Can't parse type defined inside unnamed constant

[davidpdrsn]

For proc-macros is quite common to put the generated code inside unnamed constants, to not pollute the surrounding namespace. However the name of types defined inside such constants cannot be parsed by syn::Type:

use std::any::type_name;
use syn::Type;

trait A {
    type T;
}

struct Foo;

const _: () = {
    struct Bar;

    impl A for Foo {
        type T = Bar;
    }
};

fn main() {
    syn::parse_str::<Type>(type_name::<<Foo as A>::T>()).unwrap();
}

This panics with

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error("expected identifier")', src/main.rs:6:58
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The value of type_name::<<Foo as A>::T>() is foo::_::Bar (where foo is the crate), so maybe syn doesn't support ::_::. I hadn't seen that before myself either 😅

[dtolnay]

This is behaving correctly as far as I can tell.