"Include what you use" means this: for every symbol (type, function variable,
 or macro) that you use in foo.cc, either foo.cc or foo.h should #include a .h
 file that exports the declaration of that symbol. The include-what-you-use
 tool is a program that can be built with the clang libraries in order to
 analyze #includes of source files to find include-what-you-use violations,
 and suggest fixes for them.
 .
 The main goal of include-what-you-use is to remove superfluous #includes.
 It does this both by figuring out what #includes are not actually needed for
 this file (for both .cc and .h files), and replacing #includes with
 forward-declares when possible.