feature(const_try)]
diff --git a/library/std/src/path.rs b/library/std/src/path.rs
index 470d300d2d9..5ccea1c972c 100644
--- a/library/std/src/path.rs
+++ b/library/std/src/path.rs
@@ -2264,11 +2264,13 @@ impl Path {
     /// assert_eq!(from_string, from_path);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    pub fn new<S: AsRef<OsStr> + ?Sized>(s: &S) -> &Path {
+    #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+    pub const fn new<S: [const] AsRef<OsStr> + ?Sized>(s: &S) -> &Path {
         unsafe { &*(s.as_ref() as *const OsStr as *const Path) }
     }
 
-    fn from_inner_mut(inner: &mut OsStr) -> &mut Path {
+    #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+    const fn from_inner_mut(inner: &mut OsStr) -> &mut Path {
         // SAFETY: Path is just a wrapper around OsStr,
         // therefore converting &mut OsStr to &mut Path is safe.
         unsafe { &mut *(inner as *mut OsStr as *mut Path) }
@@ -3337,7 +3339,8 @@ unsafe impl CloneToUninit for Path {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl AsRef<OsStr> for Path {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const AsRef<OsStr> for Path {
     #[inline]
     fn as_ref(&self) -> &OsStr {
         &self.inner
@@ -3507,7 +3510,8 @@ impl Ord for Path {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl AsRef<Path> for Path {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const AsRef<Path> for Path {
     #[inline]
     fn as_ref(&self) -> &Path {
         self
@@ -3515,7 +3519,8 @@ impl AsRef<Path> for Path {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl AsRef<Path> for OsStr {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const AsRef<Path> for OsStr {
     #[inline]
     fn as_ref(&self) -> &Path {
         Path::new(self)