Missing fd_close in wasm32-wasip2: Rust 1.93.0 regression

Upstream issue: rust-lang/rust#152255

Rust 1.93.0 stable produces wasm32-wasip2 components that are missing the fd_close import from wasi_snapshot_preview1. File descriptors are never released, exhausting the WASI resource table after ~100-125 file operations.

The bug is release-specific: it only exists in 1.93.0 stable (254b59607). Every nightly version tested (including 1.93.0-nightly) works correctly. This points to a bad cherry-pick or backport to the 1.93 release branch.

Quick reproduction

./scripts/reproduce.sh

This builds the same component with Rust 1.92.0 and 1.93.0, compares the WASM imports, and runs both. Requires rustup, wasmtime, and wasm-tools.

Manual reproduction

# Build the component with Rust 1.93.0
rustup run 1.93.0 cargo build --release -p wasi-component --target wasm32-wasip2

# The compiled WASM is missing fd_close:
wasm-tools print target/wasm32-wasip2/release/wasi-component.wasm | grep fd_close
# (no output)

# Run it - fails after ~100-125 iterations:
wasmtime run --dir ./test-data::/test-data target/wasm32-wasip2/release/wasi-component.wasm
# Error: Os { code: 48, kind: OutOfMemory, message: "Out of memory" }

Compare with 1.92.0:

rustup run 1.92.0 cargo build --release -p wasi-component --target wasm32-wasip2

wasm-tools print target/wasm32-wasip2/release/wasi-component.wasm | grep fd_close
# (import "wasi_snapshot_preview1" "fd_close" (func ...))  <-- present

wasmtime run --dir ./test-data::/test-data target/wasm32-wasip2/release/wasi-component.wasm
# All 1000 iterations completed successfully!

The component

Both component/ (wasm32-wasip2) and component-p1/ (wasm32-wasip1) contain identical code - a loop that opens a file, reads 64 bytes, and drops it:

use std::fs::File;
use std::io::{self, Read};

fn main() -> io::Result<()> {
    for i in 1..=1000 {
        {
            let mut file = File::open("/test-data/test.txt")?;
            let mut buf = [0u8; 64];
            let _ = file.read(&mut buf)?;
        } // file is dropped here
        if i % 100 == 0 { println!("Completed {} iterations", i); }
    }
    println!("All 1000 iterations completed successfully!");
    Ok(())
}

Root cause

The wasm32-wasip2 target uses a hybrid approach: Rust's std calls WASI Preview 1 functions (path_open, fd_read) which are bridged to Preview 2 by an adapter layer embedded in the component. In working versions, fd_close is also imported and called when File is dropped:

Working (1.92.0):  path_open → fd_read → fd_close → cleanup
Broken  (1.93.0):  path_open → fd_read → (no fd_close) → leak

Each File::open creates ~2 entries in the WASI resource table (a file descriptor and an input stream). Without fd_close, these accumulate until the table limit (~250 resources) is reached, causing error code 48 ("too many open files", reported as "Out of memory").

Affected versions

Version	Released	Result	fd_close
1.92.0 stable	Dec 8, 2025	PASS	present
nightly-2025-09-18	—	PASS	present
nightly-2025-11-18 (1.93.0-nightly)	—	PASS	present
nightly-2025-12-29 (1.94.0-nightly)	—	PASS	present
nightly-2026-01-18 (1.94.0-nightly)	—	PASS	present
1.93.0 stable	Jan 19, 2026	FAIL	missing

All 10+ nightly versions tested across a 4-month range work correctly. Only 1.93.0 stable is affected.

Workaround

• Use Rust 1.92.0: rustup default 1.92.0
• Use nightly: rustup default nightly
• Wait for Rust 1.94.0 stable

Prerequisites

• rustup with Rust 1.93.0 and wasm32-wasip2 target
• wasmtime (tested with 41.0)
• wasm-tools (for import inspection)

Project structure

component/       wasm32-wasip2 component (exhibits the bug in 1.93.0)
component-p1/    wasm32-wasip1 module (works correctly, for comparison)
host/            Custom wasmtime host supporting both P1 and P2
test-data/       Test file read by the components
scripts/
  reproduce.sh       One-command reproduction
  compare-imports.sh  Show P1 vs P2 imports side by side
  test-rust-version.sh  Test a specific Rust version
  bisect-test.sh     For use with cargo-bisect-rustc
notes/           Detailed investigation history

Additional scripts

Test any Rust version:

./scripts/test-rust-version.sh 1.92.0   # PASS
./scripts/test-rust-version.sh 1.93.0   # FAIL

Compare P1 vs P2 imports (requires both targets built):

./scripts/compare-imports.sh

System information

Affected:   rustc 1.93.0 (254b59607 2026-01-19), target wasm32-wasip2
Tested on:  wasmtime 41.0.1, wasm-tools 1.244.0, Linux x86_64

Investigation notes

The notes/ directory contains the full investigation trail:

1. Version check — initial environment
2. Adapter layer — P1→P2 bridge analysis
3. Third-party reproduction — confirmed in independent code
4. WASM import analysis — the missing fd_close
5. Version regression — bisection to 1.93.0
6. Release bisection — confirmed never on master