What is this + (plus) in trait bounds?

Explain this line of Rust code (what is +?):

pub struct BlinkAlloc<A: Allocator = +Global> {

>Solution :

I think you are looking at this:

pub struct BlinkAlloc<A: Allocator = +Global> {
    arena: ArenaLocal,
    allocator: A,
}

This is not valid Rust syntax… usually. But this struct definition is actually inside a macro call.

This crate uses a complex system of macros, which I didn’t spend the time to understand. It also doesn’t document the supported syntax, so I would only be guessing if I gave any interpretation for what it actually means.