Rollup merge of #146533 - Zalathar:array-default, r=compiler-errors

Note some previous attempts to change the Default impl for `[T; 0]`

Recently, #145457 experimented with changing the Default impl for `[T; 0]`.

Subsequently, #146531 also aimed to perform a similar experiment.

It seems like a good idea to add some links to the relevant source code, so that the historical context of this tricky topic is easier to find.

Author: matthiaskrgr

library/core/src/array/mod.rs

@@ -472,6 +472,11 @@ impl<T: Copy> SpecArrayClone for T {
 // The Default impls cannot be done with const generics because `[T; 0]` doesn't
 // require Default to be implemented, and having different impl blocks for
 // different numbers isn't supported yet.
+//
+// Trying to improve the `[T; 0]` situation has proven to be difficult.
+// Please see these issues for more context on past attempts and crater runs:
+// - https://github.com/rust-lang/rust/issues/61415
+// - https://github.com/rust-lang/rust/pull/145457
 
 macro_rules! array_impl_default {
     {$n:expr, $t:ident $($ts:ident)*} => {