Fix std::fs::copy on WASI by setting proper OpenOptions flags #150881
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When PR rust-lang#147572 switched WASI to use Unix-style filesystem APIs, the open_to_and_set_permissions function for WASI was implemented to call OpenOptions::new().open() without setting any access mode flags. This causes std::fs::copy to fail with the error: "must specify at least one of read, write, or append access" The fix is to explicitly set .write(true), .create(true), and .truncate(true) on the OpenOptions, matching the behavior of the non-WASI Unix implementation but without the permission handling that WASI doesn't support. Minimal reproduction: fn main() { std::fs::write("/src.txt", b"test").unwrap(); match std::fs::copy("/src.txt", "/dst.txt") { Ok(_) => println!("PASS: fs::copy works!"), Err(e) => println!("FAIL: {}", e), } } # Compile and run: rustc +nightly --target wasm32-wasip2 test.rs -o test.wasm wasmtime -S cli --dir . test.wasm # Before fix: FAIL: must specify at least one of read, write, or append access # After fix: PASS: fs::copy works! Note: The existing test library/std/src/fs/tests.rs::copy_file_ok would have caught this regression if the std test suite ran on WASI targets. Currently std tests don't compile for wasm32-wasip2 due to Unix-specific test code in library/std/src/sys/fd/unix/tests.rs. Fixes the regression introduced in nightly-2025-12-10.
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Fix std::fs::copy on WASI by setting proper OpenOptions flags When PR rust-lang#147572 switched WASI to use Unix-style filesystem APIs, the open_to_and_set_permissions function for WASI was implemented to call OpenOptions::new().open() without setting any access mode flags. This causes std::fs::copy to fail with the error: "must specify at least one of read, write, or append access" The fix is to explicitly set .write(true), .create(true), and .truncate(true) on the OpenOptions, matching the behavior of the non-WASI Unix implementation but without the permission handling that WASI doesn't support. Minimal reproduction: ```rs fn main() { std::fs::write("/src.txt", b"test").unwrap(); match std::fs::copy("/src.txt", "/dst.txt") { Ok(_) => println!("PASS: fs::copy works!"), Err(e) => println!("FAIL: {}", e), } } ``` # Compile and run: rustc +nightly --target wasm32-wasip2 test.rs -o test.wasm wasmtime -S cli --dir . test.wasm # Before fix: FAIL: must specify at least one of read, write, or append access # After fix: PASS: fs::copy works! Note: The existing test library/std/src/fs/tests.rs::copy_file_ok would have caught this regression if the std test suite ran on WASI targets. Currently std tests don't compile for wasm32-wasip2 due to Unix-specific test code in library/std/src/sys/fd/unix/tests.rs. Fixes the regression introduced in nightly-2025-12-10. r? @alexcrichton
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Rollup of 9 pull requests Successful merges: - #149318 (Implement partial_sort_unstable for slice) - #150805 (Fix ICE in inline always warning emission.) - #150822 (Fix for ICE: eii: fn / macro rules None in find_attr()) - #150853 (std: sys: fs: uefi: Implement File::read) - #150855 (std: sys: fs: uefi: Implement File::tell) - #150881 (Fix std::fs::copy on WASI by setting proper OpenOptions flags) - #150891 (Fix a trivial typo in def_id.rs) - #150892 (Don't check `[mentions]` paths in submodules from tidy) - #150894 (cg_llvm: add a pause to make comment less confusing) r? @ghost
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Rollup merge of #150881 - fix-wasi-fs-copy, r=alexcrichton Fix std::fs::copy on WASI by setting proper OpenOptions flags When PR #147572 switched WASI to use Unix-style filesystem APIs, the open_to_and_set_permissions function for WASI was implemented to call OpenOptions::new().open() without setting any access mode flags. This causes std::fs::copy to fail with the error: "must specify at least one of read, write, or append access" The fix is to explicitly set .write(true), .create(true), and .truncate(true) on the OpenOptions, matching the behavior of the non-WASI Unix implementation but without the permission handling that WASI doesn't support. Minimal reproduction: ```rs fn main() { std::fs::write("/src.txt", b"test").unwrap(); match std::fs::copy("/src.txt", "/dst.txt") { Ok(_) => println!("PASS: fs::copy works!"), Err(e) => println!("FAIL: {}", e), } } ``` # Compile and run: rustc +nightly --target wasm32-wasip2 test.rs -o test.wasm wasmtime -S cli --dir . test.wasm # Before fix: FAIL: must specify at least one of read, write, or append access # After fix: PASS: fs::copy works! Note: The existing test library/std/src/fs/tests.rs::copy_file_ok would have caught this regression if the std test suite ran on WASI targets. Currently std tests don't compile for wasm32-wasip2 due to Unix-specific test code in library/std/src/sys/fd/unix/tests.rs. Fixes the regression introduced in nightly-2025-12-10. r? @alexcrichton
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When PR #147572 switched WASI to use Unix-style filesystem APIs, the open_to_and_set_permissions function for WASI was implemented to call OpenOptions::new().open() without setting any access mode flags.
This causes std::fs::copy to fail with the error:
"must specify at least one of read, write, or append access"
The fix is to explicitly set .write(true), .create(true), and .truncate(true) on the OpenOptions, matching the behavior of the non-WASI Unix implementation but without the permission handling that WASI doesn't support.
Minimal reproduction:
Note: The existing test library/std/src/fs/tests.rs::copy_file_ok would have caught this regression if the std test suite ran on WASI targets. Currently std tests don't compile for wasm32-wasip2 due to Unix-specific test code in library/std/src/sys/fd/unix/tests.rs.
Fixes the regression introduced in nightly-2025-12-10.
r? @alexcrichton