If in the previous section you compiled the program in release mode and actually looked at the disassembly, you probably noticed that the delay function is optimized away and never gets called from within main.

LLVM decided that the function wasn't doing anything worthwhile and just removed it.

There is a way to prevent LLVM from optimizing the for loop delay: add a volatile assembly instruction. Any instruction will do but NOP (No OPeration) is a particular good choice in this case because it has no side effect.

Your for loop delay would become:

# #![allow(unused_variables)]
#fn main() {
fn delay(_tim6: &tim6::RegisterBlock, ms: u16) {
    const K: u16 = 3; // this value needs to be tweaked
    for _ in 0..(K * ms) {

And this time delay won't be compiled away by LLVM when you compile your program in release mode:

$ cargo build --release
$ arm-none-eabi-objdump -Cd target/thumbv7em-none-eabihf/release/clocks-and-timers
08000548 <clocks_and_timers::delay>:
 8000548:       2000            movs    r0, #0
 800054a:       3001            adds    r0, #1
 800054c:       bf00            nop
 800054e:       b281            uxth    r1, r0
 8000550:       2996            cmp     r1, #150        ; 0x96
 8000552:       d3fa            bcc.n   800054a <clocks_and_timers::delay+0x2>
 8000554:       4770            bx      lr

Now, test this: Compile the program in debug mode and run it, then compile the program in release mode and run it. What's the difference between them? What do you think is the main cause of the difference? Can you think of a way to make them equivalent or at least more similar again?