SI-8040 Warn unused flags

Introduce `-Ywarn-unused:x,y,z` and exploit `-Ywarn-unused:patvars`.

Although the tree attachment for shielding patvars from warnings
is not structural, sneaking the settings flag into the reflection
internal TreeGen is awkward.

Add test to ensure isolation of patvars warning from others.

`-Ywarn-unused-import` is an alias for `-Ywarn-unused:imports`.

`-Xlint:unused` is an alias for `-Ywarn-unused`, but not enabled
yet. The help text advises to use `-Ywarn-unused`. The future can
decide if `-Xlint:unused-imports` is warranted.

4 files changed, 65 insertions, 1 deletions

diff --git a/test/files/neg/warn-unused-imports/warn-unused-imports_2.scala b/test/files/neg/warn-unused-imports/warn-unused-imports_2.scala
index ded1186209..56ad3393a1 100644
--- a/test/files/neg/warn-unused-imports/warn-unused-imports_2.scala
+++ b/test/files/neg/warn-unused-imports/warn-unused-imports_2.scala
@@ -96,7 +96,7 @@ trait Warn {
 trait Nested {
   {
     import p1._   // warn
-    trait Warn {  // warn about unused local trait for good measure
+    trait Warn {  // don't warn about unused local trait with -Ywarn-unused:imports
       import p2._
       println(new A)
       println("abc".bippy)
diff --git a/test/files/neg/warn-unused-patvars.check b/test/files/neg/warn-unused-patvars.check
new file mode 100644
index 0000000000..002f7a07ca
--- /dev/null
+++ b/test/files/neg/warn-unused-patvars.check
@@ -0,0 +1,12 @@
+warn-unused-patvars.scala:7: warning: private val x in trait Boundings is never used
+  private val x = 42                      // warn, sanity check
+              ^
+warn-unused-patvars.scala:26: warning: local val x in method v is never used
+    val D(x) = d                          // warn, fixme
+         ^
+warn-unused-patvars.scala:30: warning: local val x in method w is never used
+    val D(x @ _) = d                      // warn, fixme (valdef pos is different)
+         ^
+error: No warnings can be incurred under -Xfatal-warnings.
+three warnings found
+one error found
diff --git a/test/files/neg/warn-unused-patvars.flags b/test/files/neg/warn-unused-patvars.flags
new file mode 100644
index 0000000000..d5bd86a658
--- /dev/null
+++ b/test/files/neg/warn-unused-patvars.flags
@@ -0,0 +1 @@
+-Ywarn-unused:-patvars,_ -Xfatal-warnings
diff --git a/test/files/neg/warn-unused-patvars.scala b/test/files/neg/warn-unused-patvars.scala
new file mode 100644
index 0000000000..6f4620c0c7
--- /dev/null
+++ b/test/files/neg/warn-unused-patvars.scala
@@ -0,0 +1,51 @@
+
+case class C(a: Int, b: String, c: Option[String])
+case class D(a: Int)
+
+trait Boundings {
+
+  private val x = 42                      // warn, sanity check
+
+  def c = C(42, "hello", Some("world"))
+  def d = D(42)
+
+  def f() = {
+    val C(x, y, Some(z)) = c              // no warn
+    17
+  }
+  def g() = {
+    val C(x @ _, y @ _, Some(z @ _)) = c  // no warn
+    17
+  }
+  def h() = {
+    val C(x @ _, y @ _, z @ Some(_)) = c  // no warn for z?
+    17
+  }
+
+  def v() = {
+    val D(x) = d                          // warn, fixme
+    17
+  }
+  def w() = {
+    val D(x @ _) = d                      // warn, fixme (valdef pos is different)
+    17
+  }
+
+}
+
+trait Forever {
+  def f = {
+    val t = Option((17, 42))
+    for {
+      ns <- t
+      (i, j) = ns                        // no warn
+    } yield (i + j)
+  }
+  def g = {
+    val t = Option((17, 42))
+    for {
+      ns <- t
+      (i, j) = ns                        // no warn
+    } yield 42
+  }
+}