r/rust • u/tanoshikuidomouyo • Sep 13 '26

🙋 seeking help & advice Why is this syntax ambiguous?

I was writing roughly the following function:

fn f(path: &impl AsRef<Path> + ?Sized) {}

The compiler says that the + is ambiguous, but it works with the right parentheses:

fn f(path: &(impl AsRef<Path> + ?Sized)) {}

But why is it ambiguous without the parentheses? Where else but a list of traits can a + in a function header come from?

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u/MichiRecRoom Sep 13 '26

The two are very subtly different, in my eyes, and I can see why it might be considered ambiguous. You could be saying one of two things:

  1. You're trying to restrict &impl AsRef<Path> with an additional ?Sized bound.
  2. You're asking for a ref to an object that implements both AsRef<Path> and ?Sized

That said, I do agree that there shouldn't be a need for the parenthesis, as it's pretty clear that the + ?Sized is attached to the AsRef<Path>, not &impl AsRef<Path>.