The `slice::chunks_exact` iterator does not optimize when the initial slice is non-empty

I tried this code:

use std::hint::black_box;

pub fn demo(data: &[u8]) {
    assert!(!data.is_empty());

    let mut chunks = data.chunks_exact(1024);

    let last_block = if chunks.remainder().is_empty() {
        chunks.next_back().expect("this is impossible")
    } else {
        chunks.remainder()
    };

    black_box(last_block);
}

When compiled with optimizations enabled and viewing the assembly code, I expected to not see a reference to the string "this is impossible". Since there's an assert that the input data is non-empty, then either there will be a remainder, there will be at least one chunk, or both.

Instead, the assembly contains a reference to the expect string and there's a conditional call to expect_failed.

Meta

• 1.79.0
• 1.81.0-nightly (2024-07-12 c6727fc9b5c64cefa726)

Start by compiling the provided chunks_exact example with optimizations enabled and inspecting its assembly, then trace the relevant slice::chunks_exact implementation. The fix is complete when the impossible expect string and conditional expect_failed call are no longer present for the asserted non-empty input, with regression coverage for this case.