False positive in nonstandard_macro_braces involving macros

[dtolnay]

// src/main.rs

#[derive(testing::Derive)]
pub struct S;

fn main() {}

// src/lib.rs

use proc_macro::TokenStream;

#[proc_macro_derive(Derive)]
pub fn derive(input: TokenStream) -> TokenStream {
    let _ = input;
    "fn _f() -> Vec<u8> { vec![] }".parse().unwrap()
}

$ cargo clippy
warning: use of irregular braces for `vec!` macro
 --> src/main.rs:1:10
  |
1 | #[derive(testing::Derive)]
  |          ^^^^^^^^^^^^^^^
  |
  = note: `#[warn(clippy::nonstandard_macro_braces)]` on by default
help: consider writing `testing::Derive`
 --> src/main.rs:1:10
  |
1 | #[derive(testing::Derive)]
  |          ^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#nonstandard_macro_braces
  = note: this warning originates in the derive macro `testing::Derive` (in Nightly builds, run with -Z macro-backtrace for more info)

vec![] seems like the standard braces. The consider writing `testing::Derive`​ doesn't make sense.

@DevinR528 @llogiq

Meta

• cargo clippy -V: clippy 0.1.55 (798baeb 2021-07-02)
• rustc -Vv:

  rustc 1.55.0-nightly (798baebde 2021-07-02)
  binary: rustc
  commit-hash: 798baebde1fe77e5a660490ec64e727a5d79970d
  commit-date: 2021-07-02
  host: x86_64-unknown-linux-gnu
  release: 1.55.0-nightly
  LLVM version: 12.0.1
  

[DevinR528]

I'm looking into it, my guess is that it's finding the macro then comparing the snippet to macro_name!(/[/{ but what the snippet found is the outer macro so test::Derive and since it isn't the same it passes. I wonder if adding a check for the actual macro name in the snippet and bailing out if not found would work well enough?