SI-6476 Accept backslash at end of interpolation

Everyone wants to say `raw"\hello, world.\"`.

Parser accepts arbitrary escaped quotes optionally followed by
a quote. That is, an escaped quote is deemed to terminate the
interpolated string if it is not followed by another quote on
the same line.

src/compiler/scala/tools/nsc/ast/parser/Scanners.scala

@@ -718,6 +718,14 @@ trait Scanners extends ScannersCommon {
         next.lastOffset = charOffset - 1
         next.offset = charOffset - 1
       }
+      def isLastQuoteOnLine: Boolean = {
+        val lookahead = lookaheadReader
+        var c: Char = 0
+        do {
+          c = lookahead.getc()
+        } while (c != '"' && c != LF && c != SU)
+        (c != '"')
+      }
       @tailrec def loop(escaped: Boolean): Unit = ch match {
         case '\\' if !multiLine =>
           putChar(ch)
@@ -730,7 +738,18 @@ trait Scanners extends ScannersCommon {
             token = STRINGLIT
           } else
             loop(escaped = false)
-        case '"' | '$' if escaped =>
+        case '"' if escaped =>
+          // at \", unescape and finish if this is the last quote on the line
+          if (!multiLine && isLastQuoteOnLine) {
+            nextChar()
+            setStrVal()
+            token = STRINGLIT
+          } else {
+            putChar(ch)
+            nextRawChar()
+            loop(escaped = false)
+          }
+        case '$' if escaped =>
           putChar(ch)
           nextRawChar()
           loop(escaped = false)

test/files/neg/t5510.check

@@ -1,25 +1,22 @@
-t5510.scala:2: error: unclosed string literal (\" is an escaped quote, not the end of string)
-  val sa = s"x\"y
-             ^
-t5510.scala:3: error: unclosed string literal (\" is an escaped quote, not the end of string)
+t5510.scala:5: error: unclosed string literal (\" is an escaped quote, not the end of string)
   val sb = "x\"y
            ^
-t5510.scala:4: error: unclosed string literal
+t5510.scala:6: error: unclosed string literal
   val s1 = s"xxx
              ^
-t5510.scala:5: error: unclosed string literal
+t5510.scala:7: error: unclosed string literal
   val s2 = s"xxx $x
                    ^
-t5510.scala:6: error: unclosed string literal
+t5510.scala:8: error: unclosed string literal
   val s3 = s"xxx $$
              ^
-t5510.scala:7: error: unclosed string literal
+t5510.scala:9: error: unclosed string literal
   val s4 = ""s"
                ^
-t5510.scala:8: error: unclosed multi-line string literal
+t5510.scala:10: error: unclosed multi-line string literal
   val s5 = ""s""" $s1 $s2 s"
                          ^
-t5510.scala:9: error: unclosed multi-line string literal
+t5510.scala:11: error: unclosed multi-line string literal
 }
  ^
-8 errors found
+7 errors found

test/files/neg/t5510.scala

@@ -1,5 +1,7 @@
-object Test {
-  val sa = s"x\"y
+trait TypeErrors {
+  val sa = s"x\"y  // see t6476.scala
+}
+trait ParseErrors {
   val sb = "x\"y
   val s1 = s"xxx
   val s2 = s"xxx $x

test/files/neg/t6476.check

@@ -0,0 +1,4 @@
+t6476.scala:2: error: value y is not a member of String
+  val sa = s"x\"y
+                ^
+one error found

test/files/neg/t6476.scala

@@ -0,0 +1,3 @@
+trait TypeErrors {
+  val sa = s"x\"y
+}

test/junit/scala/StringContextTest.scala

@@ -85,6 +85,9 @@ class StringContextTest {
     val i = 42
     assertEquals("""Forty-two is "42"""", s"Forty-two is \"42\"")
     assertEquals("""dir\""", s"dir\\")
+    assertEquals("""dir\""", raw"dir\")
+    assertEquals("""1 \ 42""", raw"1 \ $i")
+    assertEquals("""1 \ 42 \""", raw"1 \ $i \")
     assertEquals("""\\""", s"\\\\")
     // colon separator, in case we like to escape dollar someday
     assertEquals("""\\\:42""", raw"\\\:$i")