go build cache.

The -modcache flag causes clean to remove the entire module
download cache, including unpacked source code of versioned
dependencies.

The -fuzzcache flag causes clean to remove files stored in the Go build
cache for fuzz testing. The fuzzing engine caches files that expand
code coverage, so removing them may make fuzzing less effective until

// The subdirectory may not exist.
//
// This directory is managed by the internal/fuzz package. Files in this
// directory aren't removed by the 'go clean -cache' command or by Trim.
// They may be removed with 'go clean -fuzzcache'.
//
// TODO(#48526): make Trim remove unused files from this directory.
func (c *DiskCache) FuzzDir() string {
	return filepath.Join(c.dir, "fuzz")

specifically, values that expanded code coverage when passed to a
fuzz function. These values are not used for regular building and
testing, but they're stored in a subdirectory of the build cache.
Running 'go clean -fuzzcache' removes all cached fuzzing values.
This may make fuzzing less effective, temporarily.

The GODEBUG environment variable can enable printing of debugging
information about the state of the cache: