feat(fuzz): add option to use only defined fixtures

[Dzejkop]

Component

Forge

Have you ensured that all of these are up to date?

• Foundry
• Foundryup

What version of Foundry are you on?

forge 0.3.0 (5a8bd89 2024-12-20T08:45:53.204298000Z)

What command(s) is the bug in?

forge test

Operating System

macOS (Apple Silicon)

Describe the bug

Minimal example repo

Docs suggest we can setup fixtures for testing with specific values using storage arrays or functions named like fixtureParamName. This doesn't work.

Code from the repo above:

// SPDX-License-Identifier: UNLICENSED
pragma solidity ^0.8.13;

import {Test} from "forge-std/Test.sol";

contract FixtureTest is Test {
    function fixtureA() public returns (uint256[] memory) {
      uint256[] memory nums = new uint256[](1);
      nums[0] = 1;
      return nums;
    }

    function fixture_a() public returns (uint256[] memory) {
      uint256[] memory nums = new uint256[](1);
      nums[0] = 1;
      return nums;
    }

    uint256[] fixtureB = [1];
    uint256[] fixture_b = [1];

    function testFuzz_DoesNotWorkFun(uint256 a) public {
      assertEq(a, 1);
    }

    function testFuzz_DoesNotWorkStorage(uint256 b) public {
      assertEq(b, 1);
    }
}

Example test output:

Ran 2 tests for test/Fixture.t.sol:FixtureTest
[FAIL: assertion failed: 3 != 1; counterexample: calldata=0x0a5b9e0a0000000000000000000000000000000000000000000000000000000000000003 args=[3]] testFuzz_DoesNotWorkFun(uint256) (runs: 1, μ: 3[17](https://github.com/Dzejkop/forge-test-fuzz-not-working-minimal-example/actions/runs/12651348861/job/35251762954#step:7:18)2, ~: 3172)
Traces:
  [3164] FixtureTest::testFuzz_DoesNotWorkFun(3)
    ├─ [0] VM::assertEq(3, 1) [staticcall]
    │   └─ ← [Revert] assertion failed: 3 != 1
    └─ ← [Revert] assertion failed: 3 != 1

[FAIL: assertion failed: 36[18](https://github.com/Dzejkop/forge-test-fuzz-not-working-minimal-example/actions/runs/12651348861/job/35251762954#step:7:19) != 1; counterexample: calldata=0x90e665580000000000000000000000000000000000000000000000000000000000000e22 args=[3618]] testFuzz_DoesNotWorkStorage(uint256) (runs: 0, μ: 0, ~: 0)
Traces:
  [3141] FixtureTest::testFuzz_DoesNotWorkStorage(3618)
    ├─ [0] VM::assertEq(3618, 1) [staticcall]
    │   └─ ← [Revert] assertion failed: 3618 != 1
    └─ ← [Revert] assertion failed: 3618 != 1

[grandizzy]

@Dzejkop per https://book.getfoundry.sh/forge/fuzz-testing#fuzz-test-fixtures

Fuzz test fixtures can be defined when you want to make sure a certain set of values is used as inputs for fuzzed parameters.

which means that is guaranteed that fixture values will be used as fuzz inputs, along other inputs (see https://book.getfoundry.sh/images/fuzzer.png fixtures represents 40% from fuzzed inputs)

We could extend this and have an option to use only fixtures when fuzzing, lmk your thoughts. thank you!

[Dzejkop]

I see, makes sense!

Maybe it would make sense then to split this into fuzzing and prop testing. Perhaps explicit testFuzz and testProp prefixes?