Auto merge of #73819 - euclio:rustdoc-summaries, r=jyn514,GuillaumeGomez

rustdoc: do not use plain summary for trait impls

Fixes #38386.
Fixes #48332.
Fixes #49430.
Fixes #62741.
Fixes #73474.

Unfortunately this is not quite ready to go because the newly-working links trigger a bunch of linkcheck failures. The failures are tough to fix because the links are resolved relative to the implementor, which could be anywhere in the module hierarchy.

(In the current docs, these links end up rendering as uninterpreted markdown syntax, so I don't think these failures are any worse than the status quo. It might be acceptable to just add them to the linkchecker whitelist.)

Ideally this could be fixed with intra-doc links ~~but it isn't working for me: I am currently investigating if it's possible to solve it this way.~~ Opened #73829.

EDIT: This is now ready!

library/core/src/slice/mod.rs

@@ -3319,7 +3319,7 @@ pub unsafe trait SliceIndex<T: ?Sized>: private_slice_index::Sealed {
     /// Calling this method with an out-of-bounds index or a dangling `slice` pointer
     /// is *[undefined behavior]* even if the resulting reference is not used.
     ///
-    /// [undefined behavior]: ../../reference/behavior-considered-undefined.html
+    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     #[unstable(feature = "slice_index_methods", issue = "none")]
     unsafe fn get_unchecked(self, slice: *const T) -> *const Self::Output;
 
@@ -3328,7 +3328,7 @@ pub unsafe trait SliceIndex<T: ?Sized>: private_slice_index::Sealed {
     /// Calling this method with an out-of-bounds index or a dangling `slice` pointer
     /// is *[undefined behavior]* even if the resulting reference is not used.
     ///
-    /// [undefined behavior]: ../../reference/behavior-considered-undefined.html
+    /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
     #[unstable(feature = "slice_index_methods", issue = "none")]
     unsafe fn get_unchecked_mut(self, slice: *mut T) -> *mut Self::Output;