ICE when trying to implement Drop generically

[raphaelcohn]

I've defined a trait CompletionQueue<'a>, which wraps various FFI *mut xxx pointers. All of these pointers can be freed by the same FFI function, so to reduce code duplication, I defined a pointer() method on my CompletionQueue<'a> trait, and tried to define a generic implementation of Drop that is basically unsafe { ff_drop_function(self.pointer()) }. This causes the compiler to panic...

     Running `rustc --crate-name rdma_core src/lib.rs --crate-type lib --emit=dep-info,link -C debuginfo=2 -C metadata=7b223b004c70695f -C extra-filename=-7b223b004c70695f --out-dir /Volumes/Source/GitHub/lemonrock/rdma-core/.cargo/target/debug/deps -L dependency=/Volumes/Source/GitHub/lemonrock/rdma-core/.cargo/target/debug/deps --extern rust_extra=/Volumes/Source/GitHub/lemonrock/rdma-core/.cargo/target/debug/deps/librust_extra-c243164c893672bb.rlib --extern errno=/Volumes/Source/GitHub/lemonrock/rdma-core/.cargo/target/debug/deps/liberrno-e26062265f10ba2f.rlib --extern syscall_alt=/Volumes/Source/GitHub/lemonrock/rdma-core/.cargo/target/debug/deps/libsyscall_alt-e072c8ee531427e2.rlib --extern libc=/Volumes/Source/GitHub/lemonrock/rdma-core/.cargo/target/debug/deps/liblibc-b057f0656f58b4ca.rlib --extern rdma_core_sys=/Volumes/Source/GitHub/lemonrock/rdma-core/.cargo/target/debug/deps/librdma_core_sys-9e84b480a6be2488.rlib`
error[E0119]: conflicting implementations of trait `std::ops::Drop` for type `std::boxed::Box<_>`:
  --> src/CompletionQueue.rs:5:1
   |
5  | / impl<'a, T: Sized + CompletionQueue<'a>> Drop for T
6  | | {
7  | | 	#[inline(always)]
8  | | 	fn drop(&mut self)
...  |
11 | | 	}
12 | | }
   | |_^
   |
   = note: conflicting implementation in crate `alloc`

error[E0120]: the Drop trait may only be implemented on structures
 --> src/CompletionQueue.rs:5:51
  |
5 | impl<'a, T: Sized + CompletionQueue<'a>> Drop for T
  |                                                   ^ implementing Drop requires a struct

error: internal compiler error: src/librustc_typeck/check/dropck.rs:64: should have been rejected by coherence check: T
  --> src/CompletionQueue.rs:5:1
   |
5  | / impl<'a, T: Sized + CompletionQueue<'a>> Drop for T
6  | | {
7  | | 	#[inline(always)]
8  | | 	fn drop(&mut self)
...  |
11 | | 	}
12 | | }
   | |_^

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: run with `RUST_BACKTRACE=1` for a backtrace

thread 'rustc' panicked at 'Box<Any>', src/librustc_errors/lib.rs:375
stack backtrace:
   0: std::sys::imp::backtrace::tracing::imp::unwind_backtrace
   1: std::panicking::default_hook::{{closure}}
   2: std::panicking::default_hook
   3: std::panicking::rust_panic_with_hook
   4: std::panicking::begin_panic
   5: rustc_errors::Handler::span_bug
   6: rustc::session::opt_span_bug_fmt::{{closure}}
   7: rustc::session::span_bug_fmt
   8: rustc_typeck::check::dropck::check_drop_impl
   9: core::ops::FnMut::call_mut
  10: rustc::ty::util::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::calculate_dtor::{{closure}}
  11: rustc::ty::util::<impl rustc::ty::context::TyCtxt<'a, 'gcx, 'tcx>>::calculate_dtor
  12: rustc_typeck::check::adt_destructor
  13: rustc::ty::maps::<impl rustc::ty::maps::queries::adt_destructor<'tcx>>::try_get
  14: rustc::ty::util::<impl rustc::ty::ParameterEnvironment<'tcx>>::can_type_implement_copy
  15: rustc_typeck::coherence::coherent_trait
  16: rustc::ty::maps::<impl rustc::ty::maps::queries::coherent_trait<'tcx>>::try_get
  17: rustc::ty::maps::<impl rustc::ty::context::TyCtxt<'a, 'tcx, 'lcx>>::coherent_trait
  18: rustc_typeck::coherence::check_coherence
  19: rustc_typeck::check_crate
  20: rustc_driver::driver::phase_3_run_analysis_passes::{{closure}}
  21: rustc_driver::driver::phase_3_run_analysis_passes
  22: rustc_driver::driver::compile_input
  23: rustc_driver::run_compiler
  24: std::panicking::try::do_call
  25: __rust_maybe_catch_panic
  26: <F as alloc::boxed::FnBox<A>>::call_box
  27: std::sys::imp::thread::Thread::new::thread_start
  28: _pthread_body
  29: _pthread_start

[raphaelcohn]

Background: rust-nightly, version rustc 1.19.0-nightly (777ee2079 2017-05-01), running on Mac OS X (Darwin Kernel Version 15.6.0) on a x86_64 bit Mac Pro cylinder. Native target.

[raphaelcohn]

Also present in rustc 1.19.0-nightly (e17a1227a 2017-05-12)

[djzin]

Implementing Drop (or any trait you do not control) generically in this way is not possible because it violates the coherence rules. (What if someone implemented CompletionQueue and Drop seperately?) You probably want to wrap your types in a newtype with a forwarding implementation of the CompletionQueue trait.

With that said, the compiler probably shouldn't ICE on this!

[raphaelcohn]

Thank you. I was trying to avoid writing and maintaining a lot of tedious, duplicated boilerplate...

[Mark-Simulacrum]

Can you provide a code example (preferably minimal?)

[raphaelcohn]

Nope; it was a long time ago.

[tmccombs]

I encountered this same issue on rustc 1.21 (with a very similar use case).

Unfortunately, while I can reproduce it in my project, I can't come up with a minimal example that reproduces the ICE.

The full otuput look like

error[E0119]: conflicting implementations of trait `std::ops::Drop` for type `std::boxed::Box<_>`:
  --> src/stream.rs:76:1
   |
76 | / impl<T> Drop for T where T: Stream {
77 | |     fn drop(&mut self) {
78 | |         unsafe {
79 | |             nn_close(self.as_raw_fd());
80 | |         }
81 | |     }
82 | | }
   | |_^
   |
   = note: conflicting implementation in crate `alloc`

error[E0120]: the Drop trait may only be implemented on structures
  --> src/stream.rs:76:18
   |
76 | impl<T> Drop for T where T: Stream {
   |                  ^ implementing Drop requires a struct

error: internal compiler error: /checkout/src/librustc_typeck/check/dropck.rs:64: should have been rejected by coherence check: T
  --> src/stream.rs:76:1
   |
76 | / impl<T> Drop for T where T: Stream {
77 | |     fn drop(&mut self) {
78 | |         unsafe {
79 | |             nn_close(self.as_raw_fd());
80 | |         }
81 | |     }
82 | | }
   | |_^

note: the compiler unexpectedly panicked. this is a bug.

note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports

note: rustc 1.21.0 (3b72af97e 2017-10-09) running on x86_64-unknown-linux-gnu

thread 'rustc' panicked at 'Box<Any>', /checkout/src/librustc_errors/lib.rs:437:8

[tmccombs]

Actually, I was able to get a minimal reproduction: https://play.rust-lang.org/?gist=3c42e05707afa39f805997dc2f2f4f08&version=undefined

[sinkuu]

Further reduced (playground):

#[derive(Copy, Clone)]
struct Flags;

trait A {
}

impl<T> Drop for T where T: A {
    fn drop(&mut self) {
    }
}