[move] `tests/` modules should be treated as test-only for OTW construction without requiring `#[test_only]`

[0xNeshi]

Steps to Reproduce Issue

Create a package with a test module placed in the tests/ directory (so it is compiled only in test mode) whose name matches an OTW-shaped struct, and construct that OTW manually:

// tests/my_module_tests.move
module my_pkg::my_module_tests;

public struct MY_MODULE_TESTS has drop {}

fun new_then_drop_otw() {
    let _otw = MY_MODULE_TESTS {}; // manual OTW construction
}

#[test]
fun some_test() {
    new_then_drop_otw();
}

1. Run sui move test --build-env testnet. Confirm failure.

$ sui move test --build-env testnet
INCLUDING DEPENDENCY MoveStdlib
INCLUDING DEPENDENCY Sui
BUILDING my_pkg
error[Sui E02005]: invalid one-time witness usage
  ┌─ ./sources/my_module_tests.move:7:16
  │
7 │     let _otw = MY_MODULE_TESTS {}; // manual OTW construction
  │                ^^^^^^^^^^^^^^^^^^ Invalid one-time witness construction. One-time witness types cannot be created manually, but are passed as an argument 'init'
  │
  = One-time witness types are structs with the following requirements: their name is the upper-case version of the module's name, they have no fields (or a single boolean field), they have no type parameters, and they have only the 'drop' ability.

2. Add #[test_only] to the module (#[test_only] module my_pkg::my_module_tests;) and run sui move test --build-env testnet again.

$ sui move test --build-env testnet
INCLUDING DEPENDENCY MoveStdlib
INCLUDING DEPENDENCY Sui
BUILDING my_pkg
Running Move unit tests
[ PASS    ] my_pkg::my_module_tests::some_test
Test result: OK. Total tests: 1; passed: 1; failed: 0

3. Add #[test_only] to the new_then_drop_otw function (#[test_only] fun new_then_drop_otw()) and run sui move test --build-env testnet again.

$ sui move test --build-env testnet
INCLUDING DEPENDENCY MoveStdlib
INCLUDING DEPENDENCY Sui
BUILDING my_pkg
Running Move unit tests
[ PASS    ] my_pkg::my_module_tests::some_test
Test result: OK. Total tests: 1; passed: 1; failed: 0

Expected Result

All 3 runs should compile. A module in tests/ is compiled only in test mode, so #[test_only] is semantically redundant there, and the existing OTW test-code carve-out should apply either way.

Expected the OTW manual construction to be allowed in step 1 (no #[test_only]), just as it is in steps 2 and 3.

Actual Result

Step 1 (no #[test_only]) fails to compile:

Invalid one-time witness construction. One-time witness types cannot be created
manually, but are passed as an argument 'init'

Steps 2 and 3 (with #[test_only] on module or function level) compiles fine. The carve-out is gated on the #[test]/#[test_only] attribute of the enclosing module/function (context.in_test in external-crates/move/crates/move-compiler/src/sui_mode/typing.rs, set only from attributes.is_test_or_test_only()), not on whether the code is compiled in test mode / lives in tests/. The same gate also governs the "cannot call init directly" check.

Suggested fix: treat all modules compiled in test-only mode (everything under tests/) as in_test = true, so the attribute is genuinely optional there.

System Information

• OS: macOS 26.5.1 (build 25F80)
• Compiler: sui 1.72.5-5445323ef25aq

[github-actions]

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[0x-j]

hi @tnowacki @cgswords can you guys take a look at this issue?

[tnowacki]

Hi @0xNeshi,
I am not able to reproduce the error as described. You state "Run sui move test. Confirm failure.", but I have run exactly the code provided and cannot reproduce the error.

Can you share the full output from the compiler after running sui move test?

The gating code linked is correct in that it is looking at the test attributes. While we only include the modules in tests/ during test compilation, this does not imply the code is in mode(test) (test_only). In your provided example, you have the test marked as #[test] which will disable the error relating to OTW witness construction.

[0xNeshi]

Hi @tnowacki , I just noticed a mistake in the example code I provided.
I updated the example to one that actually fails on sui move test. The problem appears when the OTW is created inside a regular function in the test module (see new_then_drop_otw in the example).

[tnowacki]

Gotcha, I think I understand now, thanks!
I'll discuss the idea with the team

[0xNeshi]

@tnowacki thanks. I also found that also adding #[test_only] just to the new_then_drop_otw function the test compiles just fine.
Updated the issue description again.