Re-entrancy

Where can re-entrancy occur? Turns out that with global singletons it can occur about anywhere; you don't need interrupts (preemption) to cause re-entrancy. See below:

#![allow(unused)]
fn main() {
extern crate defmt;
let x = 0u8;
defmt::info!("The answer is {=?}!", x /*: Struct */);
}

As you have seen before this will first send the string index of "The answer is {=?}!" and then call x's Format::format method. The re-entrancy issue arises if the Format implementation calls a logging macro:

#![allow(unused)]
fn main() {
extern crate defmt;
struct X;
impl defmt::Format for X {
    fn format(&self, f: defmt::Formatter) {
        //           ^ this is a handle to the global logger
        defmt::info!("Hello!");
        // ..
    }
}
}

f is a handle to the global logger. The info! call inside the format method is trying to access the global logger again. If info! succeeds then you have two exclusive handles (Formatter) to the logger and that's UB. If info! uses a spinlock to access the logger then this will deadlock.