declare

(declare declaration...)

Declarations never change what a program computes: the whole declare form evaluates to nil and its arguments are never evaluated or validated, so any standard declaration (ignore, ignorable, type, optimize, inline, special, ...) is accepted anywhere in a body, and source code written for other Common Lisp implementations loads unchanged.

Two declaration families do affect compilation. A (declare (special ...)) marks a variable dynamically scoped, as in standard CL. And on the WASM backend, a type declaration that names an array type -- (simple-array (unsigned-byte 8) (*)), simple-vector, simple-string, ... -- lets the compiler emit that one representation's element accessors directly, which makes the compiled module smaller and faster. Results are unaffected by a correct declaration; a false one, which is undefined behavior in Common Lisp, traps at the access on WASM while the other backends continue to ignore it.