Rust: Unify handling of struct and tuple constructors

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

@@ -50,5 +50,10 @@ module Impl {
       or
       result = this.getVariant().getStructField(name)
     }
+
+    /** Gets the `i`th struct field of the instantiated struct or variant. */
+    StructField getNthStructField(int i) {
+      result = [this.getStruct().getNthStructField(i), this.getVariant().getNthStructField(i)]
+    }
   }
 }

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

@@ -35,6 +35,12 @@ module Impl {
     /** Gets a record field, if any. */
     StructField getAStructField() { result = this.getStructField(_) }
 
+    /** Gets the `i`th struct field, if any. */
+    pragma[nomagic]
+    StructField getNthStructField(int i) {
+      result = this.getFieldList().(StructFieldList).getField(i)
+    }
+
     /** Gets the `i`th tuple field, if any. */
     pragma[nomagic]
     TupleField getTupleField(int i) { result = this.getFieldList().(TupleFieldList).getField(i) }

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

@@ -42,6 +42,13 @@ module Impl {
       )
     }
 
+    /** Gets the `i`th struct field of the instantiated struct or variant. */
+    StructField getNthStructField(int i) {
+      exists(PathResolution::ItemNode item | item = this.getResolvedPath(_) |
+        result = [item.(Struct).getNthStructField(i), item.(Variant).getNthStructField(i)]
+      )
+    }
+
     /** Gets the struct pattern for the field `name`. */
     pragma[nomagic]
     StructPatField getPatField(string name) {

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

@@ -32,6 +32,12 @@ module Impl {
       result.getName().getText() = name
     }
 
+    /** Gets the `i`th struct field, if any. */
+    pragma[nomagic]
+    StructField getNthStructField(int i) {
+      result = this.getFieldList().(StructFieldList).getField(i)
+    }
+
     /** Gets the `i`th tuple field, if any. */
     pragma[nomagic]
     TupleField getTupleField(int i) { result = this.getFieldList().(TupleFieldList).getField(i) }