custom-declare-variable is a compiled Lisp function in `custom.el'.

(custom-declare-variable SYMBOL DEFAULT DOC &rest ARGS)

Like `defcustom', but SYMBOL and DEFAULT are evaluated as normal arguments.
DEFAULT should be an expression to evaluate to compute the default value,
not the default value itself.

DEFAULT is stored as SYMBOL's standard value, in SYMBOL's property
`standard-value'. At the same time, SYMBOL's property `force-value' is
set to nil, as the value is no longer rogue.