Memory leak in LocalWaker

[RalfJung]

The following code leaks memory:

#![feature(local_waker)]
use std::rc::Rc;
use std::task::LocalWake;
use std::task::LocalWaker;

fn main() {
    struct NoopWaker;

    impl LocalWake for NoopWaker {
        fn wake(self: Rc<Self>) {}
    }

    let _waker = LocalWaker::from(Rc::new(NoopWaker));
}

Miri error:

error: memory leaked: alloc1518 (Rust heap, size: 16, align: 8), allocated here:
   --> /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/alloc.rs:100:9
    |
100 |         __rust_alloc(layout.size(), layout.align())
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: BACKTRACE:
    = note: inside `std::alloc::alloc` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/alloc.rs:100:9: 100:52
    = note: inside `std::alloc::Global::alloc_impl` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/alloc.rs:183:73: 183:86
    = note: inside `<std::alloc::Global as std::alloc::Allocator>::allocate` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/alloc.rs:243:9: 243:39
    = note: inside `alloc::alloc::exchange_malloc` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/alloc.rs:332:11: 332:34
    = note: inside `std::boxed::Box::<std::rc::RcBox<main::NoopWaker>>::new` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/boxed.rs:218:9: 218:20
    = note: inside `std::rc::Rc::<main::NoopWaker>::new` at /playground/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/alloc/src/rc.rs:398:27: 398:94
note: inside `main`
   --> src/main.rs:13:35
    |
13  |     let _waker = LocalWaker::from(Rc::new(NoopWaker));
    |                                   ^^^^^^^^^^^^^^^^^^

The equivalent code with Waker is fine, so I think this is a bug.

Cc @dtolnay @tvallotton

[RalfJung]

I guess this is not too surprising... Waker has

#[stable(feature = "futures_api", since = "1.36.0")]
impl Drop for Waker {
    #[inline]
    fn drop(&mut self) {
        // SAFETY: This is safe because `Waker::from_raw` is the only way
        // to initialize `drop` and `data` requiring the user to acknowledge
        // that the contract of `RawWaker` is upheld.
        unsafe { (self.waker.vtable.drop)(self.waker.data) }
    }
}

but LocalWaker has nothing like that.

[tvallotton]

Oh, that's may bad. I assumed that RawWaker handled ownership. I can push a PR today in the evening.

[RalfJung]

This leads to a miri-test-libstd failure every day, would be nice to get a fix soon. :)