SI-7707 SI-7712 Exclude unused warnings from -Xlint

Experience building open source projects like Specs that use
`-Xlint` suggests that this warning is too noisy to lump in
with the others.

We are lacking in more fine-grained control of these things, so
simply turning of `-Xlint` in favour of its underlying `-Y` options
ends up *losing* some other important warnings that are predicated
directly on `-Xlint`.

Furthermore, bug reports against M8, SI-7707 SI-7712, show that
unused private/local warnings, while far less noisy, are still
in need of polish.

This commit moves these warnings to a pair of new -Y options,
neither of which is part of  `-Xlint`..

Let's ask people to opt in for 2.11, and as it stabilizes, we can
consider adding it to Xlint (or the desirable evolution of that)
in the next release.

7 files changed, 12 insertions, 19 deletions

diff --git a/src/compiler/scala/tools/nsc/settings/Warnings.scala b/src/compiler/scala/tools/nsc/settings/Warnings.scala
index 791d44153c..5214f6485f 100644
--- a/src/compiler/scala/tools/nsc/settings/Warnings.scala
+++ b/src/compiler/scala/tools/nsc/settings/Warnings.scala
@@ -30,6 +30,8 @@ trait Warnings {
     warnNullaryUnit,
     warnAdaptedArgs,
     warnInferAny
+    // warnUnused       SI-7712, SI-7707 warnUnused not quite ready for prime-time
+    // warnUnusedImport currently considered too noisy for general use
   )
 
   // Warning groups.
@@ -56,6 +58,8 @@ trait Warnings {
   val warnInaccessible     = BooleanSetting   ("-Ywarn-inaccessible", "Warn about inaccessible types in method signatures.")
   val warnNullaryOverride  = BooleanSetting   ("-Ywarn-nullary-override", "Warn when non-nullary overrides nullary, e.g. `def foo()` over `def foo`.")
   val warnInferAny         = BooleanSetting   ("-Ywarn-infer-any", "Warn when a type argument is inferred to be `Any`.")
+  val warnUnused           = BooleanSetting   ("-Ywarn-unused", "Warn when local and private vals, vars, defs, and types are are unused")
+  val warnUnusedImport     = BooleanSetting   ("-Ywarn-unused-import", "Warn when imports are unused")
 
   // Backward compatibility.
   @deprecated("Use fatalWarnings", "2.11.0") def Xwarnfatal            = fatalWarnings      // used by sbt
diff --git a/src/compiler/scala/tools/nsc/typechecker/Analyzer.scala b/src/compiler/scala/tools/nsc/typechecker/Analyzer.scala
index 5c02516c47..323fe1c171 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Analyzer.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Analyzer.scala
@@ -102,7 +102,9 @@ trait Analyzer extends AnyRef
           unit.body = typer.typed(unit.body)
           if (global.settings.Yrangepos && !global.reporter.hasErrors) global.validatePositions(unit.body)
           for (workItem <- unit.toCheck) workItem()
-          if (settings.lint)
+          if (settings.warnUnusedImport)
+            warnUnusedImports(unit)
+          if (settings.warnUnused)
             typer checkUnused unit
         }
         finally {
diff --git a/src/compiler/scala/tools/nsc/typechecker/Contexts.scala b/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
index c065fb54b7..974c0842d3 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Contexts.scala
@@ -1210,7 +1210,7 @@ trait Contexts { self: Analyzer =>
   trait ImportContext extends Context {
     private val impInfo: ImportInfo = {
       val info = new ImportInfo(tree.asInstanceOf[Import], outerDepth)
-      if (settings.lint && !isRootImport) // excludes java.lang/scala/Predef imports
+      if (settings.warnUnusedImport && !isRootImport) // excludes java.lang/scala/Predef imports
         allImportInfos(unit) ::= info
       info
     }
@@ -1319,7 +1319,7 @@ trait Contexts { self: Analyzer =>
         if (result == NoSymbol)
           selectors = selectors.tail
       }
-      if (settings.lint && selectors.nonEmpty && result != NoSymbol && pos != NoPosition)
+      if (settings.warnUnusedImport && selectors.nonEmpty && result != NoSymbol && pos != NoPosition)
         recordUsage(current, result)
 
       // Harden against the fallout from bugs like SI-6745
diff --git a/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala b/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
index 71e6454931..dfa1b6db0f 100644
--- a/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/TypeDiagnostics.scala
@@ -518,8 +518,6 @@ trait TypeDiagnostics {
       }
 
       def apply(unit: CompilationUnit) = {
-        warnUnusedImports(unit)
-
         val p = new UnusedPrivates
         p traverse unit.body
         val unused = p.unusedTerms
diff --git a/test/files/neg/warn-unused-imports.check b/test/files/neg/warn-unused-imports.check
index 1b938f4fd7..36c6dd03c3 100644
--- a/test/files/neg/warn-unused-imports.check
+++ b/test/files/neg/warn-unused-imports.check
@@ -1,11 +1,3 @@
-warn-unused-imports.scala:7: warning: it is not recommended to define classes/objects inside of package objects.
-If possible, define class A in package p1 instead.
-  class A
-        ^
-warn-unused-imports.scala:13: warning: it is not recommended to define classes/objects inside of package objects.
-If possible, define class A in package p2 instead.
-  class A
-        ^
 warn-unused-imports.scala:57: warning: Unused import
     import p1.A // warn
               ^
@@ -36,9 +28,6 @@ warn-unused-imports.scala:98: warning: Unused import
 warn-unused-imports.scala:118: warning: Unused import
     import p1.A   // warn
               ^
-warn-unused-imports.scala:99: warning: local trait Warn is never used
-    trait Warn {  // warn about unused local trait for good measure
-          ^
 error: No warnings can be incurred under -Xfatal-warnings.
-13 warnings found
+10 warnings found
 one error found
diff --git a/test/files/neg/warn-unused-imports.flags b/test/files/neg/warn-unused-imports.flags
index 954eaba352..24db705df1 100644
--- a/test/files/neg/warn-unused-imports.flags
+++ b/test/files/neg/warn-unused-imports.flags
@@ -1 +1 @@
--Xfatal-warnings -Xlint
+-Xfatal-warnings -Ywarn-unused-import
diff --git a/test/files/neg/warn-unused-privates.flags b/test/files/neg/warn-unused-privates.flags
index 7949c2afa2..25474aefb3 100644
--- a/test/files/neg/warn-unused-privates.flags
+++ b/test/files/neg/warn-unused-privates.flags
@@ -1 +1 @@
--Xlint -Xfatal-warnings
+-Ywarn-unused -Xfatal-warnings