llvm-project/libcxx/test/lit.cfg.py

# All the Lit configuration is handled in the site configs -- this file is only
# left as a canary to catch invocations of Lit that do not go through llvm-lit.
#
# Invocations that go through llvm-lit will automatically use the right Lit
# site configuration inside the build directory.

lit_config.fatal(
    "You seem to be running Lit directly -- you should be running Lit through "
    "<build>/bin/llvm-lit, which will ensure that the right Lit configuration "
    "file is used. See https://libcxx.llvm.org/TestingLibcxx.html#usage for "
    "how to run the libc++ tests."
)