all: remove unused enum value and improve error message for `@` tokens in scanner (#6751)

vlib/v/checker/checker.v

@@ -3053,8 +3053,9 @@ fn (mut c Checker) at_expr(mut node ast.AtExpr) table.Type {
 			}
 			node.val = c.vmod_file_content
 		}
-		.unknown, ._end_ {
-			c.error('unknown @ identifier: $node.name', node.pos)
+		.unknown {
+			c.error('unknown @ identifier: ${node.name}. Available identifiers: $token.valid_at_tokens',
+				node.pos)
 		}
 	}
 	return table.string_type

vlib/v/scanner/scanner.v

@@ -760,7 +760,12 @@ fn (mut s Scanner) text_scan() token.Token {
 					return s.new_token(.at, '@' + name, name.len + 1)
 				}
 				if !token.is_key(name) {
-					s.error('@ must be used before keywords (e.g. `@type string`)')
+					mut at_error_msg := '@ must be used before keywords or compile time variables (e.g. `@type string` or `@FN`)'
+					// If name is all uppercase, the user is probably looking for a compile time variable ("at-token")
+					if name.is_upper() {
+						at_error_msg += '\nAvailable compile time variables:\n$token.valid_at_tokens'
+					}
+					s.error(at_error_msg)
 				}
 				return s.new_token(.name, name, name.len)
 			}

vlib/v/token/token.v

@@ -164,11 +164,9 @@ pub enum AtKind {
 	column_nr
 	vhash
 	vmod_file
-	_end_
 }
 const (
 	valid_at_tokens = ['@FN','@MOD','@STRUCT','@VEXE','@FILE','@LINE','@COLUMN','@VHASH','@VMOD_FILE']
-	//valid_at_tokens_len = int(AtKind._end_)
 )
 
 // build_keys genereates a map with keywords' string values: