Rustfmt break macros which match on empty angle brackets

[JulianKnodt]

Summary

If a macro needs empty angle brackets after an ident, rustfmt may end up deleting them.

I tried to format this code:

macro_rules! example {
    ($func: ident <>) => {{
        $func();
    }};
}

fn log() {
    println!("Hi");
}

fn main() {
    example!(log<>);
}

Expected behavior

The angle brackets should remain.

Actual behavior

The angle brackets are deleted, leading the code to no longer compile after formatting.

Configuration

Reproduces on the playground.

Reproduction Steps

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=4787f0b7d4242108cd3982d5c7748ba1

Meta

I checked if this bug was reported previously by looking for "angle brackets", and I didn't see any similar threads.

[jieyouxu]

There's some funny behavior related to macro_rules! matchers, empty generics and turbofish... Some more cases:

Case 1

macro_rules! m { ($f:ident <>) => { $f(); } }
fn log() {}
fn main() {
    m!(log<>);
}

Formats to compile-fail: m!(log);

Case 2

macro_rules! m { ($f:ident :: <>) => { $f(); } }
fn log() {}
fn main() {
    m!(log::<>);
}

Formats to compile-fail: m!(log);

Case 3

macro_rules! m { ($f:ident :: <> ($($a:tt)*)) => {} }
fn main() {
    m!(S::<>());
}

Formats to compile-fail: m!(S());

Case 4

macro_rules! m { ($m:ident :: $n:ident <>) => {} }
fn main() {
    m!(N::Foo<>);
}

Formats to compile-fail: m!(N::Foo);

Case 5

macro_rules! m { ($f:ident <>) => {} }
fn main() {
    m!(Foo< >);
}

Formats to compile-fail: m!(Foo);

Case 6

macro_rules! m { ($a:ident <>, $b:ident <>) => {} }
fn main() {
    m!(log<>, foo<>);
}

Formats to compile-fail: m!(log, foo);

Case 7

macro_rules! m { ($a:ident as $t:ident <>) => {} }
fn main() {
    m!(a as Foo<>);
}

Formats to compile-fail: m!(a as Foo);