ctfe validation on unaligned references does not mention expected / provided alignment

[gnzlbg]

This code (playground):

fn main() {
    unsafe {
        use std::mem::transmute;
        let a = 0_u8;
        let b = &a as *const u8 as *const u64;
        //let _c: &u64 = transmute(b);
        let _d = *b;
    }
}

errors under miri with

tried to access memory with alignment 1, but alignment 8 is required

This snippet, however (playground):

fn main() {
    unsafe {
        use std::mem::transmute;
        let a = 0_u8;
        let b = &a as *const u8 as *const u64;
        let _c: &u64 = transmute(b);
        //let _d = *b;
    }
}

errors with

type validation failed: encountered unaligned reference

It would be great if this error about the validity of references would also state that "alignment 1, but alignment 8 is required" or similar.

cc @oli-obk @RalfJung

[oli-obk]

I think all we need is

rust/src/librustc_mir/interpret/validity.rs

Lines 360 to 361 in 0e25a68

	EvalErrorKind::AlignmentCheckFailed { .. } =>
	return validation_failure!("unaligned reference", self.path),

to actually use the fields of the AlignmentCheckFailed and add their information to the validation failure string.

[pmccarter]

I'd like to do it.

[oli-obk]

It's all your's, let me know if you have any questions

[pmccarter]

Hmm both of these snippets seem to compile in the playground (when unused use removed).

[gnzlbg]

Yes the examples compile fine. The issue is triggered by running them under miri.

[oli-obk]

You can probably repro the same thing in rustc with const eval by making the final value be such an invalid reference

[oli-obk]

Jup, here's a repro (that you can use as a regression test, too):

#![feature(const_transmute)]
const FOO: &u64 = unsafe {
    use std::mem::transmute;
    let a = &0_u8;
    let b = a as *const u8 as *const u64;
    transmute(b)
};

[hellow554]

Note, that clippy does emit an error:

error: casting from *const u8 to a more-strictly-aligned pointer (*const u64)

[gnzlbg]

@hellow554 thanks, i've reported the issue there tiL rust-lang/rust-clippy#3797