special formsince v1.9.293Edit
Parser code @ clojurescript:src/main/clojure/cljs/analyzer.cljc
(defmethod parse 'ns*
  [_ env [_ quoted-specs :as form] _ opts]
  (when-let [not-quoted (->> (remove keyword? quoted-specs)
                          (remove #(and (seq? %) (= 'quote (first %))) )
    (throw (error env (str "Arguments to " (name (first quoted-specs))
                        " must be quoted. Offending spec: " not-quoted))))
  (when-not *allow-ns*
    (throw (error env (str "Calls to `" (name (first quoted-specs))
                        "` must appear at the top-level."))))
  (let [specs        (if (= :import (first quoted-specs))
                       (canonicalize-import-specs quoted-specs)
                       (canonicalize-specs quoted-specs))
        name         (-> env :ns :name)
        args         (desugar-ns-specs
                       #?(:clj  (list (process-rewrite-form
                          :cljs (list specs)))
        {:keys [as-aliases] args :libspecs} (nses/elide-aliases-from-ns-specs args)
        {excludes :excludes core-renames :renames} (parse-ns-excludes env args)
        core-renames (reduce (fn [m [original renamed]]
                               (assoc m renamed (symbol "cljs.core" (str original))))
                       {} core-renames)
        deps         (atom [])
        ;; as-aliases can only be used *once* because they are about the reader
        aliases      (atom {:fns as-aliases :macros as-aliases})
        spec-parsers {:require        (partial parse-require-spec env false deps aliases)
                      :require-macros (partial parse-require-spec env true deps aliases)
                      :use            (comp (partial parse-require-spec env false deps aliases)
                                        (partial use->require env))
                      :use-macros     (comp (partial parse-require-spec env true deps aliases)
                                        (partial use->require env))
                      :import         (partial parse-import-spec env deps)}
        reload       (atom {:use nil :require nil :use-macros nil :require-macros nil})
        reloads      (atom {})
        {uses :use requires :require renames :rename
         use-macros :use-macros require-macros :require-macros
         rename-macros :rename-macros imports :import :as params}
          (fn [m [k & libs]]
            ;; check for spec type reloads
            (when-not (= :import k)
              (when (some? (some #{:reload} libs))
                (swap! reload assoc k :reload))
              (when (some? (some #{:reload-all} libs))
                (swap! reload assoc k :reload-all)))
            ;; check for individual ns reloads from REPL interactions
            (when-some [xs (seq (filter #(-> % meta :reload) libs))]
              (swap! reloads assoc k
                (zipmap (map first xs) (map #(-> % meta :reload) xs))))
            (apply merge-with merge m
              (map (spec-parsers k)
                (remove #{:reload :reload-all} libs))))
          {} (remove (fn [[r]] (= r :refer-clojure)) args))]
    (set! *cljs-ns* name)
    (let [require-info
          {:as-aliases     as-aliases
           :name           name
           :excludes       excludes
           :use-macros     use-macros
           :require-macros require-macros
           :rename-macros  rename-macros
           :uses           uses
           :requires       requires
           :renames        (merge renames core-renames)
           :imports        imports}]
      (swap! env/*compiler* update-in [::namespaces name] merge-ns-info require-info env)
      (merge {:op      :ns*
              :env     env
              :form    form
              :deps    (into [] (distinct @deps))
              :reload  @reload
              :reloads @reloads}
        (cond-> require-info
          (@reload :use)
          (update-in [:uses]
            (fn [m] (with-meta m {(@reload :use) true})))
          (@reload :require)
          (update-in [:requires]
            (fn [m] (with-meta m {(@reload :require) true}))))))))

Emitting code @ clojurescript:src/main/clojure/cljs/compiler.cljc
(defmethod emit* :ns*
  [{:keys [name requires uses require-macros reloads env deps]}]
  (load-libs requires nil (:require reloads) deps name)
  (load-libs uses requires (:use reloads) deps name)
  (when (:repl-env env)
    (emitln "'nil';")))