constify Iterator, take IV

[fee1-dead]

Like its predecessors (#92433, #102225, #106541), this PR allows one to make Iterator implementations const, and thus enables the ability to have for loops in const contexts. I've also included constifying Option as IntoIterator, option::IntoIter as Iterator as a minimal dogfooding example.

But unlike its predecessors, it uses a new attribute (not unsound anymore!) that prevents any Iterator extension methods from being called. This is intentionally made minimal for an initial approval, the fun stuff like .fold, Range as Iterator could be done later.

cc @rust-lang/wg-const-eval, cc @oli-obk to review the compiler parts

cc #92476

[rustbot]

Some changes occurred in compiler/rustc_hir/src/attrs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred in compiler/rustc_passes/src/check_attr.rs

cc @jdonszelmann, @JonathanBrouwer

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann, @JonathanBrouwer

[rustbot]

r? @scottmcm

rustbot has assigned @scottmcm.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer