Source

File: wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php

public function update_item_permissions_check( $request ) {
		$user = $this->get_user( $request['id'] );
		if ( is_wp_error( $user ) ) {
			return $user;
		}

		if ( ! empty( $request['roles'] ) ) {
			if ( ! current_user_can( 'promote_user', $user->ID ) ) {
				return new WP_Error(
					'rest_cannot_edit_roles',
					__( 'Sorry, you are not allowed to edit roles of this user.' ),
					array( 'status' => rest_authorization_required_code() )
				);
			}

			$request_params = array_keys( $request->get_params() );
			sort( $request_params );
			// If only 'id' and 'roles' are specified (we are only trying to
			// edit roles), then only the 'promote_user' cap is required.
			if ( array( 'id', 'roles' ) === $request_params ) {
				return true;
			}
		}

		if ( ! current_user_can( 'edit_user', $user->ID ) ) {
			return new WP_Error(
				'rest_cannot_edit',
				__( 'Sorry, you are not allowed to edit this user.' ),
				array( 'status' => rest_authorization_required_code() )
			);
		}

		return true;
	}