Add regression tests for #23649

[frewsxcv]

Closes #23649

[rust-highfive]

r? @alexcrichton

(rust_highfive has picked a reviewer for you, use r? to override)

[arielb1]

I would prefer a test that the generated code is correct, e.g.

use std::mem;

pub struct X([u8]);

fn _f(x: &X) -> usize { match *x { X(ref x) =>  { x.len() } } }

fn main() {
    let b: &[u8] = &[11; 42];
    let v: &X = unsafe { mem::transmute(b) };
    assert_eq!(_f(v), 42);
}

Also, use assert_eq! instead of println!.

[frewsxcv]

I would prefer a test that the generated code is correct

@arielb1 what do you mean by this?

[frewsxcv]

@arielb1 I'm just creating a regression test. The code that I have committed is the exact code that previous caused an ICE. Altering it could result in a no longer accurate regression test.

[alexcrichton]

@frewsxcv could you add the code that @arielb1 posted as well to a test case here? Other than that this looks good to go, thanks!

[frewsxcv]

Done

I altered src/test/run-pass/issue-23649-1.rs

[alexcrichton]

@bors: r+ 68bb164

[bors]

⌛ Testing commit 68bb164 with merge dd2151c...

[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-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt