diagnostic::do_not_recommend can emit an error if it encounters a keyword

[mejrs]

I tried this code:

#![crate_type = "lib"]

pub trait X{}
pub struct Y;

#[diagnostic::do_not_recommend(if)]
impl X for Y {}

I expected to see this happen: a warning

Instead, this happened: an error

Meta

current nightly

this attribute is never allowed to error, this is a corner case I missed in #151516

cc @JonathanBrouwer @jdonszelmann

@rustbot claim

[Krishna-js45]

Thanks for the clear repro and quick fix.

I can confirm that on current nightly the attribute errors instead of warning
when encountering a keyword, and that this matches the described corner case.
The linked PR approach makes sense to keep the attribute non-erroring.