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Fix non-hygienic hazard in count_idents! macro #23

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Fix non-hygienic hazard in count_idents! macro #23

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14 changes: 7 additions & 7 deletions text/blk-counting.md
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Original file line number Diff line number Diff line change
Expand Up @@ -125,22 +125,22 @@ This approach can be used where you need to count a set of mutually distinct ide

```rust
macro_rules! count_idents {
($($idents:ident),* $(,)*) => {
() => (0);

($first:ident $(,$idents:ident)* $(,)*) => {
{
#[allow(dead_code, non_camel_case_types)]
enum Idents { $($idents,)* __CountIdentsLast }
const COUNT: u32 = Idents::__CountIdentsLast as u32;
COUNT
enum Idents { $($idents,)* $first }
(Idents::$first as usize) + 1
}
};
}
#
# fn main() {
# const COUNT: u32 = count_idents!(A, B, C);
# const COUNT: usize = count_idents!(A, B, C);
# assert_eq!(COUNT, 3);
# }
```

This method does have two drawbacks. First, as implied above, it can *only* count valid identifiers (which are also not keywords), and it does not allow those identifiers to repeat.
This method does have a drawback though. As implied above, it can *only* count valid identifiers (which are also not keywords), and it does not allow those identifiers to repeat.

Secondly, this approach is *not* hygienic, meaning that if whatever identifier you use in place of `__CountIdentsLast` is provided as input, the macro will fail due to the duplicate variants in the `enum`.