Copying the tree

By default, cargo-mutants copies your tree to a temporary directory before mutating and building it. This behavior is turned of by the --in-place option, which builds mutated code in the original source directory.

When the --jobs option is used, one build directory is created per job.

Some filters are applied while copying the tree, which can be configured by options.

Troubleshooting tree copies

If the baseline tests fail in the copied directory it is a good first debugging step to try building with --in-place.

.git and other version control directories

By default, files or directories matching these patterns are not copied, because they can be large and typically are not needed to build the source:

.git
.hg
.jj
.bzr
.svn
_darcs
.pijul

If your tree's build or tests require the VCS directory then it can be copied with --copy-vcs=true or by setting copy_vcs = true in .cargo/mutants.toml.

.gitignore

The --gitignore=true command line option or gitignore = true in .cargo/mutants.toml enables gitignore filtering, meaning that files matching gitignore patterns will be excluded from copying from the source tree to the build directory.

This option will make copying slightly faster (and use less temporary space) if your tree contains a large number of ignored files that aren't needed to build the source for mutation testing.

gitignore filtering is only used within trees containing a .git directory.

The filter, based on the ignore crate, also respects global git ignore configuration in the home directory, as well as .gitignore files within the tree.

The target/ directory is excluded by default, regardless of gitignore settings, to avoid copying large build artifacts that are typically not needed for mutation testing. This can be overridden with --copy-target=true if your tests depend on existing build artifacts, or by setting copy_target = true in .cargo/mutants.toml.

Note that if you set --gitignore=true and --copy-target=true and your target/ is excluded by gitignore files (which is common) then it will not be copied.

Prior to cargo-mutants 25.0.2, gitignore was on by default.

mutants.out

mutants.out and mutants.out.old are never copied, even if they're not covered by .gitignore.