Do not start a DerefableState as a side-effect of printing

See: b80e1fe4b1/src/clj/clojure/core_print.clj (L422-L446)

src/mount/core.cljc

@@ -104,6 +104,9 @@
     (swap! running dissoc state)
     (update-meta! [state :status] #{:stopped})))
 
+(defn running-states []
+  (set (keys @running)))
+
 (deftype DerefableState [name]
   #?(:clj clojure.lang.IDeref
      :cljs IDeref)
@@ -113,7 +116,13 @@
     (let [{:keys [status inst] :as state} (@meta-state name)]
       (when-not (:started status)
         (up name state (atom #{})))
-      @inst)))
+      @inst))
+  #?(:clj clojure.lang.IPending
+     :cljs IPending)
+  (#?(:clj isRealized
+      :cljs -realized?)
+    [_]
+    (boolean ((running-states) name))))
 
 (defn on-reload-meta [s-var]
   (or (-> s-var meta :on-reload)
@@ -181,9 +190,6 @@
           (with-ns ns name))
         v)))
 
-(defn running-states []
-  (set (keys @running)))
-
 (defn- unvar-state [s]
   (->> s (drop 2) (apply str)))  ;; magic 2 is removing "#'" in state name
 

test/core/mount/test.cljc

@@ -13,6 +13,7 @@
     mount.test.start-without
     mount.test.start-with
     mount.test.start-with-states
+    mount.test.printing
     ))
 
 #?(:clj (alter-meta! *ns* assoc ::load false))
@@ -31,6 +32,7 @@
                'mount.test.start-without
                'mount.test.start-with
                'mount.test.start-with-states
+               'mount.test.printing
                ))
 
 (defn run-tests []

test/core/mount/test/printing.cljc

@@ -0,0 +1,17 @@
+(ns mount.test.printing
+  (:require
+   #?@(:cljs [[cljs.test :as t :refer-macros [is are deftest testing use-fixtures]]
+              [mount.core :as mount :refer-macros [defstate]]]
+       :clj  [[clojure.test :as t :refer [is are deftest testing use-fixtures]]
+              [mount.core :as mount :refer [defstate]]])))
+
+#?(:clj (alter-meta! *ns* assoc ::load false))
+
+(defstate foo
+  :start (do (println "Starting!") 42))
+
+(deftest test-printing-has-no-side-effects
+  ;; Test that printing an unstarted DerefableState does not have the
+  ;; side-effect of starting it
+  (println foo)
+  (is (not= 42 foo)))