The other replies explained the situation, but for the curious here's Rust's basic code for slice iteration: https://github.com/rust-lang/rust/blob/master/src/l

The other replies explained the situation, but for the curious here's Rust's basic code for slice iteration: https://github.com/rust-lang/rust/blob/master/src/libcore/slice.rs#L1221 https://github.com/rust-lang/rust/blob/master/src/libcore/sl... (unfortunately buried in some macros). `if self.ptr == self.end` is where the guarantee is made, then the offsets are done unsafely. The other branch in that function is optimized away statically.