SI-7622 Plugins can be not enabled

Plugins can interrogate options and declare themselves not
enabled.  The plugin itself can return false from its init
if the options do not compute.  A plugin phase component
can declare itself not enabled, same as an internal phase.

No one exploits this facility at this commit.

4 files changed, 35 insertions, 27 deletions

diff --git a/src/compiler/scala/tools/nsc/plugins/Plugin.scala b/src/compiler/scala/tools/nsc/plugins/Plugin.scala
index 4fd6ba7d9d..1578caff26 100644
--- a/src/compiler/scala/tools/nsc/plugins/Plugin.scala
+++ b/src/compiler/scala/tools/nsc/plugins/Plugin.scala
@@ -41,12 +41,31 @@ abstract class Plugin {
    */
   val global: Global
 
-  /** Handle any plugin-specific options.  The `-P:plugname:` part
-   *  will not be present.
+  def options: List[String] = {
+    // Process plugin options of form plugin:option
+    def namec = name + ":"
+    global.settings.pluginOptions.value filter (_ startsWith namec) map (_ stripPrefix namec)
+  }
+
+  /** Handle any plugin-specific options.
+   *  The user writes `-P:plugname:opt1,opt2`,
+   *  but the plugin sees `List(opt1, opt2)`.
+   *  The plugin can opt out of further processing
+   *  by returning false.  For example, if the plugin
+   *  has an "enable" flag, now would be a good time
+   *  to sit on the bench.
+   *  @param options plugin arguments
+   *  @param error error function
+   *  @return true to continue, or false to opt out
    */
-  def processOptions(options: List[String], error: String => Unit) {
-    if (!options.isEmpty)
-      error("Error: " + name + " has no options")
+  def init(options: List[String], error: String => Unit): Boolean = {
+    processOptions(options, error)
+    true
+  }
+
+  @deprecated("use Plugin#init instead", since="2.11")
+  def processOptions(options: List[String], error: String => Unit): Unit = {
+    if (!options.isEmpty) error(s"Error: $name takes no options")
   }
 
   /** A description of this plugin's options, suitable as a response
diff --git a/src/compiler/scala/tools/nsc/plugins/PluginComponent.scala b/src/compiler/scala/tools/nsc/plugins/PluginComponent.scala
index c6e1af7ea4..a6df08c331 100644
--- a/src/compiler/scala/tools/nsc/plugins/PluginComponent.scala
+++ b/src/compiler/scala/tools/nsc/plugins/PluginComponent.scala
@@ -15,12 +15,10 @@ package plugins
  */
 abstract class PluginComponent extends SubComponent {
 
-  /** Internal flag to tell external from internal phases */
+  /** By definition, plugin phases are externally provided. */
   final override val internal = false
 
-  /** Phases supplied by plugins should not have to supply the
-   *  runsRightAfter constraint, but can override it.
-   */
+  /** Only plugins are granted a reprieve from specifying whether they follow. */
   val runsRightAfter: Option[String] = None
 
   /** Useful for -Xshow-phases. */
diff --git a/src/compiler/scala/tools/nsc/plugins/Plugins.scala b/src/compiler/scala/tools/nsc/plugins/Plugins.scala
index 8f7794fa90..4769705404 100644
--- a/src/compiler/scala/tools/nsc/plugins/Plugins.scala
+++ b/src/compiler/scala/tools/nsc/plugins/Plugins.scala
@@ -78,27 +78,18 @@ trait Plugins {
 
     val plugs = pick(roughPluginsList, Set(), (phasesSet map (_.phaseName)).toSet)
 
-    /* Verify requirements are present. */
+    // Verify required plugins are present.
     for (req <- settings.require.value ; if !(plugs exists (_.name == req)))
       globalError("Missing required plugin: " + req)
 
-    /* Process plugin options. */
-    def namec(plug: Plugin) = plug.name + ":"
-    def optList(xs: List[String], p: Plugin) = xs filter (_ startsWith namec(p))
-    def doOpts(p: Plugin): List[String] =
-      optList(settings.pluginOptions.value, p) map (_ stripPrefix namec(p))
+    // Verify no non-existent plugin given with -P
+    for {
+      opt <- settings.pluginOptions.value
+      if !(plugs exists (opt startsWith _.name + ":"))
+    } globalError("bad option: -P:" + opt)
 
-    for (p <- plugs) {
-      val opts = doOpts(p)
-      if (!opts.isEmpty)
-        p.processOptions(opts, globalError)
-    }
-
-    /* Verify no non-existent plugin given with -P */
-    for (opt <- settings.pluginOptions.value ; if plugs forall (p => optList(List(opt), p).isEmpty))
-      globalError("bad option: -P:" + opt)
-
-    plugs
+    // Plugins may opt out, unless we just want to show info
+    plugs filter (p => p.init(p.options, globalError) || (settings.debug && settings.isInfo))
   }
 
   lazy val plugins: List[Plugin] = loadPlugins()
diff --git a/test/files/neg/t7494-no-options.check b/test/files/neg/t7494-no-options.check
index efc91a6215..04fbf0ec72 100644
--- a/test/files/neg/t7494-no-options.check
+++ b/test/files/neg/t7494-no-options.check
@@ -1,4 +1,4 @@
-error: Error: ploogin has no options
+error: Error: ploogin takes no options
     phase name  id  description
     ----------  --  -----------
         parser   1  parse source into ASTs, perform simple desugaring