add support for rest types elements in the middle of a tuple

javascript/ql/src/semmle/javascript/TypeScript.qll

@@ -2149,18 +2149,23 @@ class TupleType extends ArrayType, @tuple_type {
   int getMinimumLength() { tuple_type_min_length(this, result) }
 
   /**
-   * Holds if this tuple type ends with a rest element, such as `[number, ...string[]]`.
+   * Gets the index of the rest element.
+   * E.g. for a type `[number, ...string[]]` the result is 1,
+   * or for a type `[...number[], string]` the result is 0.
    */
-  predicate hasRestElement() { tuple_type_rest(this) }
+  int getRestElementIndex() { tuple_type_rest_index(this, result) }
+
+  /**
+   * Holds if this tuple type has a rest element, such as `[number, ...string[]]` or `[...number[], string]`.
+   */
+  predicate hasRestElement() { exists(getRestElementIndex()) }
 
   /**
    * Gets the type of the rest element, if there is one.
    *
    * For example, the rest element of `[number, ...string[]]` is `string`.
    */
-  Type getRestElementType() {
-    hasRestElement() and result = getElementType(getNumElementType() - 1)
-  }
+  Type getRestElementType() { result = getElementType(getRestElementIndex()) }
 }
 
 /**

javascript/ql/src/semmlecode.javascript.dbscheme

@@ -799,8 +799,9 @@ tuple_type_min_length(
   int minLength: int ref
 );
 
-tuple_type_rest(
-  unique int typ: @type ref
+tuple_type_rest_index(
+  unique int typ: @type ref,
+  int index: int ref
 );
 
 // comments