Some of the code in the standard library is created by code generation based on templates. For example, many Array methods need to be implemented separately for Array<T>, ByteArray, ShortArray, IntArray, etc.
To run the code generator from the libraries directory of a kotlin checkout, use the following command:
./gradlew :tools:kotlin-stdlib-gen:run
Note: on Windows type
gradlewwithout the leading./
This then runs the script which generates a significant part of stdlib sources from the templates authored with a special kotlin based DSL.