apply_filters() and the_excerpt are giving unexpected results

Turns out the answer was in wp_trim_excerpt().

It’s defined in wp-includes/functions.php:1879:

/**
 * Generates an excerpt from the content, if needed.
 *
 * The excerpt word amount will be 55 words and if the amount is greater than
 * that, then the string ' [...]' will be appended to the excerpt. If the string
 * is less than 55 words, then the content will be returned as is.
 *
 * The 55 word limit can be modified by plugins/themes using the excerpt_length filter
 * The ' [...]' string can be modified by plugins/themes using the excerpt_more filter
 *
 * @since 1.5.0
 *
 * @param string $text Optional. The excerpt. If set to empty, an excerpt is generated.
 * @return string The excerpt.
 */
function wp_trim_excerpt($text="") {
    $raw_excerpt = $text;
    if ( '' == $text ) {
        $text = get_the_content('');

        $text = strip_shortcodes( $text );

        $text = apply_filters('the_content', $text);
        $text = str_replace(']]>', ']]>', $text);
        $excerpt_length = apply_filters('excerpt_length', 55);
        $excerpt_more = apply_filters('excerpt_more', ' ' . '[...]');
        $text = wp_trim_words( $text, $excerpt_length, $excerpt_more );
    }
    return apply_filters('wp_trim_excerpt', $text, $raw_excerpt);
}

So any text passed in doesn’t get processed; it only works if it’s called with an empty parameter.

To solve this, I added a quick filter to my theme that solves the problem:

/**
 * Allows for excerpt generation outside the loop.
 * 
 * @param string $text  The text to be trimmed
 * @return string       The trimmed text
 */
function rw_trim_excerpt( $text="" )
{
    $text = strip_shortcodes( $text );
    $text = apply_filters('the_content', $text);
    $text = str_replace(']]>', ']]>', $text);
    $excerpt_length = apply_filters('excerpt_length', 55);
    $excerpt_more = apply_filters('excerpt_more', ' ' . '[...]');
    return wp_trim_words( $text, $excerpt_length, $excerpt_more );
}
add_filter('wp_trim_excerpt', 'rw_trim_excerpt');

It’s somewhat redundant, but I like it better than opening new loops every time I want to generate an excerpt.