Rollup merge of #106541 - fee1-dead-contrib:no-const-check-no, r=thomcc

implement const iterator using `rustc_do_not_const_check`

Previous experiment: #102225.

Explanation: rather than making all default methods work under `const` all at once, this uses `rustc_do_not_const_check` as a workaround to "trick" the compiler to not run any checks on those other default methods. Any const implementations are only required to implement the `next` method. Any actual calls to the trait methods other than `next` will either error in compile time (at CTFE runs), or run the methods correctly if they do not have any non-const operations. This is extremely easy to maintain, remove, or improve.

library/core/src/lib.rs

@@ -194,6 +194,7 @@
 #![feature(cfg_target_has_atomic_equal_alignment)]
 #![feature(const_closures)]
 #![feature(const_fn_floating_point_arithmetic)]
+#![feature(const_for)]
 #![feature(const_mut_refs)]
 #![feature(const_precise_live_drops)]
 #![feature(const_refs_to_cell)]