Regression in nightly-2020-09-23: "missing `as_flags` in implementation" of OpenOptionsExt

[dtolnay]

The following code compiles fine with nightly-2020-09-22 (and stable rustc since 1.10.0) but not nightly-2020-09-23.

use std::os::unix::fs::OpenOptionsExt;

struct MyOpenOptions;

impl OpenOptionsExt for MyOpenOptions {
    fn mode(&mut self, _mode: u32) -> &mut Self {
        unimplemented!()
    }

    fn custom_flags(&mut self, _flags: i32) -> &mut Self {
        unimplemented!()
    }
}

error[E0046]: not all trait items implemented, missing: `as_flags`
 --> src/lib.rs:5:1
  |
5 | impl OpenOptionsExt for MyOpenOptions {
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing `as_flags` in implementation
  |
  = help: implement the missing item: `fn as_flags(&self) -> std::result::Result<i32, std::io::Error> { todo!() }`

This is minimized from https://github.com/async-rs/async-std/blob/e8126633a89aafea23259eb9faddb70b89f94423/src/fs/open_options.rs#L303-L313.

Mentioning #76110, #76801, @FedericoPonzi, @joshtriplett.

[jyn514]

Whoops, I didn't realize the trait itself was stable. Probably the PR should either be reverted or add a default implementation for as_flags().

[Stupremee]

Assigning P-critical as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

[yoshuawuyts]

async-std is now broken on nightly as well: async-rs/async-std#883