"< is interpreted as a start of generic arguments for '_', not a comparison"

[ghost]

Playground link: https://play.rust-lang.org/?gist=e93b00dd6c33bf6bbcc035a7be501592&version=stable

macro_rules! foo {
    (
        $(
            bar: {
                $($rest:tt)+
            }
        )+
    ) => {
        $(
            bar($($rest)+);
        )+
    };
}

macro_rules! bar {
    (baz: $baz:expr,) => {};
}

fn main() {
    foo!{
        bar: {
            baz: true,
        }
    }
}

Compiling playground v0.0.1 (file:///playground)
error: expected identifier, found keyword `true`
  --> src/main.rs:24:18
   |
24 |             baz: true,
   |                  ^^^^

error: `<` is interpreted as a start of generic arguments for `true`, not a comparison
  --> src/main.rs:24:22
   |
24 |             baz: true,
   |                      -
   |                      |
   |                      not interpreted as comparison
   |                      interpreted as generic arguments
   |
help: if you want to compare the casted value then write:
   |
24 |             (baz: true),
   |             ^^^^^^^^^^^

error: aborting due to 2 previous errors

error: Could not compile `playground`.

To learn more, run the command again with --verbose.

[kennytm]

I guess the problem is treating true as the name of a type. Simplified:

macro_rules! foo {
    ($rest:tt) => {
        bar(baz: $rest)
    }
}
fn main() {
    foo!(true);
}

Look at this mess

Standard Error

   Compiling playground v0.0.1 (file:///playground)
error: expected identifier, found keyword `true`
 --> src/main.rs:8:10
  |
8 |     foo!(true);
  |          ^^^^

error: `<` is interpreted as a start of generic arguments for `true`, not a comparison
 --> src/main.rs:3:23
  |
3 |           bar(baz: $rest)
  |                         -
  |                         |
  |  _______________________not interpreted as comparison
  | |
4 | |     }
5 | | }
6 | |
7 | | fn main() {
8 | |     foo!(true);
  | |     -----------
  | |_____|_______|
  |       |       interpreted as generic arguments
  |       in this macro invocation
  |
help: if you want to compare the casted value then write:
  |
3 |         bar((baz: $rest)
4 |     }
5 | }
6 | 
7 | fn main() {
8 |     foo!(true));
  |

error: aborting due to 2 previous errors

error: Could not compile `playground`.

To learn more, run the command again with --verbose.

cc @petrochenkov #43060

[petrochenkov]

Thanks!
#43060 does some shady things.
I tried to come up with an example where it would fail, but without success, now here it is!