CLJS-2886: count specializations for string and array

Author: mfikes

src/main/clojure/cljs/compiler.cljc

@@ -1136,8 +1136,11 @@
                                 (not ('#{any clj clj-or-nil clj-nil number string boolean function object array js} tag))
                                 (when-let [ps (:protocols (ana/resolve-existing-var env tag))]
                                   (ps protocol)))))))
+        first-arg-tag (ana/infer-tag env (first (:args expr)))
         opt-not? (and (= (:name info) 'cljs.core/not)
-                      (= (ana/infer-tag env (first (:args expr))) 'boolean))
+                      (= first-arg-tag 'boolean))
+        opt-count? (and (= (:name info) 'cljs.core/count)
+                        (boolean ('#{string array} first-arg-tag)))
         ns (:ns info)
         js? (or (= ns 'js) (= ns 'Math))
         goog? (when ns
@@ -1193,6 +1196,9 @@
        opt-not?
        (emits "(!(" (first args) "))")
 
+       opt-count?
+       (emits "((" (first args) ").length)")
+
        proto?
        (let [pimpl (str (munge (protocol-prefix protocol))
                         (munge (name (:name info))) "$arity$" (count args))]

src/test/cljs/cljs/core_test.cljs

@@ -1703,6 +1703,14 @@
   (is (= "ab" (str "a" nil "b")))
   (is (= "ahellob" (str "a" (str-fn-2865) "b"))))
 
+(deftest test-cljs-2886
+  (is (zero? (count "")))
+  (is (== 1 (count "a")))
+  (is (zero? (count #js [])))
+  (is (== 1 (count #js [1])))
+  (is (zero? (count [])))
+  (is (== 1 (count [1]))))
+
 (deftest test-cljs-2934
   (let [x (delay 1)]
     (is (= "#object[cljs.core.Delay {:status :pending, :val nil}]" (pr-str x)))