byte-compile-function-environment is a variable defined in `bytecomp.el'.
Its value is

Alist of functions defined in the file being compiled.
This is so we can inline them when necessary.
Each element looks like (FUNCTIONNAME . DEFINITION). It is
(FUNCTIONNAME . nil) when a function is redefined as a macro.
It is (FUNCTIONNAME . t) when all we know is that it was defined,
and we don't know the definition. For an autoloaded function, DEFINITION
has the form (autoload . FILENAME).