Allow \" in single-quoted string interpolations

Changing `"Hello, \"World\""` to `s"Hello, \"$who\""` no longer breaks.

Before this change, `\"` terminated single-quoted interpolated string
literals, now the string remains open. The scanner doesn't interpret
the escape sequence, string interpolators can do so (`s` and `f` do).

Breaking changes:
  - `raw"c:\"` no longer compiles, it's now an unclosed string
  - `raw"c:\" // uh"` used to evaluate to `"""c:\""", now it's
    """c:\" // uh"""

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

@@ -854,7 +854,12 @@ trait Scanners extends ScannersCommon {
       } else unclosedStringLit()
     }
 
-    private def unclosedStringLit(): Unit = syntaxError("unclosed string literal")
+    private def unclosedStringLit(seenEscapedQuoteInInterpolation: Boolean = false): Unit = {
+      val note =
+        if (seenEscapedQuoteInInterpolation) "; note that `\\\"` no longer closes single-quoted interpolated string literals since 2.13.6, you can use a triple-quoted string instead"
+        else ""
+      syntaxError(s"unclosed string literal$note")
+    }
 
     private def replaceUnicodeEscapesInTriple(): Unit = 
       if(strVal != null) {
@@ -890,7 +895,8 @@ trait Scanners extends ScannersCommon {
       }
     }
 
-    @tailrec private def getStringPart(multiLine: Boolean): Unit = {
+    // for interpolated strings
+    @tailrec private def getStringPart(multiLine: Boolean, seenEscapedQuote: Boolean = false): Unit = {
       def finishStringPart() = {
         setStrVal()
         token = STRINGPART
@@ -904,18 +910,27 @@ trait Scanners extends ScannersCommon {
             setStrVal()
             token = STRINGLIT
           } else
-            getStringPart(multiLine)
+            getStringPart(multiLine, seenEscapedQuote)
         } else {
           nextChar()
           setStrVal()
           token = STRINGLIT
         }
+      } else if (ch == '\\' && !multiLine) {
+        putChar(ch)
+        nextRawChar()
+        val q = ch == '"'
+        if (q || ch == '\\') {
+          putChar(ch)
+          nextRawChar()
+        }
+        getStringPart(multiLine, seenEscapedQuote || q)
       } else if (ch == '$') {
         nextRawChar()
         if (ch == '$' || ch == '"') {
           putChar(ch)
           nextRawChar()
-          getStringPart(multiLine)
+          getStringPart(multiLine, seenEscapedQuote)
         } else if (ch == '{') {
           finishStringPart()
           nextRawChar()
@@ -946,13 +961,14 @@ trait Scanners extends ScannersCommon {
         if (isUnclosedLiteral) {
           if (multiLine)
             incompleteInputError("unclosed multi-line string literal")
-          else
-            unclosedStringLit()
+          else {
+            unclosedStringLit(seenEscapedQuote)
+          }
         }
         else {
           putChar(ch)
           nextRawChar()
-          getStringPart(multiLine)
+          getStringPart(multiLine, seenEscapedQuote)
         }
       }
     }

test/files/neg/t6476.check

@@ -0,0 +1,4 @@
+t6476.scala:8: error: unclosed string literal; note that `\"` no longer closes single-quoted interpolated string literals since 2.13.6, you can use a triple-quoted string instead
+  mimi"\"
+       ^
+1 error

test/files/neg/t6476.scala

@@ -0,0 +1,9 @@
+// only the last one doesn't parse
+class C {
+  mimi"""\ """
+  mimi"""\\"""
+  mimi"""\"""
+  mimi"\ "
+  mimi"\\"
+  mimi"\"
+}

test/files/neg/t6476b.check

@@ -0,0 +1,7 @@
+t6476b.scala:2: error: invalid escape at terminal index 0 in "\". Use \\ for literal \.
+  val sa = s"""\"""
+               ^
+t6476b.scala:4: error: invalid escape '\ ' not one of [\b, \t, \n, \f, \r, \\, \", \', \uxxxx] at index 0 in "\ ". Use \\ for literal \.
+  val sc = s"""\ """
+               ^
+2 errors

test/files/neg/t6476b.scala

@@ -0,0 +1,8 @@
+class C {
+  val sa = s"""\"""
+  val sb = s"""\\"""
+  val sc = s"""\ """
+  val ra = raw"""\"""
+  val rb = raw"""\\"""
+  val rc = raw"""\ """
+}

test/files/neg/t8266-invalid-interp.check

@@ -1,6 +1,6 @@
 t8266-invalid-interp.scala:4: error: Trailing '\' escapes nothing.
-    f"a\",
-       ^
+    f"""a\""",
+         ^
 t8266-invalid-interp.scala:5: error: invalid escape '\x' not one of [\b, \t, \n, \f, \r, \\, \", \', \uxxxx] at index 1 in "a\xc". Use \\ for literal \.
     f"a\xc",
        ^

test/files/neg/t8266-invalid-interp.scala

@@ -1,7 +1,7 @@
 
 trait X {
   def f = Seq(
-    f"a\",
+    f"""a\""",
     f"a\xc",
     // following could suggest \u000b for vertical tab, similar for \a alert
     f"a\vc"

test/files/pos/t11966.scala

@@ -3,5 +3,5 @@
 object Test {
   val original = """\/ \/ /\"""
   val minimal = """\1234\"""
-  val alternative = raw"\1234\"
+  val alternative = raw"""\1234\"""
 }

test/files/run/interpolation-repl.check

@@ -0,0 +1,12 @@
+
+scala> raw"\""
+val res0: String = \"
+
+scala> raw"\" // this used to be a comment, but after scala/pull#8830 it's part of the string! "
+val res1: String = "\" // this used to be a comment, but after scala/pull#8830 it's part of the string! "
+
+scala> raw"\" // this used to compile, now it's unclosed
+           ^
+       error: unclosed string literal; note that `\"` no longer closes single-quoted interpolated string literals since 2.13.6, you can use a triple-quoted string instead
+
+scala> :quit

test/files/run/interpolation-repl.scala

@@ -0,0 +1,9 @@
+import scala.tools.partest.ReplTest
+
+object Test extends ReplTest {
+  def code = """
+raw"\""
+raw"\" // this used to be a comment, but after scala/pull#8830 it's part of the string! "
+raw"\" // this used to compile, now it's unclosed
+"""
+}

test/files/run/t6476.check

@@ -0,0 +1,13 @@
+"Hello", Alice
+"Hello", Alice
+"Hello", Alice
+"Hello", Alice
+\"Hello\", Alice
+\"Hello\", Alice
+\TILT\
+\TILT\
+\\TILT\\
+\TILT\
+\TILT\
+\\TILT\\
+\TILT\

test/files/run/t6476.scala

@@ -0,0 +1,23 @@
+object Test {
+  def main(args: Array[String]): Unit = {
+    val person = "Alice"
+    println(s"\"Hello\", $person")
+    println(s"""\"Hello\", $person""")
+
+    println(f"\"Hello\", $person")
+    println(f"""\"Hello\", $person""")
+
+    println(raw"\"Hello\", $person")
+    println(raw"""\"Hello\", $person""")
+
+    println(s"\\TILT\\")
+    println(f"\\TILT\\")
+    println(raw"\\TILT\\")
+
+    println(s"""\\TILT\\""")
+    println(f"""\\TILT\\""")
+    println(raw"""\\TILT\\""")
+
+    println(raw"""\TILT\""")
+  }
+}