fix(tests): add missing min_cost parameter to evalTCOExpression FFI binding

[tvolk131]

Summary

The C function rustsimplicity_0_6_evalTCOExpression has a minCost parameter (of type ubounded) between len and budget, but the Rust FFI binding in the test module was missing this parameter. This caused an argument mismatch in the foreign function call, leading to undefined behavior at runtime.

• Add min_cost: ubounded parameter to the simplicity_evalTCOExpression extern declaration in simplicity-sys/src/tests/ffi.rs to match the C signature in eval.h.
• Pass 0 as min_cost in the simplicity_evalTCOProgram Rust wrapper, matching the behavior of the C inline evalTCOProgram helper in eval.h.
• Fix budget.map() to budget.as_ref().map() in run_program (simplicity-sys/src/tests/mod.rs) — the previous code moved the Option<u32> value rather than borrowing it, which produced a dangling pointer instead of a valid pointer to the stack-local budget value.

Context

The C signatures (from depend/simplicity/eval.h):

simplicity_err rustsimplicity_0_6_evalTCOExpression(
    flags_type anti_dos_checks, UWORD* output, const UWORD* input,
    const dag_node* dag, type* type_dag, size_t len,
    ubounded minCost, const ubounded* budget, const txEnv* env);

static inline simplicity_err evalTCOProgram(
    const dag_node* dag, type* type_dag, size_t len,
    ubounded minCost, const ubounded* budget, const txEnv* env) {
  return rustsimplicity_0_6_evalTCOExpression(CHECK_ALL, NULL, NULL, dag, type_dag, len, minCost, budget, env);
}

Test plan

• Verify the Rust FFI binding now matches the C function signature
• Run the existing test suite (cargo test) to confirm no regressions

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR fixes undefined behavior in the simplicity-sys test harness by aligning the Rust test-only FFI binding for evalTCOExpression with the actual C signature and correcting how an optional budget pointer is derived.

Changes:

• Add the missing min_cost: ubounded parameter to the simplicity_evalTCOExpression extern declaration in simplicity-sys/src/tests/ffi.rs.
• Update the simplicity_evalTCOProgram wrapper to pass min_cost = 0, matching the C inline helper behavior.
• Fix run_program to use budget.as_ref().map(...) so the budget pointer refers to valid memory instead of casting the integer value into an address.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
simplicity-sys/src/tests/mod.rs	Fixes construction of budget_ptr to correctly borrow the optional budget value when passing into FFI.
simplicity-sys/src/tests/ffi.rs	Updates the FFI signature to include min_cost and adjusts the Rust wrapper call to match the C ABI.

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[apoelstra]

Good catch. This was missed in #296. I'm quite surprised it did cause test failures up til now. (There were several other changes in #296 which did.)

Can you please run cargo +nightly fmt on this commit?

Also, I appreciate the LLM disclosure but please rewrite the PR description so it's not a massive textwall. In this case the PR title is sufficient to understand the change so it didn't cause problems, but it's extra work to scroll past.