Fix error message handling

We previously got:

    an identifier expected but `=` found

even though the token that was found was a `}`! The fallacy was that
in.name is not always set to something significant. I had to refactor quite a bit
to avoid this. Anyway, I thought the code was a bit underwhelming with `null`
interpreted and an "explanation" that simply repeated what was said in the
main message.

Author: odersky

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

@@ -266,7 +266,7 @@ object Parsers {
     def accept(token: Int): Int = {
       val offset = in.offset
       if (in.token != token) {
-        syntaxErrorOrIncomplete(ExpectedTokenButFound(token, in.token, in.name))
+        syntaxErrorOrIncomplete(ExpectedTokenButFound(token, in.token))
       }
       if (in.token == token) in.nextToken()
       offset
@@ -477,7 +477,7 @@ object Parsers {
         in.nextToken()
         name
       } else {
-        syntaxErrorOrIncomplete(ExpectedTokenButFound(IDENTIFIER, in.token, in.name))
+        syntaxErrorOrIncomplete(ExpectedTokenButFound(IDENTIFIER, in.token))
         nme.ERROR
       }
 

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

@@ -1055,17 +1055,15 @@ object messages {
            |""".stripMargin
   }
 
-  case class ExpectedTokenButFound(expected: Token, found: Token, foundName: TermName)(implicit ctx: Context)
+  case class ExpectedTokenButFound(expected: Token, found: Token)(implicit ctx: Context)
   extends Message(ExpectedTokenButFoundID) {
     val kind = "Syntax"
 
     private val expectedText =
       if (Tokens.isIdentifier(expected)) "an identifier"
       else Tokens.showToken(expected)
 
-    private val foundText =
-      if (foundName != null) hl"`${foundName.show}`"
-      else Tokens.showToken(found)
+    private val foundText = Tokens.showToken(found)
 
     val msg = hl"""${expectedText} expected, but ${foundText} found"""
 
@@ -1075,10 +1073,7 @@ object messages {
            |If you necessarily want to use $foundText as identifier, you may put it in backticks.""".stripMargin
       else
         ""
-
-    val explanation =
-      s"""|The text ${foundText} may not occur at that position, the compiler expected ${expectedText}.$ifKeyword
-          |""".stripMargin
+    val explanation = s"$ifKeyword"
   }
 
   case class MixedLeftAndRightAssociativeOps(op1: Name, op2: Name, op2LeftAssoc: Boolean)(implicit ctx: Context)

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -121,10 +121,9 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val defn = ictx.definitions
 
       assertMessageCount(1, messages)
-      val ExpectedTokenButFound(expected, found, foundName) :: Nil = messages
+      val ExpectedTokenButFound(expected, found) :: Nil = messages
       assertEquals(Tokens.IDENTIFIER, expected)
       assertEquals(Tokens.VAL, found)
-      assertEquals("val", foundName.show)
     }
 
   @Test def expectedToken =