False positive unused_braces when calling a macro

[peter-lyons-kehl]

#![allow(incomplete_features)]
#![feature(generic_const_exprs)]

macro_rules! co_vec_default {
    () => {
        1
    }
}

struct V<const U: usize>
where
    [(); U]:;

trait Tr {}

impl Tr for V<{co_vec_default!()}> {}

Playground: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=058b224b06eefb23132dda5fad6f2950

The above generates a warning:

Compiling playground v0.0.1 (/playground)
warning: unnecessary braces around const expression
  --> src/lib.rs:16:15
   |
16 | impl Tr for V<{co_vec_default!()}> {}
   |               ^                 ^
   |
   = note: `#[warn(unused_braces)]` on by default
help: remove these braces

That above warning is either

• false positive, or
• directing a developer to an unimplemented rustc functionality or error, because without the braces it produces a (confusing) error. The same example code adjusted:

#![allow(incomplete_features)]
#![feature(generic_const_exprs)]

macro_rules! co_vec_default {
    () => {
        1
    }
}

struct V<const U: usize>
where
    [(); U]:;

trait Tr {}

impl Tr for V<co_vec_default!()> {}

Limited playground (I don't see a way to pass -Z macro-backtrace to the playground):
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=8b334f459ac48f56639de94f116a7705

The current output (with nightly invoked with RUSTFLAGS="-Z macro-backtrace" cargo build) is:

Compiling co_vec_macro v0.1.0 (/tmp/co_vec_macro)
error: expected type, found `1`
  --> src/lib.rs:6:9
   |
4  | / macro_rules! co_vec_default {
5  | |     () => {
6  | |         1
   | |         ^ expected type
7  | |     }
8  | | }
   | |_- in this expansion of `co_vec_default!`
...
16 |   impl Tr for V<co_vec_default!()> {}
   |                 -----------------
   |                 |
   |                 this macro call doesn't expand to a type
   |                 in this macro invocation

error[E0747]: type provided when a constant was expected
  --> src/lib.rs:16:15
   |
16 | impl Tr for V<co_vec_default!()> {}
   |               ^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0747`.

Thank you in advance for any progress.