Source

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

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

		$id = $user->ID;

		if ( ! $user ) {
			return new WP_Error(
				'rest_user_invalid_id',
				__( 'Invalid user ID.' ),
				array( 'status' => 404 )
			);
		}

		$owner_id = email_exists( $request['email'] );

		if ( $owner_id && $owner_id !== $id ) {
			return new WP_Error(
				'rest_user_invalid_email',
				__( 'Invalid email address.' ),
				array( 'status' => 400 )
			);
		}

		if ( ! empty( $request['username'] ) && $request['username'] !== $user->user_login ) {
			return new WP_Error(
				'rest_user_invalid_argument',
				__( "Username isn't editable." ),
				array( 'status' => 400 )
			);
		}

		if ( ! empty( $request['slug'] ) && $request['slug'] !== $user->user_nicename && get_user_by( 'slug', $request['slug'] ) ) {
			return new WP_Error(
				'rest_user_invalid_slug',
				__( 'Invalid slug.' ),
				array( 'status' => 400 )
			);
		}

		if ( ! empty( $request['roles'] ) ) {
			$check_permission = $this->check_role_update( $id, $request['roles'] );

			if ( is_wp_error( $check_permission ) ) {
				return $check_permission;
			}
		}

		$user = $this->prepare_item_for_database( $request );

		// Ensure we're operating on the same user we already checked.
		$user->ID = $id;

		$user_id = wp_update_user( wp_slash( (array) $user ) );

		if ( is_wp_error( $user_id ) ) {
			return $user_id;
		}

		$user = get_user_by( 'id', $user_id );

		/** This action is documented in wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php */
		do_action( 'rest_insert_user', $user, $request, false );

		if ( ! empty( $request['roles'] ) ) {
			array_map( array( $user, 'add_role' ), $request['roles'] );
		}

		$schema = $this->get_item_schema();

		if ( ! empty( $schema['properties']['meta'] ) && isset( $request['meta'] ) ) {
			$meta_update = $this->meta->update_value( $request['meta'], $id );

			if ( is_wp_error( $meta_update ) ) {
				return $meta_update;
			}
		}

		$user          = get_user_by( 'id', $user_id );
		$fields_update = $this->update_additional_fields_for_object( $user, $request );

		if ( is_wp_error( $fields_update ) ) {
			return $fields_update;
		}

		$request->set_param( 'context', 'edit' );

		/** This action is documented in wp-includes/rest-api/endpoints/class-wp-rest-users-controller.php */
		do_action( 'rest_after_insert_user', $user, $request, false );

		$response = $this->prepare_item_for_response( $user, $request );
		$response = rest_ensure_response( $response );

		return $response;
	}