std::env::temp_dir() unconditionally panics on wasm32-wasip1/wasm32-wasip2

I tried this code:

fn main() {
    println!("{:?}", std::env::temp_dir());
}

compiled with rustc --target wasm32-wasip1 main.rs (also reproduces on wasm32-wasip2) and run under wasmtime 47.0.3, with or without TMPDIR set in the environment.

I expected to see the path std would use as a scratch directory — e.g. respecting TMPDIR if set, falling back to /tmp otherwise, the same way sys/paths/unix.rs's temp_dir() behaves for every other Unix-family target.

Instead, this happened: an unconditional panic that aborts the process:

thread 'main' (1) panicked at library/std/src/sys/paths/wasi.rs:44:5:
not supported by WASI yet

sys/paths/wasi.rs implements temp_dir() as a bare panic!, unlike getcwd/chdir in the same file which have real WASI implementations via libc::getcwd/libc::chdir. There is no capability gap forcing this: reading TMPDIR via std::env::var_os and falling back to a /tmp literal, exactly like the unix implementation, requires nothing WASI doesn't already support — env::var_os and plain string literals are both already used elsewhere in the same file and module.

This is not a hypothetical: it's user-visible in real programs built for WASI. In uutils/coreutils, sort and mktemp both call std::env::temp_dir() in their normal (no-argument) code paths; sort was patched around this with an explicit #[cfg(target_os = "wasi")] fallback to a hardcoded /tmp, but mktemp was not (see uutils/coreutils source), and mktemp invoked with no arguments panics/traps immediately when run under wasm32-wasip1 or wasm32-wasip2 via wasmtime — confirmed via local build and wasmtime run.

Suggested fix: give wasi.rs's temp_dir() the same body as unix.rs's (minus the Apple/Android special cases, which don't apply to WASI):

pub fn temp_dir() -> PathBuf {
    crate::env::var_os("TMPDIR").map(PathBuf::from).unwrap_or_else(|| PathBuf::from("/tmp"))
}

Meta

rustc --version --verbose:

rustc 1.97.1 (8bab26f4f 2026-07-14)

Reproduced on both wasm32-wasip1 and wasm32-wasip2, with wasmtime 47.0.3.

Start in library/std/src/sys/paths/wasi.rs and compare temp_dir() with the implementation in library/std/src/sys/paths/unix.rs. Reproduce with wasm32-wasip1 or wasm32-wasip2 under wasmtime, checking both TMPDIR set and unset; done means std::env::temp_dir() no longer panics and returns TMPDIR or /tmp.