Erroneous drop_bounds lint

[jonhoo]

I tried this code (playground):

trait Foo where Self: Sized + Drop + 'static {
    fn drop(me: *mut Self) {
        bar(me)
    }
}
pub fn bar(_: *mut dyn Drop) {}

I expected to see this happen: the code should compile without warning.

Instead, this happened: the compiler gave the following warning:

warning: bounds on `Self: Drop` are useless, consider instead using `std::mem::needs_drop` to detect if a type has a destructor
 --> src/lib.rs:1:31
  |
1 | trait Foo where Self: Sized + Drop + 'static {
  |                               ^^^^
  |
  = note: `#[warn(drop_bounds)]` on by default

But, the Drop bound is not useless here — removing it causes the code to no longer compile. It's not clear if removing the bound should make the code stop compiling, but that's a different matter (and perhaps a different bug?).

Meta

rustc --version --verbose:

rustc 1.53.0 (53cb7b09b 2021-06-17)
binary: rustc
commit-hash: 53cb7b09b00cbea8754ffb78e7e3cb521cb8af4b
commit-date: 2021-06-17
host: x86_64-unknown-linux-gnu
release: 1.53.0
LLVM version: 12.0.1

The warning also occurs on nightly.

[scottmcm]

Hmm, dyn Drop is pretty weird. Notably, drop_in_place doesn't have a bound on Drop.

I think a warning is still appropriate for this code, even if perhaps it's in a supoptimal place.

[jonhoo]

For context, this code is used in a library that does deferred memory reclamation, so I want to store a heterogeneous collection of elements "to be dropped". I don't know what other type to use in that context than dyn Drop — it seems like exactly the right thing to use. I could introduce an empty placeholder trait instead, but that only adds unnecessary complexity.

[kpreid]

I could introduce an empty placeholder trait instead, but that only adds unnecessary complexity.

Perhaps the standard library should provide an empty trait implemented by everything. (Pointee, if stabilized, will be implemented by everything, but its actual purpose might be distracting.) Or perhaps there should be a way to write a dyn which has a vtable for drop but no actual methods; the obvious syntax is dyn with no following traits (which is already parsed but rejected with error[E0224]: at least one trait is required for an object type).