> unchecked_add to explicitly opt-in to a C-like "just do whatever the machine feels like".

Just like for signed integers in C the {integer}::unchecked_add is Undefined Behaviour if overflow occurs. So that's not "whatever the machine feels like" it's much worse.

This is an unsafe function, so you're expected to write a SAFETY rationale for the unsafe call, and that's your opportunity to justify to a future maintainer why you were sure this addition never overflows. If you find yourself struggling to justify that belief it probably means you should not have used this function.

If you're going to write guard code for overflows, and use that to justify the SAFETY claim, why not use the existing functions for that? Clippy will suggest this in many cases because it's less likely you will blow your own foot off.