The extensions:optimize-interface declaration is identical in syntax to the optimize declaration, but it specifies the policy used during compilation of code the compiler automatically generates to check the number and type of arguments supplied to a function. It is useful to specify this policy separately, since even thoroughly debugged functions are vulnerable to being passed the wrong arguments. The optimize-interface declaration can specify that arguments should be checked even when the general optimize policy is unsafe.
Note that this argument checking is the checking of user-supplied arguments to any functions defined within the scope of the declaration, not the checking of arguments to Common Lisp primitives that appear in those definitions.
The idea behind this declaration is that it allows the definition of
functions that appear fully safe to other callers, but that do no
internal error checking. Of course, it is possible that arguments may
be invalid in ways other than having incorrect type. Functions
compiled unsafely must still protect themselves against things like
user-supplied array indices that are out of bounds and improper lists.
See also the :context-declarations option to
with-compilation-unit (page 
).