Fix #3788 escape backslash for char and string show

library/src/dotty/Show.scala

@@ -20,42 +20,36 @@ object Show extends LowPrioShow {
       ev.show(v)
   }
 
-  implicit val stringShow: Show[String] = new Show[String] {
+  /** Adds escaping backslashes in a string so that it could be put in source code.
+    * For example, a newline character is shown as '\n' (without the quotes).
+    * Chars that don't have to be escaped are simply converted to a string.
+    * @param c the char to be escaped
+    * @return a string describing how to escape the give char in source code.
+    */
+  private def escapeChar(c: Char): String = c match {
     // From 2.12 spec, `charEscapeSeq`:
     // ‘\‘ (‘b‘ | ‘t‘ | ‘n‘ | ‘f‘ | ‘r‘ | ‘"‘ | ‘'‘ | ‘\‘)
-    def show(str: String) = {
-      val sb = new StringBuilder
-      sb.append("\"")
-      str.foreach {
-        case '\b' => sb.append("\\b")
-        case '\t' => sb.append("\\t")
-        case '\n' => sb.append("\\n")
-        case '\f' => sb.append("\\f")
-        case '\r' => sb.append("\\r")
-        case '\'' => sb.append("\\'")
-        case '\"' => sb.append("\\\"")
-        case c => sb.append(c)
-      }
-      sb.append("\"")
-      sb.toString
-    }
+    case '\b' => "\\b"
+    case '\t' => "\\t"
+    case '\n' => "\\n"
+    case '\f' => "\\f"
+    case '\r' => "\\r"
+    case '\'' => "\\'"
+    case '\"' => "\\\""
+    case '\\' => "\\\\"
+    case c    => c.toString
+  }
+
+  implicit val stringShow: Show[String] = new Show[String] {
+    def show(str: String) = str.flatMap(escapeChar).mkString("\"", "", "\"")
   }
 
   implicit val floatShow: Show[Float] = new Show[Float] {
     def show(f: Float) = f + "f"
   }
 
   implicit val charShow: Show[Char] = new Show[Char] {
-    def show(c: Char) = "'" + (c match {
-      case '\b' => "\\b"
-      case '\t' => "\\t"
-      case '\n' => "\\n"
-      case '\f' => "\\f"
-      case '\r' => "\\r"
-      case '\'' => "\\'"
-      case '\"' => "\\\""
-      case c    => c
-    }) + "'"
+    def show(c: Char) = s"'${escapeChar(c)}'"
   }
 
   implicit def showList[T](implicit st: Show[T]): Show[List[T]] = new Show[List[T]] {

library/test/dotty/ShowTests.scala

@@ -7,10 +7,11 @@ class ShowTests {
   import Show._
 
   @Test def showString = {
+    assertEquals(""""\\"""", "\\".show)
     assertEquals("\"\\thello world!\"", "\thello world!".show)
     assertEquals("\"\\nhello world!\"", "\nhello world!".show)
     assertEquals("\"\\rhello world!\"", "\rhello world!".show)
-    assertEquals("\"\\b\\t\\n\\f\\r\\\'\\\"\"", "\b\t\n\f\r\'\"".show)
+    assertEquals(""""\b\t\n\f\r\'\"\\"""", "\b\t\n\f\r\'\"\\".show)
   }
 
   @Test def showFloat = {
@@ -31,6 +32,7 @@ class ShowTests {
     assertEquals("'\\r'", '\r'.show)
     assertEquals("'\\''", '\''.show)
     assertEquals("'\\\"'", '\"'.show)
+    assertEquals("'\\\\'", '\\'.show)
   }
 
   @Test def showCar = {