wasip2: build uucore and env standalone, don't trap on empty argv

[Aditya1404Sal]

fixes #14561

Four commits

1. uucore: enable rustix/std so its fs and io modules compile on wasm32-wasip2
   One line in src/uucore/Cargo.toml: the optional rustix dependency gains
   features = ["std"]. std is a rustix default feature the graph already ends up with on unix
   via clap → clap_builder → terminal_size, which depends on rustix with its defaults;
   terminal_size is absent on wasm32-wasip2, so nothing enables it there and the std-backed
   API those modules use (AsFd, Arg for &Path, From<Errno> for io::Error) is not compiled.
   The requirement is put on the dependency rather than repeated across the seven features that
   activate it (fs, entries, mode, pipes, process, checksum, uptime), all of which
   use that API. Deliberately scoped to uucore: setting features = ["std"] on the workspace
   rustix works too, but changes rustix for every member. The narrow version is enough for the
   whole wasm build — including uu_tee, the one other crate whose own rustix dependency is
   live on target_os = "wasi", which picks up std through uucore in any build that contains
   both (and every uu_* crate depends on uucore).

2. env: build on wasm32-wasip2 without the unstable std::os::wasi::ffi (encoded-bytes conversions)
native_int_str.rs keeps the std::os::wasi::ffi import for target_env = "p1" and gains a
   small wasip2_ffi module for p2: as_bytes → as_encoded_bytes, into_vec →
   into_encoded_bytes, and from_vec through String::from_utf8 (lossy only for bytes that
   cannot have come from the host — the component model defines strings as UTF-8). No unsafe,
   matching the module's stated rule and the approach uucore::os_str_from_bytes /
   os_string_from_vec already take on this target. One consequence: from_native_int_representation
   returns an owned OsStr on p2 where it could borrow before, because bytes → OsStr has to go
   through str.

3. env: return to feat_common_core now that it builds on wasm32-wasip1 and wasip2
   Reverts the env half of c9077c764: env goes back into feat_common_core (so back into
   feat_wasm, which is now ["feat_common_core", "nproc"]) and out of feat_Tier1, where it had
   been moved to keep it out of the wasm build. This is what gives commit 2 CI coverage — the
   existing wasip1 and wasip2 jobs both build feat_wasm, so a regression in the env conversions
   fails the build rather than going unnoticed. The other four utilities that commit moved out stay
   where they are: df, du and more genuinely do not compile for wasip2, and tac does but
   nothing in this PR changed it, so restoring it is your call, not mine. Drop this commit if you
   would rather keep env out of the wasm set for reasons beyond the build failure.

4. uucore: don't index an empty argv on wasip2 (guests embedded as libraries have none)
   On target_os = "wasi", ARGV is initialised with a single fallback element when
   std::env::args_os() is empty, so UTIL_NAME/EXECUTION_PHRASE resolve to a stable name
   instead of panicking. Other targets are unchanged. If you prefer this unconditional (a library
   consumer on any OS can have an empty argv), it is a one-line cfg change — I kept it to wasi to
   match where it was observed.

Testing

• cargo check --target wasm32-wasip2 --no-default-features --features feat_wasm,env -p coreutils
  — the wasip2 CI job's own flags plus env: E0658 on main, clean with the patch. With commit
  3 the same holds for plain --features feat_wasm (no explicit env) on both wasm32-wasip1
  and wasm32-wasip2, which is what the two CI jobs run; --features feat_os_unix still builds
  natively.
• cargo check --target wasm32-wasip2 -p uucore -p uu_cat -p uu_ls -p uu_wc -p uu_head -p uu_sort -p uu_mkdir -p uu_rm -p uu_mv -p uu_cp -p uu_env -p uu_cut -p uu_tr -p uu_uniq -p uu_tail -p uu_tee -p uu_touch -p uu_sleep -p uu_printf — clean (12 errors → 0; env builds).
• cargo clippy -p uu_env --all-targets -- -D warnings, native and --target wasm32-wasip2 — clean;
  cargo fmt --check clean; cargo test -p uucore --lib passes.
• Under wasmtime 46 on the target, the built env.wasm behaves: wasmtime --env X=1 env.wasm
  prints X=1, and env -u X drops it.
• The equivalent of (4) has been running for two months inside a shell component that calls
  eighteen uu_* crates as library functions.

[Copilot]

🟡 Changes recommended

Add WASI Preview 2 coverage and regression tests for the new conversion and empty-argv paths.

Get a fresh assessment by requesting another Copilot review.

Pull request overview

Enables standalone wasm32-wasip2 builds for uucore and env, while preventing empty-WASI-argv traps.

Changes:

• Enables rustix/std for uucore.
• Adds WASI Preview 2 encoded-byte conversions for env.
• Restores env to common/WASI features and adds an empty-argv fallback.

File summaries

File	Description
src/uucore/src/lib/lib.rs	Prevents empty-argv indexing on WASI.
src/uucore/Cargo.toml	Enables required rustix standard-library APIs.
src/uu/env/src/native_int_str.rs	Adds WASI Preview 2 string conversions.
Cargo.toml	Includes env in common/WASI features.

Review details

Suppressed comments (1)

Cargo.toml:140

• This move does not provide the wasip2 CI coverage claimed in the PR description: .github/workflows/wasi.yml gates the feat_wasm build to wasip1 and excludes env from the wasip2 package-test list. Add a wasip2 compile/test step or correct the description before relying on this feature move for regression protection.

  "env",

• Files reviewed: 4/4 changed files
• Comments generated: 2
• Review effort level: Lite

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