[Parser] Parse contextual 'each' in expressions as PackElementExpr.

Author: hborla

lib/Parse/ParseExpr.cpp

@@ -1612,6 +1612,19 @@ ParserResult<Expr> Parser::parseExprPrimary(Diag<> ID, bool isExprBasic) {
       return makeParserResult(typeExpr);
     }
 
+    // 'each' followed by another identifier is a pack element expr.
+    if (Tok.isContextualKeyword("each") &&
+        peekToken().is(tok::identifier) &&
+        !peekToken().isAtStartOfLine()) {
+      SourceLoc loc = consumeToken();
+      ParserResult<Expr> ref = parseExpr(ID);
+      if (ref.isNull())
+        return ref;
+
+      auto *packRef = PackElementExpr::create(Context, loc, ref.get());
+      return makeParserResult(packRef);
+    }
+
     LLVM_FALLTHROUGH;
   case tok::kw_Self:     // Self
     return parseExprIdentifier();