CLJS-2864: Optimize str macro for single arity case

For the single arity case, emit code that doesn't involve constructing
a JavaScript array and a call to join, but instead directly calls the
runtime single-arity str implementation.

Author: mfikes

src/main/clojure/cljs/core.cljc

@@ -827,16 +827,22 @@
       (core/quot c 32)
       (core/inc (core/quot c 32)))))
 
-(core/defmacro str [& xs]
-  (core/let [interpolate (core/fn [x]
-                           (if (core/string? x)
-                             "~{}"
-                             "cljs.core.str.cljs$core$IFn$_invoke$arity$1(~{})"))
-             strs (core/->> xs
-                    (map interpolate)
-                    (interpose ",")
-                    (apply core/str))]
-    (list* 'js* (core/str "[" strs "].join('')") xs)))
+(core/defmacro str
+  ([] "")
+  ([x]
+   (if (core/string? x)
+     x
+     (core/list 'js* "cljs.core.str.cljs$core$IFn$_invoke$arity$1(~{})" x)))
+  ([x & ys]
+   (core/let [interpolate (core/fn [x]
+                            (if (core/string? x)
+                              "~{}"
+                              "cljs.core.str.cljs$core$IFn$_invoke$arity$1(~{})"))
+              strs        (core/->> (core/list* x ys)
+                            (map interpolate)
+                            (interpose ",")
+                            (apply core/str))]
+     (list* 'js* (core/str "[" strs "].join('')") x ys))))
 
 (core/defn- bool-expr [e]
   (vary-meta e assoc :tag 'boolean))

src/test/cljs/cljs/core_test.cljs

@@ -1626,6 +1626,13 @@
   (is (thrown? js/Error ((not empty?) "foo")))
   (is (thrown? js/Error ((not empty?) ""))))
 
+(deftest test-cljs-2864
+  (is (= "" (str)))
+  (is (= "a" (str "a")))
+  (is (= "1" (str 1)))
+  (is (= "xyzzy" (str "x" "y" "z" "z" "y")))
+  (is (= "a1b2c3" (str "a" 1 "b" 2 "c" 3))))
+
 (deftest test-cljs-2943
   (let [m1 {:a 2, :b 3, :c 5}
         m2 {:a 7, :b 11, :d 13, :e 17}