Skip to content

Fix non-hygienic hazard in count_idents! macro #23

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
RReverser wants to merge 1 commit into
base: master
Choose a base branch
from
Open

Fix non-hygienic hazard in count_idents! macro #23

Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
14 changes: 7 additions & 7 deletions text/blk-counting.md
View file
Open in desktop
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`.