Allow trailing comma in list literals (#900)

Fixes #873

Author: mattisboeckle

effekt/jvm/src/test/scala/effekt/RecursiveDescentTests.scala

@@ -125,6 +125,12 @@ class RecursiveDescentTests extends munit.FunSuite {
     parseExpr("fun() { foo(()) }")
 
     parseExpr("10.seconds")
+
+    parseExpr("[1,2,3]")
+    parseExpr("[3,2,1,]")
+    parseExpr("[]")
+    parseExpr("[,]")
+    intercept[Throwable] { parseExpr("[,1]") }
   }
 
   test("Boxing") {

effekt/shared/src/main/scala/effekt/RecursiveDescent.scala

@@ -993,7 +993,7 @@ class RecursiveDescent(positions: Positions, tokens: Seq[Token], source: Source)
   }
   def listLiteral(): Term =
     nonterminal:
-      many(expr, `[`, `,`, `]`).foldRight(NilTree) { ConsTree }
+      manyTrailing(expr, `[`, `,`, `]`).foldRight(NilTree) { ConsTree }
 
   private def NilTree: Term =
     Call(IdTarget(IdRef(List(), "Nil")), Nil, Nil, Nil)
@@ -1406,6 +1406,29 @@ class RecursiveDescent(positions: Positions, tokens: Seq[Token], source: Source)
       components.toList
     }
 
+  inline def manyTrailing[T](p: () => T, before: TokenKind, sep: TokenKind, after: TokenKind): List[T] =
+    consume(before)
+    if (peek(after)) {
+      consume(after)
+      Nil
+    } else if (peek(sep)) {
+      consume(sep)
+      consume(after)
+      Nil
+    } else {
+      val components: ListBuffer[T] = ListBuffer.empty
+      components += p()
+      while (peek(sep)) {
+        consume(sep)
+
+        if (!peek(after)) {
+          components += p()
+        }
+      }
+      consume(after)
+      components.toList
+    }
+
 
   // Positions