References are compared by pointer value, not according to their `PartialEq`/`PartialOrd` instances

[narpfel]

Summary

E. g. &str should be compared lexicographically, &u32 by dereferencing, etc. Trait objects (&dyn T) should not be comparable at all unless PartialEq or PartialOrd (depending on which comparison operator is used) is implemented for dyn T.

Reproducer

I tried this code:

#![feature(no_core)]
#![no_core]

#[inline(never)]
pub fn lt_ref(x: &u32, y: &u32) -> bool {
    x < y
}

#[inline(never)]
pub fn eq_str(s: &str) -> bool {
    s == "hello world"
}

trait T {}

#[inline(never)]
pub fn eq_trait_object(a: &dyn T, b: &dyn T) -> bool {
    a == b
}

pub fn main() -> i32 {
    let x = &42;
    let y = &27;
    lt_ref(x, y) as i32
}

Does the code make use of any (1.49) nightly feature ?

• Nightly

Godbolt link

https://godbolt.org/z/qqvhTarz5

Actual behavior

The current behaviour is that comparisons on references are miscompiled.

• lt_ref compiles to a cmp rdi, rsi, which compares the numerical pointer values of x and y.
• eq_str is compiled to a componentwise comparison of the two components in the &str fat pointer: the pointer part and the length metadata.
• eq_trait_object is compiled to a comparison of only the pointer part of the fat pointer, the vtable metadata is ignored.
• main returns 1 because x has a smaller address than y.

Expected behavior

I expected to see that the code is compiled correctly (or not at all in the case of eq_trait_object because of the missing PartialEq impl).

GCC Version

679aad3