Trailing semicolon silently ignored in expr macro

0c0aa63
Opened by mahkoh at 2020-12-08 02:06:28
fn main() {
    macro_rules! a {
        ($e:expr) => { $e; }
    }
    a!(true)
}

Error: Expected (), found bool

But:

fn main() {
    macro_rules! a {
        ($e:expr) => { $e; }
    }
    a!(true);  // <-- semicolon
}

does not warn that the ; is ignored.

So either

  • there should be a warning that the ; has no effect in the macro, or
  • the ; inside the macro should turn the expression into ().

  1. I'm not sure there should be a warning for use of a semicolon in an expression expansion. The problem is fixed when you write ($e: expr) => {{ $e; }} which indicates a block expansion.

    So the semicolon is silently ignored, but putting something after it throws a helpful error: ($e:expr) => { $e; blahblah } gives the error "macro expansion ignores token blahblah and any following".

    Michael Rosenberg at 2016-05-30 05:35:30

  2. cc me

    Jeffrey Seyfried at 2016-06-10 04:10:41

  3. Progress towards fixing this issue is tracked in https://github.com/rust-lang/rust/issues/79813

    Aaron Hill at 2020-12-08 02:06:28