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Inconsistent handling of #[derive] on invalid targets #78331
Description
The following code:
fn main() {
struct Foo {}
#[derive(Copy)] impl Foo {}
}produces the following error
error[E0774]: `derive` may only be applied to structs, enums and unions
--> src/main.rs:3:2
|
3 | #[derive(Copy)] impl Foo {}
| ^^^^^^^^^^^^^^^
However, applying #[derive] to a let statement just gives an "unused attribute" warning:
fn main() {
#[derive(Copy)] let _ = 1;
}warning: unused attribute
--> src/main.rs:2:5
|
2 | #[derive(Copy)] let _ = 1;
| ^^^^^^^^^^^^^^^
|
= note: `#[warn(unused_attributes)]` on by default
A #[derive] attribute doesn't make any more sense on a statement than it does on an item. I think we should handle these two cases in the same way.
There's been a move towards forbidding attributes on targets that don't make sense: #77015. Unless anyone objects, I think we should consistently forbid #[derive] on all invalid targets.
Activity
Aaron1011 commentedon Oct 24, 2020
The hard error dates back to 2014 (pre-1.0): ad61ff4#diff-b6281951b118c3ea5f37605f6b8bc76cb0406fff7a59d830c16c13e9d429f1d4R354-R357
The warnings (or ast least some of them) are more recent, dating back to 2018: #50092
Aaron1011 commentedon Oct 24, 2020
I opened #78338 to do a Crater run, so we can see how much code is relying on this compiling.
[-]Incosistent handling of `#[derive]` on non-items[/-][+]Incosistent handling of `#[derive]` on invaid targets[/+][-]Incosistent handling of `#[derive]` on invaid targets[/-][+]Incosistent handling of `#[derive]` on invalid targets[/+][-]Incosistent handling of `#[derive]` on invalid targets[/-][+]Inconsistent handling of `#[derive]` on invalid targets[/+]Aaron1011 commentedon Feb 14, 2021
This was turned into a hard error by #79078