unused_parens false positive on braced macro calls

[LunaBorowska]

I tried this code:

macro_rules! x {
    {} => { "q" };
}

fn f() -> String {
    (x! {}.to_string())
}

fn main() {}

I expected to see this happen: No warning about unnecessary parens

Instead, this happened: A warning about unnecessary parens suggesting to remove parens. Attempting to actually follow the advice causes an error.

warning: unnecessary parentheses around block return value
 --> src/main.rs:6:5
  |
6 |     (x! {}.to_string())
  |     ^^^^^^^^^^^^^^^^^^^ help: remove these parentheses
  |
  = note: `#[warn(unused_parens)]` on by default

This can be worked around by using x!().to_string or (x! {}).to_string() instead, however this shouldn't be necessary as the original code is fine.

This happens since Rust 1.42.

Meta

rustc --version --verbose:

rustc 1.47.0 (18bf6b4f0 2020-10-07)
binary: rustc
commit-hash: 18bf6b4f01a6feaf7259ba7cdae58031af1b7b39
commit-date: 2020-10-07
host: x86_64-unknown-linux-gnu
release: 1.47.0
LLVM version: 11.0

[apiraino]

Assigning P-medium as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

[Aaron1011]

Note that the parenthesis are required due to the fact that we have special handling for macro invocations with curly braces:

rust/compiler/rustc_parse/src/parser/stmt.rs

Lines 108 to 111 in b2d115f

	let kind = if delim == token::Brace || self.token == token::Semi || self.token == token::Eof
	{
	StmtKind::MacCall(P(MacCallStmt { mac, style, attrs }))
	} else {

Without the parenthesis, we parse x!{} as a statement, not an expression. However, x!().to_string() (without any surrounding parenthesis) compiles.

[kawadakk]

The suggestion is valid as of #88690 (included in 1.57.0).