use-package

(use-package packages &optional package)

Adds packages (a package designator or a list of them) to the use list of package (the current package by default), so the external symbols of the used packages are visible unqualified afterwards. Returns t. Using a package twice is a no-op; using a package in itself is an error, as is an unknown package (No such package: NOSUCH). It is the runtime form of the defpackage :use clause.

Packages are resolved at read/compile time here (see Packages), so a literal top-level call is consumed at compile time like in-package and takes effect for the forms that follow it — which is what makes it work on every backend. A runtime-computed call (a designator built at run time) works on the interpreter only. Internal symbols are never inherited: only what the used package :exports becomes visible.