…r=eddyb

Always try to project predicates when finding auto traits in rustdoc

Fixes rust-lang#60726

Previous, AutoTraitFinder would only try to project predicates when the
predicate type contained an inference variable. When finding auto
traits, we only project to try to unify inference variables - we don't
otherwise learn any new information about the required bounds.

However, this lead to failing to properly generate a negative auto trait
impl (indicating that a type never implements a certain auto trait) in
the following unusual scenario:

In almost all cases, a type has an (implicit) negative impl of an auto
trait due some other type having an explicit *negative* impl of that
auto trait. For example:

struct MyType<T> {
    field: *const T
}

has an implicit 'impl<T> !Send for MyType<T>', due to the explicit
negative impl (in libcore) 'impl<T: ?Sized> !Send for *const T'.

However, as exposed by the 'abi_stable' crate, this isn't always the
case. This minimzed example shows how a type can never implement
'Send', due to a projection error:

```
pub struct True;
pub struct False;

pub trait MyTrait {
    type Project;
}

pub struct MyStruct<T> {
    field: T
}

impl MyTrait for u8 {
    type Project = False;
}

unsafe impl<T> Send for MyStruct<T>
    where T: MyTrait<Project=True> {}

pub struct Wrapper {
    inner: MyStruct<u8>
}
```

In this example, `<u8 as MyTrait>::Project == True'
must hold for 'MyStruct<u8>: Send' to hold.
However, '<u8 as MyTrait>::Project == False' holds instead

To properly account for this unusual case, we need to call
'poly_project_and_unify' on *all* predicates, not just those with
inference variables. This ensures that we catch the projection error
that occurs above, and don't incorrectly determine that 'Wrapper: Send'
holds.

Rollup of 8 pull requests

Successful merges:

 - #60300 (Allow null-pointer-optimized enums in FFI if their underlying representation is FFI safe)
 - #60773 (Always try to project predicates when finding auto traits in rustdoc)
 - #60809 (Add FAQ for NLL migration)
 - #61023 (Migrate from recursion to iterate on qualify consts visitor impl)
 - #61029 (Simplify RefCell minimum_spanning_tree example)
 - #61030 (Make maybe_codegen_consume_direct iterate instead of doing recursion)
 - #61034 (rustc_metadata: parametrize schema::CrateRoot by 'tcx and rip out old unused incremental infra.)
 - #61037 (Update clippy submodule)

Failed merges:

r? @ghost