Check types in method signatures

[arielb1]

We don't check that types in method signatures are well-formed, or even when they are Sized. I mean, this compiles:

pub struct U<T: Send+Sync+Copy+::std::fmt::Display>(*const T);

pub trait Trait { }

pub trait OtherTrait {
    fn f(self, u: U<Trait>);
    fn g(self, u: [u8]);
}

impl OtherTrait for () {
    fn f(self, u: U<Trait>) { }
    fn g(self, ref u: [u8]) {}
}

fn main () {}

[arielb1]

Fixing this seems 1) important but 2) kind-of backward-compatibility breaking

[apasel422]

Related to #19182.

[nikomatsakis]

We intentionally chose not to check that argument types are Sized in trait definitions. We do check it when the method is implemented, and the full type is known, and we check that values we actually pass as arguments are sized. (Moreover, at least for now, if two types are related by subtyping, then either both are sized or neither is.) So I don't think that's a big problem. But I agree we need to cleanup some corner cases of WF.