Tweak error reporting for illegal start of statement

Author: odersky

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

@@ -311,7 +311,7 @@ object Parsers {
     def acceptStatSep(): Unit =
       if in.isNewLine then in.nextToken() else accept(SEMI)
 
-    def exitStats[T <: Tree](stats: ListBuffer[T], altEnd: Token = EOF, noPrevStat: Boolean): Boolean =
+    def exitStats[T <: Tree](stats: ListBuffer[T], noPrevStat: Boolean, altEnd: Token = EOF, what: String = "statement"): Boolean =
       def recur(sepSeen: Boolean, endSeen: Boolean): Boolean =
         if isStatSep then
           in.nextToken()
@@ -326,9 +326,10 @@ object Parsers {
           false
         else
           val found = in.token
+          val statFollows = mustStartStatTokens.contains(found)
           syntaxError(
-            if noPrevStat then IllegalStartOfStatement(isModifier)
-            else i"end of statement expected but ${showToken(found)} found")
+            if noPrevStat then IllegalStartOfStatement(what, isModifier, statFollows)
+            else i"end of $what expected but ${showToken(found)} found")
           if mustStartStatTokens.contains(found) then
             true // it's a statement that might be legal in an outer context
           else
@@ -3933,7 +3934,7 @@ object Parsers {
           stats +++= localDef(in.offset)
         else
           empty = true
-        !exitStats(stats, CASE, empty)
+        !exitStats(stats, empty, CASE)
       do ()
       stats.toList
     }

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

@@ -1824,12 +1824,16 @@ import transform.SymUtils._
     def explain = ""
   }
 
-  class IllegalStartOfStatement(isModifier: Boolean)(using Context) extends SyntaxMsg(IllegalStartOfStatementID) {
-    def msg = {
-      val addendum = if (isModifier) ": this modifier is not allowed here" else ""
-      "Illegal start of statement" + addendum
-    }
-    def explain = "A statement is either an import, a definition or an expression."
+  class IllegalStartOfStatement(what: String, isModifier: Boolean, isStat: Boolean)(using Context) extends SyntaxMsg(IllegalStartOfStatementID) {
+    def msg =
+      if isStat then
+        "this kind of statement is not allowed here"
+      else
+        val addendum = if isModifier then ": this modifier is not allowed here" else ""
+        s"Illegal start of $what$addendum"
+    def explain =
+      i"""A statement is an import or export, a definition or an expression.
+         |Some statements are only allowed in certain contexts"""
   }
 
   class TraitIsExpected(symbol: Symbol)(using Context) extends SyntaxMsg(TraitIsExpectedID) {