Rollup merge of rust-lang#92382 - clarfonthey:const_convert, r=scottmcm

Extend const_convert to rest of blanket core::convert impls

This adds constness to all the blanket impls in `core::convert` under the existing `const_convert` feature, tracked by rust-lang#88674.

Existing impls under that feature:

```rust
impl<T> const From<T> for T;
impl<T, U> const Into<U> for T where U: ~const From<T>;

impl<T> const ops::Try for Option<T>;
impl<T> const ops::FromResidual for Option<T>;

impl<T, E> const ops::Try for Result<T, E>;
impl<T, E, F> const ops::FromResidual<Result<convert::Infallible, E>> for Result<T, F> where F: ~const From<E>;
```

Additional impls:

```rust
impl<T: ?Sized, U: ?Sized> const AsRef<U> for &T where T: ~const AsRef<U>;
impl<T: ?Sized, U: ?Sized> const AsRef<U> for &mut T where T: ~const AsRef<U>;
impl<T: ?Sized, U: ?Sized> const AsMut<U> for &mut T where T: ~const AsMut<U>;

impl<T, U> const TryInto<U> for T where U: ~const TryFrom<T>;
impl<T, U> const TryFrom<U> for T where U: ~const Into<T>;
```

library/core/src/convert/mod.rs

@@ -481,9 +481,10 @@ pub trait TryFrom<T>: Sized {
 
 // As lifts over &
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized, U: ?Sized> AsRef<U> for &T
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<T: ?Sized, U: ?Sized> const AsRef<U> for &T
 where
-    T: AsRef<U>,
+    T: ~const AsRef<U>,
 {
     fn as_ref(&self) -> &U {
         <T as AsRef<U>>::as_ref(*self)
@@ -492,9 +493,10 @@ where
 
 // As lifts over &mut
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized, U: ?Sized> AsRef<U> for &mut T
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<T: ?Sized, U: ?Sized> const AsRef<U> for &mut T
 where
-    T: AsRef<U>,
+    T: ~const AsRef<U>,
 {
     fn as_ref(&self) -> &U {
         <T as AsRef<U>>::as_ref(*self)
@@ -511,9 +513,10 @@ where
 
 // AsMut lifts over &mut
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T: ?Sized, U: ?Sized> AsMut<U> for &mut T
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<T: ?Sized, U: ?Sized> const AsMut<U> for &mut T
 where
-    T: AsMut<U>,
+    T: ~const AsMut<U>,
 {
     fn as_mut(&mut self) -> &mut U {
         (*self).as_mut()
@@ -567,9 +570,10 @@ impl<T> const From<!> for T {
 
 // TryFrom implies TryInto
 #[stable(feature = "try_from", since = "1.34.0")]
-impl<T, U> TryInto<U> for T
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<T, U> const TryInto<U> for T
 where
-    U: TryFrom<T>,
+    U: ~const TryFrom<T>,
 {
     type Error = U::Error;
 
@@ -581,9 +585,10 @@ where
 // Infallible conversions are semantically equivalent to fallible conversions
 // with an uninhabited error type.
 #[stable(feature = "try_from", since = "1.34.0")]
-impl<T, U> TryFrom<U> for T
+#[rustc_const_unstable(feature = "const_convert", issue = "88674")]
+impl<T, U> const TryFrom<U> for T
 where
-    U: Into<T>,
+    U: ~const Into<T>,
 {
     type Error = Infallible;