It's surprisingly difficult to write something where the compiler doesn't detect that it's working on a constant. I had to code up a loop that pushed values onto the vector before the compiler generated the general case. "black_box" is not, apparently, opaque to the optimizer.

Ideomatic form, inner loop: [1]

    LBB4_3:
        cmp     byte ptr [rax + rcx], 0
        jne     .LBB4_4
        inc     rcx
        cmp     rdx, rcx
        jne     .LBB4_3

C-like form, inner loop: [2]

    .LBB4_3:
        cmp     byte ptr [rax + rcx], 0
        jne     .LBB4_4
        inc     rcx
        cmp     rdx, rcx
        jne     .LBB4_3

Both ways, the compiler found the optimal solution. In the C-like loop, it was able to hoist and then elide the subscript check. In the ideomatic form, it's implicit in the iterator that you won't go off the end of the array. The lambda function was expanded inline and optimized. There's zero call overhead from using a lambda function there.

Very nice work by the Rust compiler people and LLVM.

[1] https://godbolt.org/z/5hn7Y9Mf9

[2] https://godbolt.org/z/rW4MG4v3c