The tiny bug was that `FnFlags::DEPRECTAED_SAFE_2024` and `FnFlags::RUSTC_ALLOW_INCOHERENT_IMPLS` were assigned the same value.
The catastrophic effect was that every function marked as `#[rustc_allow_incoherent_impl]` was considered safe-deprecated for edition 2024, which caused it to be considered unsafe to call when called from edition 2024. And that includes `<[_]>::into_vec()`, which is called by the `vec![]` macro. So, catastrophic effect.
This innocent-looking bug probably arose from the item tree rewrite. No review would've catch that!
This refactors how we deal with items in hir-def lowering.
- It now lowers all of them through an "ExpressionStore" (kind of a misnomer as this point) as their so called *Signatures.
- We now uniformly lower type AST into TypeRefs before type inference.
- Likewise, this moves macro expansion out of type inference, resulting in a single place where we do non-defmap macro expansion.
- Finally, this PR removes a lot of information from ItemTree, making the DefMap a lot less likely to be recomputed and have it only depend on actual early name resolution related information (not 100% true, we still have ADT fields in there but thats a follow up removal).
