Spurious "this expression will panic at runtime" error (regression)

[alexcrichton]

This code:

pub trait Foo {
    fn foo(self) -> u32;
}

impl<T> Foo for T {
    fn foo(self) -> u32 {
        fn bar<T>() { loop {} }
        bar::<T> as u32
    }
}

when compiled yields:

$ rustc +nightly foo.rs --crate-type lib
error: this expression will panic at runtime
 --> foo.rs:8:9
  |
8 |         bar::<T> as u32
  |         ^^^^^^^^^^^^^^^ a raw memory access tried to access part of a pointer value as raw bytes
  |
  = note: #[deny(const_err)] on by default

error: aborting due to previous error

but it compiles on stable/beta!

cc @oli-obk, you may know where this is coming from?

[oli-obk]

Yea I do. This is fallout from making sure 1/(false as usize) lints about division by zero.

While the lint message is nonsensical, we should be linting the truncation of addresses, should we not?

[alexcrichton]

True! I'd be fine with a warning to say "you should use usize to avoid losing data"