Nightly regression.

[dpc]

Mioco works on nightly, but recent nightly (around two day ago) does not compile mioco anymore.

I have minimized code to:

https://gist.github.com/dpc/a6bd306b8187651ec700

This code compiles on stable, but breaks on nightly, so I believe this is a regression.

dpc@futex ~/t/mioco-break (master) [I]> multirust override nightly ; make
multirust: using existing install for 'nightly'
multirust: override toolchain for '/home/dpc/tmp/mioco-break' set to 'nightly'
cargo build
   Compiling mioco-break v0.1.0 (file:///home/dpc/tmp/mioco-break)
udp.rs:11:5: 13:6 error: duplicate definitions with name `try_read`: [E0201]
udp.rs:11     pub fn try_read(&mut self, buf: &mut [u8]) -> io::Result<Option<(usize, SocketAddr)>> {
udp.rs:12         Ok(None)
udp.rs:13     }
udp.rs:11:5: 13:6 help: run `rustc --explain E0201` to see a detailed explanation
lib.rs:14:5: 16:6 note: conflicting definition is here:
lib.rs:14     pub fn try_read(&mut self, buf: &mut [u8]) -> io::Result<Option<usize>> {
lib.rs:15         Ok(None)
lib.rs:16     }
error: aborting due to previous error
       error Could not compile `mioco-break`.

To learn more, run the command again with --verbose.
Makefile:2: recipe for target 'all' failed
make: *** [all] Error 101
dpc@futex ~/t/mioco-break (master) [2] [I]> multirust override stable ; make
multirust: using existing install for 'stable'
multirust: override toolchain for '/home/dpc/tmp/mioco-break' set to 'stable'
cargo build
   Compiling mioco-break v0.1.0 (file:///home/dpc/tmp/mioco-break)
udp.rs:11:5: 13:6 warning: method is never used: `try_read`, #[warn(dead_code)] on by default
udp.rs:11     pub fn try_read(&mut self, buf: &mut [u8]) -> io::Result<Option<(usize, SocketAddr)>> {
udp.rs:12         Ok(None)
udp.rs:13     }
udp.rs:11:32: 11:35 warning: unused variable: `buf`, #[warn(unused_variables)] on by default
udp.rs:11     pub fn try_read(&mut self, buf: &mut [u8]) -> io::Result<Option<(usize, SocketAddr)>> {
                                         ^~~
lib.rs:14:32: 14:35 warning: unused variable: `buf`, #[warn(unused_variables)] on by default
lib.rs:14     pub fn try_read(&mut self, buf: &mut [u8]) -> io::Result<Option<usize>> {
                                         ^~~

[dpc]

Just to comment on the code: mio::udp::UdpSocket does not implement mio::Evented so impl<MT> MioAdapter<MT> where MT: mio::Evented + mio::TryRead + 'static should not be applied.

[nikomatsakis]

The problem here is that we started applying the coherence rules to inherent impls -- the fact that we did not do so before was a bug. The coherence rules are somewhat more restrictive about negative reasoning. For example here, you are not allowed to rely on the fact that UdpSocket does not implement Evented because --- in principle, at least --- mio might add an impl of Evented for UdpSocket and that is not supposed to be a breaking change. (That is, in general, adding an impl of a trait for a type is not a breaking change -- nothing specific to mio). But it would in fact break your code given those two impls.

We probably ought to have had a warning period for this change, however.

[nikomatsakis]

cc @aturon

[nagisa]

Duplicate of #32247 I think?

[aturon]

Yes, this is effectively a dupe. I'm planning to land a change ASAP to produce a warning instead of an error for now, though we (at the moment) intend to go forward with this change.