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

@@ -887,84 +887,93 @@ trait Scanners extends ScannersCommon {
       }
     }
 
-    @tailrec private def getStringPart(multiLine: Boolean): Unit = {
+    private def getStringPart(multiLine: Boolean): Unit = {
       def finishStringPart() = {
         setStrVal()
         token = STRINGPART
         next.lastOffset = charOffset - 1
         next.offset = charOffset - 1
       }
-      if (ch == '"') {
-        if (multiLine) {
+      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)
+          nextRawChar()
+          loop(escaped = !escaped)
+        case '"' if multiLine =>
           nextRawChar()
           if (isTripleQuote()) {
             setStrVal()
             token = STRINGLIT
           } else
             getStringPart(multiLine)
-        } else {
-          nextChar()
-          setStrVal()
-          token = STRINGLIT
-        }
-      } else if (ch == '\\') {
-        if (multiLine) {
-          putChar(ch)
-          nextRawChar()
-          getStringPart(multiLine)
-        } else {
-          putChar(ch)
-          nextRawChar()
-          if (ch == '"') {
+        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)
           }
-          getStringPart(multiLine)
-        }
-      } else if (ch == '$') {
-        nextRawChar()
-        if (ch == '$') {
+        case '$' if escaped =>
           putChar(ch)
           nextRawChar()
-          getStringPart(multiLine)
-        } else if (ch == '{') {
-          finishStringPart()
-          nextRawChar()
-          next.token = LBRACE
-        } else if (ch == '_') {
-          finishStringPart()
+          loop(escaped = false)
+        case '"' =>
+          nextChar()
+          setStrVal()
+          token = STRINGLIT
+        case '$' =>
           nextRawChar()
-          next.token = USCORE
-        } else if (Character.isUnicodeIdentifierStart(ch)) {
-          finishStringPart()
-          do {
+          if (ch == '$') {
             putChar(ch)
             nextRawChar()
-          } while (ch != SU && Character.isUnicodeIdentifierPart(ch))
-          next.token = IDENTIFIER
-          next.name = newTermName(cbuf.toString)
-          cbuf.clear()
-          val idx = next.name.start - kwOffset
-          if (idx >= 0 && idx < kwArray.length) {
-            next.token = kwArray(idx)
+            loop(escaped = false)
+          } else if (ch == '{') {
+            finishStringPart()
+            nextRawChar()
+            next.token = LBRACE
+          } else if (ch == '_') {
+            finishStringPart()
+            nextRawChar()
+            next.token = USCORE
+          } else if (Character.isUnicodeIdentifierStart(ch)) {
+            finishStringPart()
+            do {
+              putChar(ch)
+              nextRawChar()
+            } while (ch != SU && Character.isUnicodeIdentifierPart(ch))
+            next.token = IDENTIFIER
+            next.name = newTermName(cbuf.toString)
+            cbuf.clear()
+            val idx = next.name.start - kwOffset
+            if (idx >= 0 && idx < kwArray.length) {
+              next.token = kwArray(idx)
+            }
+          } else {
+            syntaxError(s"invalid string interpolation $$$ch, expected: $$$$, $$identifier or $${expression}")
           }
-        } else {
-          syntaxError(s"invalid string interpolation $$$ch, expected: $$$$, $$identifier or $${expression}")
-        }
-      } else {
-        val isUnclosedLiteral = (ch == SU || (!multiLine && (ch == CR || ch == LF)))
-        if (isUnclosedLiteral) {
+        case _ if ch == SU || (!multiLine && (ch == CR || ch == LF)) =>
           if (multiLine)
             incompleteInputError("unclosed multi-line string literal")
           else
             unclosedStringLit()
-        }
-        else {
+        case _ =>
           putChar(ch)
           nextRawChar()
-          getStringPart(multiLine)
-        }
+          loop(escaped = false)
       }
+      loop(escaped = false)
     }
 
     private def fetchStringPart() = {

test/files/neg/string-interpolator.check

@@ -0,0 +1,12 @@
+string-interpolator.scala:2: error: invalid escape at terminal index 1 in "x\". Use \\ for literal \.
+  val sa = s"x\"y
+              ^
+string-interpolator.scala:2: error: postfix operator y needs to be enabled
+by making the implicit value scala.language.postfixOps visible.
+This can be achieved by adding the import clause 'import scala.language.postfixOps'
+or by setting the compiler option -language:postfixOps.
+See the Scaladoc for value scala.language.postfixOps for a discussion
+why the feature needs to be explicitly enabled.
+  val sa = s"x\"y
+                ^
+2 errors

test/files/neg/string-interpolator.scala

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

test/files/neg/t5510.check

@@ -1,19 +1,22 @@
-t5510.scala:2: error: unclosed string literal
+t5510.scala:5: error: unclosed string literal
+  val sb = "x\"y
+           ^
+t5510.scala:6: error: unclosed string literal
   val s1 = s"xxx
              ^
-t5510.scala:3: error: unclosed string literal
+t5510.scala:7: error: unclosed string literal
   val s2 = s"xxx $x
                    ^
-t5510.scala:4: error: unclosed string literal
+t5510.scala:8: error: unclosed string literal
   val s3 = s"xxx $$
              ^
-t5510.scala:5: error: unclosed string literal
+t5510.scala:9: error: unclosed string literal
   val s4 = ""s"
                ^
-t5510.scala:6: error: unclosed multi-line string literal
+t5510.scala:10: error: unclosed multi-line string literal
   val s5 = ""s""" $s1 $s2 s"
                          ^
-t5510.scala:7: error: unclosed multi-line string literal
+t5510.scala:11: error: unclosed multi-line string literal
 }
  ^
-6 errors
+7 errors

test/files/neg/t5510.scala

@@ -1,4 +1,8 @@
-object Test {
+trait TypeErrors {
+  val sa = s"x\"y  // see string-interpolator.scala
+}
+trait ParseErrors {
+  val sb = "x\"y
   val s1 = s"xxx
   val s2 = s"xxx $x
   val s3 = s"xxx $$

test/files/run/string-interpolator.check

@@ -1 +1,2 @@
 "Hello", Alice
+oh hai C:\

test/files/run/string-interpolator.scala

@@ -2,5 +2,6 @@ object Test {
   def main(args: Array[String]): Unit = {
     val person = "Alice"
     println(s"\"Hello\", $person")
+    println(raw"oh hai C:\")
   }
 }