Be super-explicit about IterMut slice lifetimes

This is the same as the previous signature, but since it's *not* `-> &'a [T]` like in `Iter`, I think it's worth emphasizing that in the signature too, not just the text.

library/core/src/slice/iter.rs

@@ -325,7 +325,10 @@ impl<'a, T> IterMut<'a, T> {
     #[must_use]
     #[stable(feature = "slice_iter_mut_as_slice", since = "1.53.0")]
     #[inline]
-    pub fn as_slice(&self) -> &[T] {
+    pub fn as_slice<'b>(&'b self) -> &'b [T]
+    where
+        'a: 'b,
+    {
         // SAFETY: the type invariant guarantees the pointer represents a valid slice
         unsafe { self.make_nonnull_slice().as_ref() }
     }
@@ -364,7 +367,10 @@ impl<'a, T> IterMut<'a, T> {
     #[must_use]
     // FIXME: Uncomment the `AsMut<[T]>` impl when this gets stabilized.
     #[unstable(feature = "slice_iter_mut_as_mut_slice", issue = "93079")]
-    pub fn as_mut_slice(&mut self) -> &mut [T] {
+    pub fn as_mut_slice<'b>(&'b mut self) -> &'b mut [T]
+    where
+        'a: 'b,
+    {
         // SAFETY: the iterator was created from a mutable slice with pointer
         // `self.ptr` and length `len!(self)`. This guarantees that all the prerequisites
         // for `from_raw_parts_mut` are fulfilled.