Also stabilize constctlz for const ctlz_nonzero.

The public methods stabilized const by this commit are:

  * `{i*,u*}::checked_pow`
  * `{i*,u*}::saturating_pow`
  * `{i*,u*}::wrapping_pow`
  * `{i*,u*}::overflowing_pow`
  * `{i*,u*}::pow`
  * `u*::next_power_of_two`
  * `u*::checked_next_power_of_two`
  * `u*::wrapping_next_power_of_two` (the method itself is still unstable)

Same for constctlz.

plus a small edit for clarity

stabilize const_int_pow

This also requires stabilizing constctlz for const ctlz_nonzero.

…-schievink

MIR visitor: Don't treat debuginfo field access as a use of the struct

Fixes #77454.

r? `@jonas-schievink`

const_generics: assert resolve hack causes an error

prevent the min_const_generics `HACK`s in resolve from triggering a fallback path which successfully compiles so that we don't have to worry about future compat issues when removing it

r? `@eddyb` cc `@varkor`

Allow using generic trait methods in `const fn`

Next step for #67792, this now also allows code like the following:

```rust
struct S;

impl const PartialEq for S {
    fn eq(&self, _: &S) -> bool {
        true
    }
}

const fn equals_self<T: PartialEq>(t: &T) -> bool {
    *t == *t
}

pub const EQ: bool = equals_self(&S);
```

This works by threading const-ness of trait predicates through trait selection, in particular through `ParamCandidate`, and exposing it in the resulting `ImplSource`.

Since this change makes two bounds `T: Trait` and `T: ?const Trait` that only differ in their const-ness be treated like different bounds, candidate winnowing has been changed to drop the `?const` candidate in favor of the const candidate, to avoid ambiguities when both a const and a non-const bound is present.

Use Option::and_then instead of open-coding it

`@rustbot` modify labels to +C-cleanup.

Reduce boilerplate with the `?` operator

`@rustbot` modify labels to +C-cleanup.

Fix typo in comment

This is trivial enough I'm just going to merge without review.

r? `@ghost`

doc typo

plus a small edit for clarity

Use Option::map instead of open coding it

r?  `@jonas-schievink` since you're frequently sniping these minor cleanups anyway.
`@rustbot` modify labels +C-cleanup  +T-compiler

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