Warn about forgotten string interpolators.

In the compiler sources this arrives with a number of false
positives, because we frequently work with strings containing
$foo where foo is an in-scope identifier. I think in normal
source code this will be less of a problem, or none at all;
but to be conservative the warning is born under -Xlint.

1 files changed, 9 insertions, 0 deletions

diff --git a/test/files/neg/forgot-interpolator.check b/test/files/neg/forgot-interpolator.check
new file mode 100644
index 0000000000..f6de4d7b3a
--- /dev/null
+++ b/test/files/neg/forgot-interpolator.check
@@ -0,0 +1,9 @@
+forgot-interpolator.scala:4: warning: looks like an interpolated String; did you forget the interpolator?
+  def f = "Put the $bippy in the $bippy!" // warn
+          ^
+forgot-interpolator.scala:14: warning: looks like an interpolated String; did you forget the interpolator?
+  def f = """Put the ${println("bippy")} in the bippy!""" // warn
+          ^
+error: No warnings can be incurred under -Xfatal-warnings.
+two warnings found
+one error found