SI-8040 Warn unused pattern vars

Warn for unused `case X(x) =>` but, as an escape hatch,
not for `case X(x @ _) =>`. The latter form is deemed
documentary. (Named args could serve a similar purpose,
`case X(x = _) =>`.)

An attachment is used to mark the bound var, and the
symbol position is used to correlate the identifier
with the variable that is introduced. This mechanism
doesn't work yet when only a single var is defined.

2 files changed, 51 insertions, 1 deletions

diff --git a/test/files/neg/warn-unused-privates.check b/test/files/neg/warn-unused-privates.check
index 6e1511d0e5..d273aa40f4 100644
--- a/test/files/neg/warn-unused-privates.check
+++ b/test/files/neg/warn-unused-privates.check
@@ -61,6 +61,24 @@ warn-unused-privates.scala:137: warning: private method x in class OtherNames is
 warn-unused-privates.scala:138: warning: private method y_= in class OtherNames is never used
   private def y_=(i: Int): Unit = ???
               ^
+warn-unused-privates.scala:153: warning: local val x in method f is never used
+    val C(x, y, Some(z)) = c              // warn
+          ^
+warn-unused-privates.scala:153: warning: local val y in method f is never used
+    val C(x, y, Some(z)) = c              // warn
+             ^
+warn-unused-privates.scala:153: warning: local val z in method f is never used
+    val C(x, y, Some(z)) = c              // warn
+                     ^
+warn-unused-privates.scala:161: warning: local val z in method h is never used
+    val C(x @ _, y @ _, z @ Some(_)) = c  // warn?
+                        ^
+warn-unused-privates.scala:166: warning: local val x in method v is never used
+    val D(x) = d                          // warn
+         ^
+warn-unused-privates.scala:170: warning: local val x in method w is never used
+    val D(x @ _) = d                      // fixme
+         ^
 warn-unused-privates.scala:97: warning: local var x in method f2 is never set - it could be a val
     var x = 100 // warn about it being a var
         ^
@@ -80,5 +98,5 @@ warn-unused-privates.scala:121: warning: local type OtherThing is never used
     type OtherThing = String // warn
          ^
 error: No warnings can be incurred under -Xfatal-warnings.
-27 warnings found
+33 warnings found
 one error found
diff --git a/test/files/neg/warn-unused-privates.scala b/test/files/neg/warn-unused-privates.scala
index 2f67882632..1b702c7555 100644
--- a/test/files/neg/warn-unused-privates.scala
+++ b/test/files/neg/warn-unused-privates.scala
@@ -140,3 +140,35 @@ class OtherNames {
 
   def f = y
 }
+
+case class C(a: Int, b: String, c: Option[String])
+case class D(a: Int)
+
+trait Boundings {
+
+  def c = C(42, "hello", Some("world"))
+  def d = D(42)
+
+  def f() = {
+    val C(x, y, Some(z)) = c              // warn
+    17
+  }
+  def g() = {
+    val C(x @ _, y @ _, Some(z @ _)) = c  // no warn
+    17
+  }
+  def h() = {
+    val C(x @ _, y @ _, z @ Some(_)) = c  // warn?
+    17
+  }
+
+  def v() = {
+    val D(x) = d                          // warn
+    17
+  }
+  def w() = {
+    val D(x @ _) = d                      // fixme
+    17
+  }
+
+}