WIP: basic mutation testing with cargo mutants

[sveitser]

Close #2227

• Update nixpkgs to get latest cargo-mutants.
• Remove nixWithFlakes from flake.nix because it would have to be changed to work with the latest version of nixpkgs and I think we no longer need it.
• Create a non-flaky test profile.
• Run with cargo mutants.
• Inspect output and ./mutants.out directory

For example ./mutants.out/missed.txt looks like this

contract-bindings/src/plonk_verifier_2.rs:463:13: replace plonk_verifier_2::<impl AbiEncode for PlonkVerifier2Calls>::encode -> Vec<u8> with vec![]
hotshot-state-prover/src/mock_ledger.rs:496:57: replace + with * in gen_circuit_for_test
hotshot-state-prover/src/mock_ledger.rs:496:53: replace * with / in gen_circuit_for_test
hotshot-state-prover/src/mock_ledger.rs:149:9: replace MockLedger::elapse_without_block with ()
contract-bindings/src/light_client_mock.rs:1928:21: replace == with != in light_client_mock::<impl ContractRevert for LightClientMockErrors>::valid_selector
contract-bindings/src/erc1967_proxy.rs:319:13: replace erc1967_proxy::<impl AbiEncode for ERC1967ProxyErrors>::encode -> ::std::vec::Vec<u8> with vec![0]
hotshot-state-prover/src/circuit.rs:236:29: replace % with / in build

To make it really useful further configuration is likely necessary, such as marking some functions to not mutate them.

[tbro]

test already inherits dev

[tbro]

Can you comment about how we are meant to read the output? What does this mean:

contract-bindings/src/plonk_verifier_2.rs:463:13: replace plonk_verifier_2::<impl AbiEncode for PlonkVerifier2Calls>::encode -> Vec<u8> with vec![]

cargo mutants made that replacement? What was the outcome? What was the expectation?

[sveitser]

I think we can use the env var NEXTEST_PROFILE

[sveitser]

cargo mutants made that replacement? What was the outcome? What was the expectation?

@tbro this is still WIP and I will document it better once I think it's useful and I understand it well enough.

cargo mutatants makes copies of the codebase, makes some modifications, compiles the code and runs the tests.

The expectation is that for every change that cargo mutants makes there is at least one test that fails as a result. If not it's considered a mutant that our tests "missed".

However to make it effective we need to annotate some code so that cargo mutants doesn't mutate it (for example it's likely not useful if some change in mock_ledger is "missed"). There is also a bug I need to sort out where I get a warning from cargo mutants that the test run exited with an unexpected status code. I think it currently leads to false positive "missed" results.