Steps to reproduce: ``` $ rustup toolchain install 1.62.1 --target wasm32-wasi $ rustup toolchain install nightly-2022-07-19 --target wasm32-wasi $ git clone https://github.com/tokio-rs/mio.git $ cd mio $ git checkout v0.8.4 $ cargo +1.62.1 build --target wasm32-wasi --all-features Compiling libc v0.2.126 Compiling log v0.4.17 Compiling cfg-if v1.0.0 Compiling wasi v0.11.0+wasi-snapshot-preview1 Compiling mio v0.8.4 (/home/ben/code/mio) Finished dev [unoptimized + debuginfo] target(s) in 0.66s $ cargo +nightly-2022-07-19 build --target wasm32-wasi --all-features Compiling log v0.4.17 Compiling libc v0.2.126 Compiling cfg-if v1.0.0 Compiling wasi v0.11.0+wasi-snapshot-preview1 Compiling mio v0.8.4 (/home/ben/code/mio) error[E0658]: use of unstable library feature 'wasi_ext' --> src/io_source.rs:5:5 | 5 | use std::os::wasi::io::AsRawFd; | ^^^^^^^^^^^^^^^^^^^^^^^^^^ | = note: see issue #71213 <https://github.com/rust-lang/rust/issues/71213> for more information = help: add `#![feature(wasi_ext)]` to the crate attributes to enable For more information about this error, try `rustc --explain E0658`. error: could not compile `mio` due to previous error ``` It's possible that this was accidentally stabilized, then feature gated later.