Hangs and timeouts

Some mutations to the tree can cause the test suite to hang. For example, in this code, cargo-mutants might try changing should_stop to always return false, but this will cause the program to hang:

#![allow(unused)]
fn main() {
    while !should_stop() {
      // something
    }
}

In general you will want to skip functions which cause a hang when mutated, either by marking them with an attribute or in the configuration file.

Timeouts

To avoid hangs, cargo-mutants will kill the build or test after a timeout and continue to the next mutant.

By default, the timeouts are set automatically, relative to the times taken to build and test the unmodified tree (baseline).

The default timeouts are:

• cargo build/cargo check: 2 times the baseline build time times the number of jobs (with a minimum of 20 seconds)
• cargo test: 5 times baseline test time (with a minimum of 20 seconds)

The build timeout scales with the number of jobs to reflect that cargo often spawns many jobs, and so builds run in parallel are likely to take longer than the baseline, which has no external parallelism.

The minimum of 20 seconds for the test timeout can be overridden by the --minimum-test-timeout option or the CARGO_MUTANTS_MINIMUM_TEST_TIMEOUT environment variable, measured in seconds.

You can set explicit timeouts with the --build-timeout, and --timeout options, also measured in seconds. If these options are specified then they are applied to the baseline build and test as well.

You can set timeout multipliers that are relative to the duration of the baseline build or test with --build-timeout-multiplier and --timeout-multiplier, respectively. Additionally, these can be configured with build_timeout_multiplier and timeout_multiplier in .cargo/mutants.toml (e.g. timeout-multiplier = 1.5). These options are only applied if the baseline is not skipped and no corresponding --build-timeout/--timeout option is specified, otherwise they are ignored.

Exceptions

The multiplier timeout options cannot be used when the baseline is skipped (--baseline=skip), or when the build is in-place (--in-place). If no explicit timeouts is provided in these cases, then a default of 300 seconds will be used.