-Znext-solver slightly strenghten deduce closure sig

[lcnr]

doesn't fully fix rust-lang/trait-system-refactor-initiative#191 due to remaining issues in rust-lang/trait-system-refactor-initiative#8. Notably, the following still breaks when switching solvers and would rely on actual eager norm to fix

trait Ref<'a, F> {
    type Input;
}

impl<'a, F> Ref<'a, F> for u32 {
    type Input = &'a u32;
}

fn needs_super<F: for<'a> Fn(<u32 as Ref<'a, F>>::Input)>(_: F) {}

fn main() {
    needs_super(|_| {});
}

this currently breaks

trait Trait {
    type Assoc;
}

struct W<T>(T);
impl<T: Trait> Trait for W<T>
where
    u32: Trait<Assoc = T::Assoc>,
{
    type Assoc = T;
}

impl<T> Trait for (T,) {
    type Assoc = T;
}

impl Trait for u32 {
    type Assoc = u32;
}

fn foo<T: Trait>(_: impl FnOnce(T::Assoc)) {}

fn main() {
    foo::<W<_>>(|(field,)| { let _ = field.count_ones(); })
}

r? @BoxyUwU

[rustbot]

changes to the core type system

cc @compiler-errors, @lcnr

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor