Drop the dereferenceable attribute when a reference is passed in an enum

[dianqk]

Considering the following code, it should print 0 with a release build, but I get a segmentation violation.

#![feature(rustc_attrs)]
#![allow(internal_features)]
#![allow(dead_code)]

#[rustc_layout_scalar_valid_range_start(1)]
#[rustc_layout_scalar_valid_range_end(0xfffffffffffffff0)]
pub struct RestrictedAddress(&'static i8);

enum E {
    A(RestrictedAddress),
    B,
    C,
}

#[no_mangle]
#[inline(never)]
fn foo(a: E) -> i8 {
    match a {
        E::A(v) => *v.0,
        _ => 0,
    }
}

fn main() {
    println!("{}", foo(std::hint::black_box(E::B)));
}

This is because we are adding the dereferenceable attribute, which allows LLVM to hoist the load instruction. In this case, we need to drop the dereferenceable attribute.

@rustbot label +requires-nightly +I-miscompile +A-codegen

[dianqk]

@rustbot label -needs-triage

[RalfJung]

It's less about the niche and more about the enum. The logic that computes the attribute has to just stop at the enum.

An argument of type RestrictedAddress should still get the attribute.

[clubby789]

While looking into this I noticed that

#![feature(rustc_attrs)]
#![allow(internal_features)]
#![allow(dead_code)]

#[rustc_layout_scalar_valid_range_start(1)]
#[rustc_layout_scalar_valid_range_end(0xfffffffffffffff0)]
pub struct RestrictedAddress(&'static i8);

enum E {
    A(RestrictedAddress),
    B,
    C,
}

fn main() {
    std::hint::black_box(E::B);
}

causes a debug-only ICE (only allows integer types in const_uint_big) when codegenning the discriminant set.

[the8472]

causes a debug-only ICE (only allows integer types in const_uint_big) when codegenning the discriminant set.

#131698 should fix that

[RalfJung]

I have a fix for this up at #132745