Warn when trying to load plugins that can't be used.

This addresses the problems seen in #1900 and #1901. It doesn't fix
them because they can't be fixed without breaking other things,
(technomancy/grenchman#34) but at least it
lets people know what is going on and suggests a workaround.

leiningen-core/src/leiningen/core/classpath.clj

@@ -17,6 +17,8 @@
   (require 'leiningen.core.main)
   (apply (resolve 'leiningen.core.main/warn) args))
 
+(def ^:private warn-once (memoize warn))
+
 (defn ^:deprecated extract-native-deps [files native-path native-prefixes]
   (doseq [file files
           :let [native-prefix (get native-prefixes file "native/")
@@ -493,6 +495,24 @@
           (doseq [[_ {:keys [native-prefix file]}] snap-deps]
             (extract-native-dep! native-path file native-prefix))))))
 
+(def ^:private bootclasspath-deps (-> "leiningen/bootclasspath-deps.clj"
+                                      io/resource slurp read-string))
+
+(defn- warn-conflicts
+  "When using the bootclasspath (for boot speed), resources already on the
+  bootclasspath cannot be overridden by plugins, so notify the user about it."
+  [project dependencies]
+  (let [warned (atom false)]
+    (doseq [[artifact version] dependencies
+            :when (and (bootclasspath-deps artifact)
+                       (not= (bootclasspath-deps artifact) version))]
+      (reset! warned true)
+      (warn-once "Tried to load" artifact "version" version "but"
+                 (bootclasspath-deps artifact) "was already loaded."))
+    (when (and @warned (not (:root project)))
+      (warn-once "You can set :eval-in :subprocess in your :user profile;"
+                 "however this will increase repl load time."))))
+
 (defn resolve-managed-dependencies
   "Delegate dependencies to pomegranate. This will ensure they are
   downloaded into ~/.m2/repository and that native components of
@@ -511,6 +531,9 @@
         jars (->> dependencies-tree
                   (aether/dependency-files)
                   (filter #(re-find #"\.(jar|zip)$" (.getName %))))]
+    (when (some #{:add-classpath?} rest)
+      (warn-conflicts project (concat (keys dependencies-tree)
+                                      (reduce into (vals dependencies-tree)))))
     (when (and (= :dependencies dependencies-key)
                (:root project))
       (extract-native-dependencies project jars dependencies-tree))

resources/leiningen/bootclasspath-deps.clj

@@ -0,0 +1,69 @@
+;; This file is used to warn users when they attempt to load a plugin that
+;; pulls in a dependency which conflicts with something already in use
+;; by Leiningen itself.
+{
+ ;; Unhelpful to warn on these:
+ ;; org.clojure/clojure "1.8.0"
+ ;; leiningen-core "2.7.2-SNAPSHOT"
+ bultitude "0.2.8"
+ cheshire "5.6.3"
+ clj-http "2.0.1"
+ clj-stacktrace "0.2.7"
+ clj-tuple "0.2.2"
+ clojure-complete "0.2.4"
+ com.cemerick/drawbridge "0.0.6"
+ com.cemerick/pomegranate "0.3.1"
+ com.fasterxml.jackson.core/jackson-core "2.7.5"
+ com.fasterxml.jackson.dataformat/jackson-dataformat-cbor "2.7.5"
+ com.fasterxml.jackson.dataformat/jackson-dataformat-smile "2.7.5"
+ com.hypirion/io "0.3.1"
+ commons-codec "1.10"
+ commons-io "2.5"
+ commons-lang "2.6"
+ commons-logging "1.1.3"
+ jline "2.12.1"
+ net.cgrand/parsley "0.9.3"
+ net.cgrand/regex "1.1.0"
+ net.cgrand/sjacket "0.1.1"
+ org.apache.httpcomponents/httpclient "4.5"
+ org.apache.httpcomponents/httpcore "4.4.1"
+ org.apache.httpcomponents/httpmime "4.5"
+ org.apache.maven.wagon/wagon-http "2.10"
+ org.apache.maven.wagon/wagon-http-shared "2.10"
+ org.apache.maven.wagon/wagon-provider-api "2.2"
+ org.apache.maven/maven-aether-provider "3.0.4"
+ org.apache.maven/maven-model "3.0.4"
+ org.apache.maven/maven-model-builder "3.0.4"
+ org.apache.maven/maven-repository-metadata "3.0.4"
+ org.clojure/data.xml "0.0.8"
+ org.clojure/tools.cli "0.3.1"
+ org.clojure/tools.macro "0.1.5"
+ org.clojure/tools.nrepl "0.2.12"
+ org.codehaus.plexus/plexus-classworlds "2.4"
+ org.codehaus.plexus/plexus-component-annotations "1.5.5"
+ org.codehaus.plexus/plexus-interpolation "1.14"
+ org.codehaus.plexus/plexus-utils "2.0.6"
+ org.flatland/classlojure "0.7.1"
+ org.jsoup/jsoup "1.7.2"
+ org.sonatype.aether/aether-api "1.13.1"
+ org.sonatype.aether/aether-connector-file "1.13.1"
+ org.sonatype.aether/aether-connector-wagon "1.13.1"
+ org.sonatype.aether/aether-impl "1.13.1"
+ org.sonatype.aether/aether-spi "1.13.1"
+ org.sonatype.aether/aether-util "1.13.1"
+ org.sonatype.sisu/sisu-guice "3.0.3" :classifier "no_aop"
+ org.sonatype.sisu/sisu-inject-bean "2.2.3"
+ org.sonatype.sisu/sisu-inject-plexus "2.2.3"
+ org.tcrawley/dynapath "0.2.5"
+ pedantic "0.2.0"
+ potemkin "0.4.1"
+ quoin "0.1.2"
+ reply "0.3.7"
+ riddley "0.1.10"
+ robert/hooke "1.3.0"
+ scout "0.1.1"
+ slingshot "0.12.2"
+ stencil "0.5.0"
+ tigris "0.1.1"
+ trptcolin/versioneer "0.1.1"
+ }