The `#[simd]` attribute has been deprecated since c8b6d5b2 back in 2015. Any nightly crates using it have had ample time to switch to `#[repr(simd)]`, and if they didn't they're likely broken by now anyway.

The existing flags did not consider `'static` to be "free". This then
fed into what was "erasable" -- but `'static` is most certainly
erasable.

Nested generics (aka method generics) in trait methods don't have an
*additional* Self parameter in their own type parameter list (they have
a Self parameter in the parent generics), so don't try to check we're
correctly adjusting for it.

Fixes #46568.

Previously the code was somewhat duplicated.

The closure kinds themselves are now completely found in the `ClosureSubsts`.

Similar to how freshen handled things, but "always happening"; we can
thus remove the corresponding code from `freshen`.

Instead, just search the param env predicates directly. This is
equivalent to what we were doing before but more efficient.

This new way is **slightly** less expressive (I would be shocked if it
affects any code, though) when it comes to higher-ranked bounds or a
few other weird tricks. But we don't handle those consistently
regardless, and the new way does not require normalization and is just
wildly simpler.

DefaultImpl is a highly confusing name for what we now call auto impls,
as in `impl Send for ..`. The name auto impl is not formally decided
but for sanity anything is better than `DefaultImpl` which refers
neither to `default impl` nor to `impl Default`.

This custom tracking is now replaced by the red/green algorithm.

Fix https://github.com/rust-lang/rust/issues/45471

The cache was broken anyhow and this computation doesn't look that
expensive. These public accessors could potentially become queries,
but we'd have to add some more complex logic around lift. I'd prefer
to have some test cases to profile with before doing that.

Fixes #44402.