Validate all #[inline] attributes earlier

[jseyfried]

Today, the following compiles:

struct MyStruct<T>(T);

impl<T> MyStruct<T> {
    #[inline(foo)]
    pub fn f(self) {}
}

[jseyfried]

cc @SergioBenitez

[kennytm]

It does error when you actually instantiate MyStruct<T>::f.

struct MyStruct<T>(T);

impl<T> MyStruct<T> {
    #[inline(foo)]
    pub fn f(self) {}
}

fn main() {
    let f = MyStruct(4);
    f.f();  // <-- call this to raise error.
}

The problem is that the check is performed in syntax::attr::{requests_inline, find_inline_attr} which were called only during trans (and MIR inlining pass but it is disabled by default), so nothing will be checked if this is not monomorphized.

[nikomatsakis]

Closing in favor of #45738, which has mentoring instructions.