Rollup merge of #54687 - scottmcm:more-elision, r=dtolnay

Use impl_header_lifetime_elision in libcore

The feature is approved for stabilization, so let's use it to remove about 300 `'a`s.

Tracking issue for the feature: https://github.com/rust-lang/rust/issues/15872

src/test/ui/coherence/coherence-impls-copy.stderr

@@ -14,7 +14,7 @@ LL | impl Copy for &'static NotSync {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: conflicting implementation in crate `core`:
-           - impl<'a, T> std::marker::Copy for &'a T
+           - impl<'_, T> std::marker::Copy for &T
              where T: ?Sized;
 
 error[E0119]: conflicting implementations of trait `std::marker::Copy` for type `&[NotSync]`:
@@ -24,7 +24,7 @@ LL | impl Copy for &'static [NotSync] {}
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: conflicting implementation in crate `core`:
-           - impl<'a, T> std::marker::Copy for &'a T
+           - impl<'_, T> std::marker::Copy for &T
              where T: ?Sized;
 
 error[E0206]: the trait `Copy` may not be implemented for this type

src/test/ui/e0119/issue-28981.stderr

@@ -5,7 +5,7 @@ LL | impl<Foo> Deref for Foo { } //~ ERROR must be used
    | ^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: conflicting implementation in crate `core`:
-           - impl<'a, T> std::ops::Deref for &'a T
+           - impl<'_, T> std::ops::Deref for &T
              where T: ?Sized;
 
 error[E0210]: type parameter `Foo` must be used as the type parameter for some local type (e.g. `MyStruct<Foo>`)