Add error message regarding yield and do keywords in for comprehensions

Author: Sebastian Harko

src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1326,7 +1326,7 @@ object Parsers {
       if (in.token == YIELD) { in.nextToken(); ForYield(enums, expr()) }
       else if (in.token == DO) { in.nextToken(); ForDo(enums, expr()) }
       else {
-        if (!wrappedEnums) syntaxErrorOrIncomplete("`yield' or `do' expected")
+        if (!wrappedEnums) syntaxError(YieldOrDoExpectedInForComprehension())
         ForDo(enums, expr())
       }
     }

src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -512,4 +512,39 @@ object messages {
         |}""".stripMargin
   }
 
+  case class YieldOrDoExpectedInForComprehension()(implicit ctx: Context) extends Message(19) {
+    val kind = "Syntax"
+    val msg = hl"${"yield"} or ${"do"} expected"
+    
+    val code1 = "val numbers = for i <- 1 to 3 yield i"
+    val code2 = "val numbers = for (i <- 1 to 3) yield i"
+    val code3 = "for (i <- 1 to 3) println(i)"
+    val code4 = "for i <- 1 to 3 do println(i) // notice the 'do' keyword"
+    
+    val explanation =
+      hl"""When the enumerators in a for comprehension are not placed in parentheses or braces, a ${"do"} or ${"yield"} statement
+           |is required after the enumerators section of the comprehension.
+           |
+           |You can save some keystrokes by omitting the parentheses and writing
+           |
+           |$code1
+           |
+           |  instead of
+           |
+           |$code2
+           |
+           |but the ${"yield"} keyword is still required.
+           |
+           |For comprehensions that simply perform a side effect without yielding anything can also be written without parentheses
+           |but a ${"do"} keyword has to be included. For example,
+           |
+           |$code3
+           |
+           |  can be written as
+           |
+           |$code4
+           |
+           |""".stripMargin
+  }
+ 
 }