Fix cast-related ICEs

[sanxiyn]

Fix #13993.
Fix #17167.

[rust-highfive]

r? @Aatch

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[alexcrichton]

cc @nrc

[nrc]

lgtm, r=me with the comments addressed

[sanxiyn]

Addressed review comments.

[arielb1]

I would also add a test for cast from a ?Sized type parameter:

fn nullptr<T: ?Sized>() -> *const T {
    0 as *const _
}
fn main() {
    nullptr::<[();3]>();
    nullptr::<[()]>();
}

Also, &_ as usize is a non-scalar cast anyway.

[arielb1]

There are more interesting cases:

use std::fmt;
use std::iter::IntoIterator;

fn ptr_cast<U: ?Sized, V: ?Sized>(u: *const U) -> *const V
{
   u as *const _
}

fn main() {
    let i0 = [5u32];
    let i1 = i0.into_iter();
    let i2 : &Iterator<Item=&u32> = &i1;
    let i3 = i2 as *const Iterator<Item=&u32>;
    let p: *const fmt::Display = ptr_cast::<_, fmt::Display>(i3);
}

I guess this needs a separate issue.

[nrc]

@bors r+ e2ff188

[bors]

⌛ Testing commit e2ff188 with merge 287a544...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-32-nopt-t, auto-win-32-opt, auto-win-64-nopt-t, auto-win-64-opt