Assertion failed while type-casting

[jethrogb]

While trying to write an offsetof that works in a constant expr, the compiler gets a low-level assertion failure.

struct Foo {
    a: u64,
    b: u32,
    c: u8
}

macro_rules! offset_of {
  ($t:ty, $field:ident) => {
    unsafe {
      (&(*(0 as *const $t)).$field) as *const _ as *const _
    }
  }
}

const OFFSET_OF_C: *const () = offset_of!(Foo, c);

fn main() {
  println!("offset of c: {}" , OFFSET_OF_C as usize);
}

With rust-nightly 2015011204074bed1e8trusty:

$rustc test.rs 
test.rs:2:5: 2:11 warning: struct field is never used: `a`, #[warn(dead_code)] on by default
test.rs:2     a: u64,
              ^~~~~~
test.rs:3:5: 3:11 warning: struct field is never used: `b`, #[warn(dead_code)] on by default
test.rs:3     b: u32,
              ^~~~~~
rustc: /build/buildd/rust-nightly-201501120407~4bed1e8~trusty/src/llvm/include/llvm/Support/Casting.h:237: typename llvm::cast_retty<X, Y*>::ret_type llvm::cast(Y*) [with X = llvm::GlobalVariable; Y = llvm::Value; typename llvm::cast_retty<X, Y*>::ret_type = llvm::GlobalVariable*]: Assertion `isa<X>(Val) && "cast<Ty>() argument of incompatible type!"' failed.
Aborted

Via @edef1c

[tamird]

Triage: still ICEs. Reproduces without using macros:

struct Foo;

const OFFSET_OF_C: *const () = unsafe {
    &*(0 as *const Foo)as *const _ as *const _
};

fn main() {
    println!("offset of c {}" , OFFSET_OF_C as usize)
}

$ rustc main.rs
main.rs:8:33: 8:44 error: internal compiler error: constant expression should not reach expr::trans_def
main.rs:8     println!("offset of c {}" , OFFSET_OF_C as usize)

[IvanUkhov]

Here is another example:

const TAG: &'static [u8] = b"ABCD";

macro_rules! tag(
    ($value:expr) => (unsafe {
        let value: [u8; 4] = ::std::mem::transmute($value);
        &vec![value[3], value[2], value[1], value[0]][..]
    })
);

fn main() {
    match tag!(42) {
        TAG => println!("Yes."),
        _ => println!("No."),
    }
}

Assertion failed: (isa<X>(Val) && "cast<Ty>() argument of incompatible type!"), function cast, file /Users/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-mac/build/src/llvm/include/llvm/Support/Casting.h, line 237.

Regards,
Ivan

[arielb1]

Simpler:

const TAG: &'static [u8] = b"ABCD";

fn main() {
    match &[][..] {
        TAG => println!("Yes."),
        _ => println!("No."),
    }
}

Seems like a missing slice-patterns check.

[jethrogb]

The first two examples no longer ICE. The last two still do (SIGILL on stable, LLVM assertion failed on nightly).

[jonas-schievink]

Still ICEs on rustc 1.11.0-nightly (34505e222 2016-06-08), but not with -Zorbit, so it looks like this will be fixed with MIR trans

[DemiMarie]

Is this undefined behavior?

[steveklabnik]

This ICE seems to be fixed on

rustc 1.12.0-nightly (b30eff7ba 2016-08-05)

I have a hunch that this is due to MIR being turned on, though I'm not sure. Please let me know if this still ICEs for you, but closing for now 🎊