Bad codegen with functions returning `Result`.

I tried this code:
https://godbolt.org/z/WGK9WTKec

type Result<T> = core::result::Result<T, u32>;
//                                       ^--- note: all other integer types compile as expected

fn map<T, F, U>(value: Result<T>, op: F) -> Result<U>
where 
    F: FnOnce(T) -> U
{
    value.map(op)
}

pub fn assign(val: [u8; 4], out: &mut u32) -> Result<()> {
    let val = map(Ok(val), u32::from_le_bytes)?;

    *out = val;

    Ok(())
}

I expected to see this:

example[91f12ae00899f36c]::assign:
        mov     dword ptr [rsi], edi
        xor     eax, eax
        ret

Instead, this happened:

example[91f12ae00899f36c]::assign:
        mov     eax, edi
        shr     eax, 24     ; elaborate no-op
        shl     edi, 8      ; elaborate no-op
        shrd    edi, eax, 8 ; elaborate no-op
                            ; edi = edi (when the call started)
        mov     dword ptr [rsi], edi
        xor     eax, eax
        ret

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rustc --version --verbose:

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