Auto merge of #59527 - matklad:sized-index, r=<try>

remove ?Sized bounds from Index

I've noticed that we have an `Idx: ?Sized` bound on the **index** in the `Index`, which seems strange given that we accept index by value. My guess is that it was meant to be removed in #23601, but was overlooked.

If I remove this bound, `./x.py src/libstd/ src/libcore/` passes, which means at least that this is not covered by test.

I think there's three things we can do here:

* run crater with the bound removed to check if there are any regressions, and merge this, to be consistent with other operator traits
* run crater, get regressions, write a test for this with a note that "hey, we tried to fix it, its unfixable"
* decide, in the light of by-value DSTs, that this is a feature rather than a bug, and add a test

cc @rust-lang/libs

EDIT: the forth alternative is that there exist a genuine reason why this is the case, but I failed to see it :D

src/libcore/ops/index.rs

@@ -58,7 +58,7 @@
 #[doc(alias = "]")]
 #[doc(alias = "[")]
 #[doc(alias = "[]")]
-pub trait Index<Idx: ?Sized> {
+pub trait Index<Idx> {
     /// The returned type after indexing.
     #[stable(feature = "rust1", since = "1.0.0")]
     type Output: ?Sized;
@@ -163,7 +163,7 @@ see chapter in The Book <https://doc.rust-lang.org/book/ch08-02-strings.html#ind
 #[doc(alias = "[")]
 #[doc(alias = "]")]
 #[doc(alias = "[]")]
-pub trait IndexMut<Idx: ?Sized>: Index<Idx> {
+pub trait IndexMut<Idx>: Index<Idx> {
     /// Performs the mutable indexing (`container[index]`) operation.
     #[stable(feature = "rust1", since = "1.0.0")]
     fn index_mut(&mut self, index: Idx) -> &mut Self::Output;