Bug: undefined behavior in PartialEq implementations

[leddoo]

Which crate is this about?

windows

Crate version

0.42.0 but doesn't matter, same on master

Summary

some implementations of PartialEq use memcmp. 1) this causes undefined behavior in structs with padding. 2) this puts needless stress on the optimizer (to remove the memcmp, which is worse for small structs).

here's DWRITE_SCRIPT_ANALYSIS, which is where i discovered the issue: it caused non-determinism in my program. replacing the use of eq with field-wise comparison solved the issue (i verified that field-wise comparison & eq disagreed).

pub struct DWRITE_SCRIPT_SHAPES(pub u32);

pub struct DWRITE_SCRIPT_ANALYSIS {
    pub script: u16,
    // undefined_padding: u16, !!!
    pub shapes: DWRITE_SCRIPT_SHAPES,
}

impl ::core::cmp::PartialEq for DWRITE_SCRIPT_ANALYSIS {
    fn eq(&self, other: &Self) -> bool {
        unsafe { ::windows::core::memcmp(self as *const _ as _, other as *const _ as _, core::mem::size_of::<DWRITE_SCRIPT_ANALYSIS>()) == 0 }
    }
}

Toolchain version/configuration

No response

Reproducible example

No response

Crate manifest

No response

Expected behavior

No response

Actual behavior

No response

Additional comments

No response

[kennykerr]

Yes that's a problem - I'll get that fixed up.

[kennykerr]

Here you go: https://github.com/microsoft/windows-rs/pull/2168

Thanks for reporting!