While it's an internal function, it is easy to create invalid Arc/Rcs to
a dangling pointer with it.

Fixes rust-lang#89740

Helps avoid rightward drift.

…value"

This reverts commit 0a2b7d7, reversing
changes made to 47c1bd1.
This caused several unforeseen problems:
- rust-lang#91029
- rust-lang#89764 (comment)

…=Mark-Simulacrum

Mark `Arc::from_inner` / `Rc::from_inner` as unsafe

While it's an internal function, it is easy to create invalid Arc/Rcs to
a dangling pointer with it.

Fixes rust-lang#89740

Adopt let_else in more places in rustc_mir_build

Helps avoid rightward drift.

followup of rust-lang#89933

…ebank

Suggest `await` in more situations where infer types are involved

Currently we use `TyS::same_type` in diagnostics that suggest adding `.await` to opaque future types.

This change makes the suggestion slightly more general, when we're comparing types like `Result<T, E>` and `Result<_, _>` which happens sometimes in places like `match` patterns or `let` statements with partially-elaborated types.

----

Question:
1. Is this change worthwhile? Totally fine if it doesn't make sense adding.
2. Should `same_type_modulo_infer` live in `rustc_infer::infer::error_reporting` or alongside the other method in `rustc_middle::ty::util`?
3. Should we generalize this change? I wanted to change all usages, but I don't want erroneous suggestions when adding `.field_name`...

Revert "require full validity when determining the discriminant of a value"

This reverts commit 0a2b7d7, reversing
changes made to 47c1bd1.
This caused several unforeseen problems:
- rust-lang#91029
- rust-lang#89764 (comment)

So I think it's best to revert for now while we keep discussing the MIR semantics of getting a discriminant.

r? `@oli-obk`