[SPARK-5040][SQL] Support expressing unresolved attributes using $"attribute...

[SPARK-5040][SQL] Support expressing unresolved attributes using $"attribute name" notation in SQL DSL.

Author: Reynold Xin <rxin@databricks.com>

Closes #3862 from rxin/stringcontext-attr and squashes the following commits:

9b10f57 [Reynold Xin] Rename StrongToAttributeConversionHelper
72121af [Reynold Xin] [SPARK-5040][SQL] Support expressing unresolved attributes using $"attribute name" notation in SQL DSL.

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/dsl/package.scala

@@ -135,6 +135,15 @@ package object dsl {
     implicit def symbolToUnresolvedAttribute(s: Symbol): analysis.UnresolvedAttribute =
       analysis.UnresolvedAttribute(s.name)
 
+    /** Converts $"col name" into an [[analysis.UnresolvedAttribute]]. */
+    implicit class StringToAttributeConversionHelper(val sc: StringContext) {
+      // Note that if we make ExpressionConversions an object rather than a trait, we can
+      // then make this a value class to avoid the small penalty of runtime instantiation.
+      def $(args: Any*): analysis.UnresolvedAttribute = {
+        analysis.UnresolvedAttribute(sc.s(args :_*))
+      }
+    }
+
     def sum(e: Expression) = Sum(e)
     def sumDistinct(e: Expression) = SumDistinct(e)
     def count(e: Expression) = Count(e)

sql/core/src/test/scala/org/apache/spark/sql/DslQuerySuite.scala

@@ -56,6 +56,18 @@ class DslQuerySuite extends QueryTest {
     )
   }
 
+  test("convert $\"attribute name\" into unresolved attribute") {
+    checkAnswer(
+      testData.where($"key" === 1).select($"value"),
+      Seq(Seq("1")))
+  }
+
+  test("convert Scala Symbol 'attrname into unresolved attribute") {
+    checkAnswer(
+      testData.where('key === 1).select('value),
+      Seq(Seq("1")))
+  }
+
   test("select *") {
     checkAnswer(
       testData.select(Star(None)),