False unused_braces lint warning when using non-proc macro

[jeekobu]

I tried this code:

#![allow(dead_code)]
#[derive(Debug)]
struct Foo;
#[derive(Debug)]
struct Bar;
enum SomeWrapperEnum {
    Foo(Foo),
    Bar(Bar),
}

macro_rules! match_all {
    ($on:expr, $with:ident, $body:tt, $($var:ident),*) => {
        match $on {
            $(
                SomeWrapperEnum::$var($with) => { $body },
            )*
        }
    }
}

macro_rules! dispatch {
    ($on:expr, $with:ident, $body:tt) => {
        match_all!($on, $with, $body, Foo, Bar)
    }
}

fn main() {
    let quz = SomeWrapperEnum::Foo(Foo);
    dispatch!(quz, inner, { println!("{:?}", inner) } );
}

I expected to see this happen: No warnings, or applying hints from warnings improves the code.

Instead, this happened: The unused_braces lint fired, and applying the hint breaks the code.

The hint:

29 |     dispatch!(quz, inner, { println!("{:?}", inner) } );
   |                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: remove these braces

If the braces are removed:

29 |     dispatch!(quz, inner, println!("{:?}", inner));
   |                                  ^ no rules expected this token in macro call

Note: Adding a semicolon after the println!() (and leaving the braces) results in no warning and the code still compiles.

I'm using stable, but reproduced on stable, debug, and nightly on play.rust-lang.org. The lint was added in #70081. I see #70717, but there are no explicit refs here. And I also see #70814, but these are not proc macros.

rustc --version --verbose:

rustc 1.44.0 (49cae5576 2020-06-01)
binary: rustc
commit-hash: 49cae55760da0a43428eba73abcb659bb70cf2e4
commit-date: 2020-06-01
host: x86_64-unknown-linux-gnu
release: 1.44.0
LLVM version: 9.0

[lcnr]

Also happens with unused_parens. Not sure how to fix this while still linting against

let _: Vec<i32> = vec![0; { 1 << 8 }];
let _: Vec<i32> = vec![0; (1 << 8)];

[camsteffen]

This seems to be fixed. The hint now points to the braces in the macro body which are indeed unnecessary IIUC.