Feature Description

Currently the Speculative Loading plugin provides a plsr_speculation_rules_href_exclude_paths filter to exclude URLs (and URL patterns) from being speculatively loaded. However, two situations came up recently where this was not sufficient.

1. In Products added to cart in WooCommerce without clicking on Add to Cart button when Speculative Loading active #1140, WooCommerce was found to create non-idempotent add-to-cart links which add products to cart when prefetched/prerendered. WooCommerce seems to try to get around the non-idempotency by adding rel=nofollow to the links. However, there is no way to exclude links via such an attribute without manually modifying the default rules, which was done in Exclude rel=nofollow links from prefetch/prerender #1142. (This was to avoid having to add a WooCommerce-specific URLPattern to exclude URLs with an add-to-cart query param, since excluding links with rel=nofollow may be generally advisable: Should we exclude rel=nofollow by default? WICG/nav-speculation#309).
2. In a comment on the aforementioned issue, a user wanted to customize the speculation rules so that specific URLs were opted-in for eager prerendering. That is, instead of specifying a list of URLs/URLPatterns to exclude, they wanted to provide a list of URLs/URLPatterns to include. This is not possible at present either.

To account for these two use cases, I suggest that the entire set of speculation rules (speculation ruleset?) be filterable, doing something like this:

--- a/plugins/speculation-rules/helper.php
+++ b/plugins/speculation-rules/helper.php
@@ -109,5 +109,13 @@ function plsr_get_speculation_rules() {
 		);
 	}
 
-	return array( $mode => $rules );
+	$ruleset = array( $mode => $rules );
+
+	/**
+	 * Filters the entire speculation ruleset.
+	 *
+	 * @param array  $ruleset Ruleset.
+	 * @param string $mode    Mode used to apply speculative prerendering. Either 'prefetch' or 'prerender'.
+	 */
+	return (array) apply_filters( 'plsr_speculation_ruleset', $ruleset, $mode );
 }

Also, @swissspidy suggested in #1144 (comment) that a JSON Schema could be written which could validate whatever is being returned by the filter. If not valid, it could trigger a _doing_it_wrong().