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Run tests with statements and method calls removed to help identify broken tests

Necessist currently supports Anchor (TS), Foundry, Go, Hardhat (TS), and Rust.

A paper on Necessist (Test Harness Mutilation) will appear in Mutation 2024. (slides, preprint)



System requirements:

Install pkg-config and sqlite3 development files on your system, e.g., on Ubuntu:

sudo apt install pkg-config libsqlite3-dev

Install Necessist from

cargo install necessist

Install Necessist from

cargo install --git --branch release


Necessist iteratively removes statements and method calls from tests and then runs them. If a test passes with a statement or method call removed, it could indicate a problem in the test. Or worse, it could indicate a problem in the code being tested.


This example is from rust-openssl. The verify_untrusted_callback_override_ok test checks that a failed certificate validation can be overridden by a callback. But if the callback were never called (e.g., because of a failed connection), the test would still pass. Necessist reveals this fact by showing that the test passes without the call to set_verify_callback:

fn verify_untrusted_callback_override_ok() {
    let server = Server::builder().build();

    let mut client = server.client();
        .set_verify_callback(SslVerifyMode::PEER, |_, x509| { //
            assert!(x509.current_cert().is_some());           // Test passes without this call
            true                                              // to `set_verify_callback`.
        });                                                   //


Following this discovery, a flag was added to the test to record whether the callback is called. The flag must be set for the test to succeed:

fn verify_untrusted_callback_override_ok() {
    static CALLED_BACK: AtomicBool = AtomicBool::new(false);  // Added

    let server = Server::builder().build();

    let mut client = server.client();
        .set_verify_callback(SslVerifyMode::PEER, |_, x509| {
  , Ordering::SeqCst);        // Added

    assert!(CALLED_BACK.load(Ordering::SeqCst));              // Added

Comparison to conventional mutation testing

Conventional mutation testing tries to identify gaps in test coverage, whereas Necessist tries to identify bugs in existing tests.

Conventional mutation testing tools (such a universalmutator) randomly inject faults into source code, and see whether the code's tests still pass. If they do, it could mean the code's tests are inadequate.

Notably, conventional mutation testing is about finding deficiencies in the set of tests as a whole, not in individual tests. That is, for any given test, randomly injecting faults into the code is not especially likely to reveal bugs in that test. This is unfortunate since some tests are more important than others, e.g., because ensuring the correctness of some parts of the code is more important than others.

By comparison, Necessist's approach of iteratively removing statements and method calls does target individual tests, and thus can reveal bugs in individual tests.

Of course, there is overlap in the sets of problems the two approaches can uncover, e.g., a failure to find an injected fault could indicate a bug in a test. Nonetheless, for the reasons just given, we see the two approaches as complementary, not competing.

Possible theoretical foundation

The following criterion (*) comes close to describing the statements that Necessist aims to remove:

  • (*) Statement S's weakest precondition P has the same context (e.g., variables in scope) as S's postcondition Q, and P does not imply Q.

The notion that (*) tries to capture is: a statement that affects a subsequent assertion. In this section, we explain and motivate this choice. For concision, we focus on statements, but the remarks in this section apply to method calls as well.

Recall the two kinds of predicate transformer semantics: weakest precondition and strongest postcondition. With the former, one reasons about the weakest precondition that could hold prior to a statement, given a postcondition that holds after the statement. With the latter, one reasons about the strongest postcondition that could hold after a statement, given a precondition that holds prior to the statement. Generally speaking, the former is more common (see Aldrich 2013 for an explanation), and it is the one we use here.

Consider a test through this lens. A test is a function with no inputs or outputs. Thus, an alternative procedure for determining whether a test passes is the following. Starting with True, iteratively work backwards through the test's statements, computing the weakest precondition of each. If the precondition arrived at for the test's first statement is True, then the test passes. If the precondition is False, the test fails.

Now, imagine we were to apply this procedure, and consider a statement S that violates (*). We argue that it might not make sense to remove S:

Case 1: S adds or removes variables from the scope (e.g., S is a declaration), or S changes a variable's type. Then removing S would likely result in a compilation failure. (On top of that, since S's precondition and postcondition have different contexts, it's not clear how to compare them.)

Case 2: S's precondition is stronger than its postcondition (e.g., S is an assertion). Then S imposes constraints on the environments in which it executes. Put another way, S tests something. Thus, removing S would likely detract from the test's overarching purpose.

Conversely, consider a statement S that satisfies (*). Here is why it might make sense to remove S. Think of S as shifting the set of valid environments, rather than constraining them. More precisely, if S's weakest precondition P does not imply Q, and if Q is satisfiable, the there is an assignment to P and Q's free variables that satisfies both P and Q. If such an assignment results from each environment in which S is actually executed, then the necessity of S is called into question.

The main utility of (*) is in helping to select the functions, macros, and method calls that Necessist ignores. Necessist ignores certain of these by default. Suppose that, for one of the frameworks, we are considering whether Necessist should ignore some function foo. If we imagine a predicate transformer semantics for the framework's testing language, we can ask: if statement S were a call to foo, would S satisfy (*)? If the answer is "no," then Necessist should likely ignore foo.

Consider Rust's clone method, for example. A call to clone can be unnecessary. However, if we imagine a predicate transformer semantics for Rust, a call to clone is unlikely to satisfy (*). For this reason, Necessist does not attempt to remove clone calls.

In addition to helping to select the functions, etc. that Necessist ignores, (*) has other nice consequences. For example, the rule that the last statement in a test should be ignored follows from (*). To see this, note that such a statement's postcondition Q is always True. Thus, if the statement doesn't change the context, then its weakest precondition necessarily implies Q.

Having said all this, (*) doesn't quite capture what Necessist actually does. Consider a statement like x -= 1;. Necessist will remove such a statement unconditionally, but (*) says maybe Necessist shouldn't. Assuming overflow checks are enabled, computing this statement's weakest precondition would look something like the following:

{ Q[(x - 1)/x] ^ x >= 1 }
x -= 1;
{ Q }

Note that x -= 1; does not change the context, and that Q[(x - 1)/x] ^ x >= 1 could imply Q. For example, if Q does not contain x, then Q[(x - 1)/x] = Q and Q ^ x >= 1 implies Q.

Given the discrepancy between (*) and Necessist's current behavior, one can ask: which of the two should be adjusted? Put another way, should Necessist remove a statement like x -= 1; unconditionally?

One way to look at this question is: which statements are worth removing, i.e., which statements are "interesting?" As implied above, (*) considers a statement "interesting" if its removal could affect a subsequent assertion. But there are other possible, useful definitions of an "interesting" statement. For example, one could consider strongest postconditions (mentioned above), or frameworks besides Hoare logic entirely.

To be clear, Necessist does not apply (*) formally, e.g., Necessist does not actually compute weakest preconditions. The current role of (*) is to help guide which statements Necessist should ignore, and (*) seems to do well in that role. As such, we leave resolving the aforementioned discrepancy to future work.


Usage: necessist [OPTIONS] [TEST_FILES]... [-- <ARGS>...]

  [TEST_FILES]...  Test files to mutilate (optional)
  [ARGS]...        Additional arguments to pass to each test command

      --allow <WARNING>        Silence <WARNING>; `--allow all` silences all warnings
      --default-config         Create a default necessist.toml file in the project's root directory
      --deny <WARNING>         Treat <WARNING> as an error; `--deny all` treats all warnings as errors
      --dump                   Dump sqlite database contents to the console
      --dump-candidates        Dump removal candidates and exit (for debugging)
      --framework <FRAMEWORK>  Assume testing framework is <FRAMEWORK> [possible values: anchor, auto, foundry, go, hardhat, rust]
      --no-dry-run             Do not perform dry runs
      --no-sqlite              Do not output to an sqlite database
      --quiet                  Do not output to the console
      --reset                  Discard sqlite database contents
      --resume                 Resume from the sqlite database
      --root <ROOT>            Root directory of the project under test
      --timeout <TIMEOUT>      Maximum number of seconds to run any test; 60 is the default, 0 means no timeout
      --verbose                Show test outcomes besides `passed`
  -h, --help                   Print help
  -V, --version                Print version


By default, Necessist outputs to the console only when tests pass. Passing --verbose causes Necessist to instead output all of the removal outcomes below.

Outcome Meaning (With the statement/method call removed...)
passed The test(s) built and passed.
timed-out The test(s) built but timed-out.
failed The test(s) built but failed.
nonbuildable The test(s) did not build.

By default, Necessist outputs to both the console and to an sqlite database. For the latter, a tool like sqlitebrowser can be used to filter/sort the results.


Generally speaking, Necessist will not attempt to remove a statement if it is one the following:

  • a statement containing other statements (e.g., a for loop)
  • a declaration (e.g., a local or let binding)
  • a break, continue, or return
  • the last statement in a test

Similarly, Necessist will not attempt to remove a method call if:

  • It is the primary effect of an enclosing statement (e.g.,;).
  • It appears in the argument list of an ignored function, method, or macro (see below).

Also, for some frameworks, certain statements and methods are ignored. Click on a framework to see its specifics.

Ignored functions

  • assert
  • Anything beginning with assert. (e.g., assert.equal)
  • Anything beginning with console. (e.g., console.log)
  • expect

Ignored methods

  • toNumber
  • toString

In addition to the below, the Foundry framework ignores:

  • a statement immediately following a use of vm.prank or any form of vm.expect (e.g., vm.expectRevert)
  • an emit statement

Ignored functions

  • Anything beginning with assert (e.g., assertEq)
  • Anything beginning with vm.expect (e.g., vm.expectCall)
  • Anything beginning with console.log (e.g., console.log, console.logInt)
  • Anything beginning with console2.log (e.g., console2.log, console2.logInt)
  • vm.getLabel
  • vm.label

In addition to the below, the Go framework ignores:

  • defer statements

Ignored functions

  • Anything beginning with assert. (e.g., assert.Equal)
  • Anything beginning with require. (e.g., require.Equal)
  • panic

Ignored methods*

  • Close
  • Error
  • Errorf
  • Fail
  • FailNow
  • Fatal
  • Fatalf
  • Log
  • Logf
  • Parallel
  • Skip
  • Skipf
  • SkipNow

* This list is based primarily on testing.T's methods. However, some methods with commonplace names are omitted to avoid colliding with other types' methods.

The ignored functions and methods are the same as for Anchor TS above.

Ignored macros

  • assert
  • assert_eq
  • assert_matches
  • assert_ne
  • eprint
  • eprintln
  • panic
  • print
  • println
  • unimplemented
  • unreachable

Ignored methods*

  • as_bytes
  • as_encoded_bytes
  • as_mut
  • as_mut_os_str
  • as_mut_os_string
  • as_mut_slice
  • as_mut_str
  • as_os_str
  • as_path
  • as_ref
  • as_slice
  • as_str
  • borrow
  • borrow_mut
  • clone
  • cloned
  • copied
  • deref
  • deref_mut
  • expect
  • expect_err
  • into_boxed_bytes
  • into_boxed_os_str
  • into_boxed_path
  • into_boxed_slice
  • into_boxed_str
  • into_bytes
  • into_encoded_bytes
  • into_os_string
  • into_owned
  • into_path_buf
  • into_string
  • into_vec
  • iter
  • iter_mut
  • success
  • to_os_string
  • to_owned
  • to_path_buf
  • to_string
  • to_vec
  • unwrap
  • unwrap_err

* This list is essentially the watched trait and inherent methods of Dylint's unnecessary_conversion_for_trait lint, with the following additions:

Configuration files

A configuration file allows one to tailor Necessist's behavior with respect to a project. The file must be named necessist.toml, appear in the project's root directory, and be toml encoded. The file may contain one more of the options listed below.

  • ignored_functions, ignored_methods, ignored_macros: A list of strings interpreted as patterns. A function, method, or macro (respectively) whose path matches a pattern in the list is ignored. Note that ignored_macros is used only by the Rust framework currently.

  • ignored_path_disambiguation: One of the strings Either, Function, or Method. For a path that could refer to a function or method (see below), this option influences whether the function or method is ignored.

    • Either (default): Ignore if the path matches either an ignored_functions or ignored_methods pattern.

    • Function: Ignore only if the path matches an ignored_functions pattern.

    • Method: Ignore only if the path matches an ignored_methods pattern.

  • ignored_tests: A list of strings. A test whose name exactly matches a string in the list is ignored. For Mocha-based frameworks (e.g., Anchor and Hardhat), a test name is consider to be a message passed to it.


A pattern is a string composed of letters, numbers, ., _, or *. Each character, other than *, is treated literally and matches itself only. A * matches any string, including the empty string.

The following are examples of patterns:

  • assert: matches itself only
  • assert_eq: matches itself only
  • assertEqual: matches itself only
  • assert.Equal: matches itself only
  • assert.*: matches assert.Equal, but not assert, assert_eq, or assertEqual
  • assert*: matches assert, assert_eq, assertEqual, and assert.Equal
  • *.Equal: matches assert.Equal, but not Equal


  • Patterns match paths, not individual identifiers.
  • . is treated literally like in a glob pattern, not like in regular expression.


A path is a sequence of identifiers separated by .. Consider this example (from Chainlink):

    to: operator.address,

In the above, operator.connect and signer.sendTransaction are paths.

Note, however, that paths like operator.connect are ambiguous:

  • If operator refers to package or module, then operator.connect refers to a function.
  • If operator refers to an object, then operator.connect refers to a method.

By default, Necessist ignores such a path if it matches either an ignored_functions or ignored_methods pattern. Setting the ignored_path_disambiguation option above to Function or Method causes Necessist ignore the path only if it matches an ignored_functions or ignored_methods pattern (respectively).


  • Slow. Modifying tests requires them to be rebuilt. Running Necessist on even moderately sized codebases can take several hours.

  • Triage requires intimate knowledge of the source code. Generally speaking, Necessist does not produce "obvious" bugs. In our experience, deciding whether a statement/method call should be necessary requires intimate knowledge of the code under test. Necessist is best run on codebases for which one has (or intends to have) such knowledge.

Semantic versioning policy

We reserve the right to change the following, and to consider such changes non-breaking:

  • the syntax that Necessist ignores by default

Changes to the following will be accompanied by a bump of at least Necessist's minor version:

  • the order in which removal candidates are output
  • the order in which records are stored in necessist.db


  • If a project uses a supported framework, then cding into the project's directory and typing necessist (with no arguments) should produce meaningful output.


  • Become a general-purpose mutation testing tool. Good such tools already exist (e.g., universalmutator).


  • Groce, A., Ahmed, I., Jensen, C., McKenney, P.E., Holmes, J.: How verified (or tested) is my code? Falsification-driven verification and testing. Autom. Softw. Eng. 25, 917–960 (2018). A preprint is available. See Section 2.3.


Necessist is licensed and distributed under the AGPLv3 license. Contact us if you're looking for an exception to the terms.