Minimal reproducer:

#[no_mangle] pub fn f1(v: [&mut usize; 1]) { for x in v { *x += 1; } }

Results in:

f1:  test rdi, rdi  je .LBB0_2  inc qword ptr [rdi] .LBB0_2:  ret

Rust's semantics don't allow v to ever be NULL! I don't know where the test rdi, rdi is coming from!

This regressed between 1.78 and 1.79.