"Expected operator, found /" when passing operator to macro

[jonas-schievink]

macro_rules! apply_op {
    ( $op:tt ) => {{ 4 $op 5 }};
}

fn main() {
    let _ = apply_op!(/);
}

gives:

<anon>:2:24: 2:27 error: expected one of `.`, `;`, `}`, or an operator, found `/`
<anon>:2     ( $op:tt ) => {{ 4 $op 5 }};
                                ^~~

The same thing happens for other operators as well as . and ;.

[Stebalien]

That's because rust currently needs to be told how to interpret the token:

macro_rules! expr_identity {
    ($e:expr) => { $e }
}
macro_rules! apply_op {
    ( $op:tt ) => { expr_identity!(4 $op 5)};
}

fn main() {
    let _ = apply_op!(/);
}

dup of #27231

[jonas-schievink]

Ah, so these are caused by the same issue. The error message has definitely confused me there.

[steveklabnik]

tagging as 'diagnostics' since it's not actually incorrect code

[huonw]

Dupe of #27231 and #5846. Thanks for filing!