Rollup merge of #139141 - mejrs:on_unimpl, r=Noratrieb

Switch some rustc_on_unimplemented uses to diagnostic::on_unimplemented

The use on the SliceIndex impl appears unreachable, there is no mention of "vector indices" in any test output and I could not get it to show up in error messages.

Author: jhpratt

Diff for: library/alloc/src/vec/mod.rs

@@ -3360,10 +3360,6 @@ impl<T: Hash, A: Allocator> Hash for Vec<T, A> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
-    message = "vector indices are of type `usize` or ranges of `usize`",
-    label = "vector indices are of type `usize` or ranges of `usize`"
-)]
 impl<T, I: SliceIndex<[T]>, A: Allocator> Index<I> for Vec<T, A> {
     type Output = I::Output;
 
@@ -3374,10 +3370,6 @@ impl<T, I: SliceIndex<[T]>, A: Allocator> Index<I> for Vec<T, A> {
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_on_unimplemented(
-    message = "vector indices are of type `usize` or ranges of `usize`",
-    label = "vector indices are of type `usize` or ranges of `usize`"
-)]
 impl<T, I: SliceIndex<[T]>, A: Allocator> IndexMut<I> for Vec<T, A> {
     #[inline]
     fn index_mut(&mut self, index: I) -> &mut Self::Output {

Diff for: library/core/src/iter/traits/accum.rs

@@ -10,7 +10,7 @@ use crate::num::Wrapping;
 /// [`sum()`]: Iterator::sum
 /// [`FromIterator`]: iter::FromIterator
 #[stable(feature = "iter_arith_traits", since = "1.12.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "a value of type `{Self}` cannot be made by summing an iterator over elements of type `{A}`",
     label = "value of type `{Self}` cannot be made by summing a `std::iter::Iterator<Item={A}>`"
 )]
@@ -31,7 +31,7 @@ pub trait Sum<A = Self>: Sized {
 /// [`product()`]: Iterator::product
 /// [`FromIterator`]: iter::FromIterator
 #[stable(feature = "iter_arith_traits", since = "1.12.0")]
-#[rustc_on_unimplemented(
+#[diagnostic::on_unimplemented(
     message = "a value of type `{Self}` cannot be made by multiplying all elements of type `{A}` from an iterator",
     label = "value of type `{Self}` cannot be made by multiplying all elements from a `std::iter::Iterator<Item={A}>`"
 )]