Rust: fix parent/child relationship for format entities

rust/ql/lib/codeql/rust/elements/internal/FormatArgsExprImpl.qll

@@ -16,16 +16,16 @@ module Impl {
   /**
    * A FormatArgsExpr. For example:
    * ```rust
-   * todo!()
+   * format_args!("no args");
+   * format_args!("{} foo {:?}", 1, 2);
+   * format_args!("{b} foo {a:?}", a=1, b=2);
+   * let (x, y) = (1, 42);
+   * format_args!("{x}, {y}");
    * ```
    */
   class FormatArgsExpr extends Generated::FormatArgsExpr {
-    /**
-     * Gets the `index`th format of this `FormatArgsExpr`'s formatting template (0-based).
-     */
-    Format getFormat(int index) {
-      result =
-        rank[index + 1](Format f, int i | f.getParent() = this and f.getIndex() = i | f order by i)
+    override Format getFormat(int index) {
+      result.getParent() = this and result.getIndex() = index + 1
     }
   }
 }

rust/ql/lib/codeql/rust/elements/internal/FormatArgumentImpl.qll

@@ -39,6 +39,8 @@ module Impl {
     override string toString() { result = name }
 
     override Format getParent() { result = Synth::TFormat(parent, index, _, _) }
+
+    override FormatTemplateVariableAccess getVariable() { result.getArgument() = this }
   }
 
   private class FormatSynthLocationImpl extends FormatArgument, LocatableImpl::SynthLocatable {

rust/ql/lib/codeql/rust/elements/internal/FormatImpl.qll

@@ -38,6 +38,8 @@ module Impl {
 
     override int getIndex() { result = index }
 
+    override FormatArgument getArgument() { result.getParent() = this }
+
     /**
      * Gets the name or position reference of this format, if any. For example `name` and `0` in:
      * ```rust