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, 30 insertions, 15 deletions

diff --git a/library/core/src/bstr/mod.rs b/library/core/src/bstr/mod.rs
index 13127d645a2..e13dc5cd44d 100644
--- a/library/core/src/bstr/mod.rs
+++ b/library/core/src/bstr/mod.rs
@@ -63,14 +63,16 @@ impl ByteStr {
     /// ```
     #[inline]
     #[unstable(feature = "bstr", issue = "134915")]
-    pub fn new<B: ?Sized + AsRef<[u8]>>(bytes: &B) -> &Self {
+    #[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+    pub const fn new<B: ?Sized + [const] AsRef<[u8]>>(bytes: &B) -> &Self {
         ByteStr::from_bytes(bytes.as_ref())
     }
 
     #[doc(hidden)]
     #[unstable(feature = "bstr_internals", issue = "none")]
     #[inline]
-    pub fn from_bytes(slice: &[u8]) -> &Self {
+    #[rustc_const_unstable(feature = "bstr_internals", issue = "none")]
+    pub const fn from_bytes(slice: &[u8]) -> &Self {
         // SAFETY: `ByteStr` is a transparent wrapper around `[u8]`, so we can turn a reference to
         // the wrapped type into a reference to the wrapper type.
         unsafe { &*(slice as *const [u8] as *const Self) }
@@ -79,7 +81,8 @@ impl ByteStr {
     #[doc(hidden)]
     #[unstable(feature = "bstr_internals", issue = "none")]
     #[inline]
-    pub fn from_bytes_mut(slice: &mut [u8]) -> &mut Self {
+    #[rustc_const_unstable(feature = "bstr_internals", issue = "none")]
+    pub const fn from_bytes_mut(slice: &mut [u8]) -> &mut Self {
         // SAFETY: `ByteStr` is a transparent wrapper around `[u8]`, so we can turn a reference to
         // the wrapped type into a reference to the wrapper type.
         unsafe { &mut *(slice as *mut [u8] as *mut Self) }
@@ -88,20 +91,23 @@ impl ByteStr {
     #[doc(hidden)]
     #[unstable(feature = "bstr_internals", issue = "none")]
     #[inline]
-    pub fn as_bytes(&self) -> &[u8] {
+    #[rustc_const_unstable(feature = "bstr_internals", issue = "none")]
+    pub const fn as_bytes(&self) -> &[u8] {
         &self.0
     }
 
     #[doc(hidden)]
     #[unstable(feature = "bstr_internals", issue = "none")]
     #[inline]
-    pub fn as_bytes_mut(&mut self) -> &mut [u8] {
+    #[rustc_const_unstable(feature = "bstr_internals", issue = "none")]
+    pub const fn as_bytes_mut(&mut self) -> &mut [u8] {
         &mut self.0
     }
 }
 
 #[unstable(feature = "bstr", issue = "134915")]
-impl Deref for ByteStr {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const Deref for ByteStr {
     type Target = [u8];
 
     #[inline]
@@ -111,7 +117,8 @@ impl Deref for ByteStr {
 }
 
 #[unstable(feature = "bstr", issue = "134915")]
-impl DerefMut for ByteStr {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const DerefMut for ByteStr {
     #[inline]
     fn deref_mut(&mut self) -> &mut [u8] {
         &mut self.0
@@ -185,7 +192,8 @@ impl fmt::Display for ByteStr {
 }
 
 #[unstable(feature = "bstr", issue = "134915")]
-impl AsRef<[u8]> for ByteStr {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const AsRef<[u8]> for ByteStr {
     #[inline]
     fn as_ref(&self) -> &[u8] {
         &self.0
@@ -193,7 +201,8 @@ impl AsRef<[u8]> for ByteStr {
 }
 
 #[unstable(feature = "bstr", issue = "134915")]
-impl AsRef<ByteStr> for ByteStr {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const AsRef<ByteStr> for ByteStr {
     #[inline]
     fn as_ref(&self) -> &ByteStr {
         self
@@ -203,7 +212,8 @@ impl AsRef<ByteStr> for ByteStr {
 // `impl AsRef<ByteStr> for [u8]` omitted to avoid widespread inference failures
 
 #[unstable(feature = "bstr", issue = "134915")]
-impl AsRef<ByteStr> for str {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const AsRef<ByteStr> for str {
     #[inline]
     fn as_ref(&self) -> &ByteStr {
         ByteStr::new(self)
@@ -211,7 +221,8 @@ impl AsRef<ByteStr> for str {
 }
 
 #[unstable(feature = "bstr", issue = "134915")]
-impl AsMut<[u8]> for ByteStr {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const AsMut<[u8]> for ByteStr {
     #[inline]
     fn as_mut(&mut self) -> &mut [u8] {
         &mut self.0
@@ -225,7 +236,8 @@ impl AsMut<[u8]> for ByteStr {
 // `impl Borrow<ByteStr> for str` omitted to avoid widespread inference failures
 
 #[unstable(feature = "bstr", issue = "134915")]
-impl Borrow<[u8]> for ByteStr {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const Borrow<[u8]> for ByteStr {
     #[inline]
     fn borrow(&self) -> &[u8] {
         &self.0
@@ -235,7 +247,8 @@ impl Borrow<[u8]> for ByteStr {
 // `impl BorrowMut<ByteStr> for [u8]` omitted to avoid widespread inference failures
 
 #[unstable(feature = "bstr", issue = "134915")]
-impl BorrowMut<[u8]> for ByteStr {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl const BorrowMut<[u8]> for ByteStr {
     #[inline]
     fn borrow_mut(&mut self) -> &mut [u8] {
         &mut self.0
@@ -303,7 +316,8 @@ impl<'a> Default for &'a mut ByteStr {
 // }
 
 #[unstable(feature = "bstr", issue = "134915")]
-impl<'a> TryFrom<&'a ByteStr> for &'a str {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl<'a> const TryFrom<&'a ByteStr> for &'a str {
     type Error = crate::str::Utf8Error;
 
     #[inline]
@@ -313,7 +327,8 @@ impl<'a> TryFrom<&'a ByteStr> for &'a str {
 }
 
 #[unstable(feature = "bstr", issue = "134915")]
-impl<'a> TryFrom<&'a mut ByteStr> for &'a mut str {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl<'a> const TryFrom<&'a mut ByteStr> for &'a mut str {
     type Error = crate::str::Utf8Error;
 
     #[inline]