ARM Big Endian partialEq bug

At opt-level=0, derived PartialEq on enums produces incorrect results on armebv7r-none-eabi (big-endian) when built with -Z build-std=core. The comparison returns true for non-equal values. opt-level≥1 or manual match-based PartialEq works correctly.
#[repr(u8)] does not mitigate it.

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
#[repr(u8)]
pub enum MyEnum { A, B, C, D }

let a = MyEnum::A;

// BUG: returns true at opt-level=0 on armebv7r-none-eabi
if a == MyEnum::B {
    // reached
}

// OK: match always works
match a {
    MyEnum::A => { /* reached correctly */ }
    MyEnum::B => { /* not reached correctly */ }
    _ => {}
}

Minimal error reproduction's repo:

https://github.com/NielsOuvrard/rust-arm-be-partialeq-bug

Workaround

Set opt-level = 1 in Cargo.toml

Start with the minimal reproduction repository and build it for armebv7r-none-eabi with opt-level=0, then compare the derived PartialEq behavior with opt-level≥1 and the manual match workaround. Trace the Rust compiler path that lowers derived enum comparisons on big-endian targets; done means non-equal enum values compare unequal at opt-level=0 and the behavior has regression coverage.