wp-includes/link-template.php:4107Outputs rel=canonical for singular queries.
function rel_canonical() { if ( ! is_singular() ) { return; } $id = get_queried_object_id(); if ( 0 === $id ) { return; } $url = wp_get_canonical_url( $id ); if ( ! empty( $url ) ) { echo '<link rel="canonical" href="' . esc_url( $url ) . '" />' . "\n"; }}Introduced in 2.9.0. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/link-template.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.