Macros with path fragments fail to expand if the path has a non-turbofish generic

[mrobakowski]

Environment

• IntelliJ Rust plugin version: 0.2.121.3059-201-stable
• Rust toolchain version: 1.45.0-nightly
• IDE name and version: CLion 2020.1
• Operating system: NixOS 20.03

Problem description

When you have a macro_rules! macro with a path fragment $x:path, the macro fails to expand if you pass a path containing a non-turbofish generic (i.e. Foo<T> instead of Foo::<T>) to it. I only checked with the experimental macro expander. I do not know how the classic one behaves.

It seems like the macro expander uses an incorrect grammar for the path fragments. According to the rust reference the :: before generic args should be optional.

Steps to reproduce

Minimal-ish example:

trait Foo<T> {}

macro_rules! foo {
    ($p:path) => {
        struct Bar<P: $p> {
            inner: P,
        }
    };
}

foo!(u32); // able to expand
foo!(Foo::<u32>); // able to expand
foo!(Foo<u32>); // ERROR: Failed to expand the macro