Symbol-mangling (v0) should show difference between async / closure / generator

[Swatinem]

The following code:

pub async fn fn_with_nested_block() -> Backtrace {
    None.unwrap_or_else(|| async { Backtrace::force_capture() })
        .await
}

Generates the following demangled symbol with legacy demangling:

async_codegen::fn_with_nested_block::{{closure}}::{{closure}}::{{closure}}

(v0 mangling is similar, but with {closure#0} instead)

Whereas Windows mangling gives this fn name:

async_codegen::fn_with_nested_block::async_fn$0::closure$0::async_block$0

This makes it clear what is an async fn, closure or async block. v0 symbol mangling should make a similar distinction.

#104333 is a first attempt to thread the necessary information through to the place that does the symbol mangling, but actually extending the v0 symbol mangling format / spec is necessary as well to fully solve this.

[jyn514]

v0 is a public spec that is already been implemented by other tools (e.g. gdb). is this proposed change backwards compatible? If not, is it enough to extend the tools or does there need to be a new v1 format?