Rollup merge of #155054 - Lars-Schumann:const-vec-ops, r=dtolnay

constify `Index(Mut)`, `Deref(Mut)` for `Vec`

Relevant tracking issues
const_convert: https://github.com/rust-lang/rust/issues/143773
const_index: https://github.com/rust-lang/rust/issues/143775

library/alloc/src/lib.rs

@@ -106,6 +106,7 @@
 #![feature(const_destruct)]
 #![feature(const_eval_select)]
 #![feature(const_heap)]
+#![feature(const_index)]
 #![feature(const_option_ops)]
 #![feature(const_try)]
 #![feature(copied_into_inner)]

library/alloc/src/vec/mod.rs

@@ -3747,7 +3747,8 @@ unsafe fn spec_extend_from_within(&mut self, src: Range<usize>) {
 ////////////////////////////////////////////////////////////////////////////////
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T, A: Allocator> ops::Deref for Vec<T, A> {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl<T, A: Allocator> const ops::Deref for Vec<T, A> {
     type Target = [T];
 
     #[inline]
@@ -3757,7 +3758,8 @@ fn deref(&self) -> &[T] {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T, A: Allocator> ops::DerefMut for Vec<T, A> {
+#[rustc_const_unstable(feature = "const_convert", issue = "143773")]
+impl<T, A: Allocator> const ops::DerefMut for Vec<T, A> {
     #[inline]
     fn deref_mut(&mut self) -> &mut [T] {
         self.as_mut_slice()
@@ -3822,7 +3824,8 @@ fn hash<H: Hasher>(&self, state: &mut H) {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T, I: SliceIndex<[T]>, A: Allocator> Index<I> for Vec<T, A> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T, I: [const] SliceIndex<[T]>, A: Allocator> const Index<I> for Vec<T, A> {
     type Output = I::Output;
 
     #[inline]
@@ -3832,7 +3835,8 @@ fn index(&self, index: I) -> &Self::Output {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-impl<T, I: SliceIndex<[T]>, A: Allocator> IndexMut<I> for Vec<T, A> {
+#[rustc_const_unstable(feature = "const_index", issue = "143775")]
+impl<T, I: [const] SliceIndex<[T]>, A: Allocator> const IndexMut<I> for Vec<T, A> {
     #[inline]
     fn index_mut(&mut self, index: I) -> &mut Self::Output {
         IndexMut::index_mut(&mut **self, index)

tests/ui/consts/issue-94675.rs

@@ -9,8 +9,7 @@ struct Foo<'a> {
 impl<'a> Foo<'a> {
     const fn spam(&mut self, baz: &mut Vec<u32>) {
         self.bar[0] = baz.len();
-        //~^ ERROR: `Vec<usize>: [const] Index<_>` is not satisfied
-        //~| ERROR: `Vec<usize>: [const] IndexMut<usize>` is not satisfied
+        //~^ ERROR: `IndexMut` is not yet stable as a const trait
     }
 }
 

tests/ui/consts/issue-94675.stderr

@@ -1,21 +1,13 @@
-error[E0277]: the trait bound `Vec<usize>: [const] Index<_>` is not satisfied
-  --> $DIR/issue-94675.rs:11:9
+error: `IndexMut` is not yet stable as a const trait
+  --> $DIR/issue-94675.rs:11:17
    |
 LL |         self.bar[0] = baz.len();
-   |         ^^^^^^^^^^^
+   |                 ^^^
    |
-note: trait `Index` is implemented but not `const`
-  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
-
-error[E0277]: the trait bound `Vec<usize>: [const] IndexMut<usize>` is not satisfied
-  --> $DIR/issue-94675.rs:11:9
+help: add `#![feature(const_index)]` to the crate attributes to enable
    |
-LL |         self.bar[0] = baz.len();
-   |         ^^^^^^^^^^^
+LL + #![feature(const_index)]
    |
-note: trait `IndexMut` is implemented but not `const`
-  --> $SRC_DIR/alloc/src/vec/mod.rs:LL:COL
 
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0277`.