implement naive recursive @apply

Currently what it will do is re-run the apply code. If it turns out that
there are @apply rules left, then it will re-apply those. If there are
no @apply rules left, this means that we are "done".

One big downside is that we are (at the time of this commit) running the
apply code for the test at least 4 times. The good part is that we are
doing it on the exact same tree so we don't need to re-parse it.

This is the first implementation which can be improved!

Author: RobinMalfait

src/lib/expandApplyAtRules.js

@@ -9,7 +9,10 @@ function buildApplyCache(applyCandidates, context) {
     }
 
     if (context.classCache.has(candidate)) {
-      context.applyClassCache.set(candidate, context.classCache.get(candidate))
+      context.applyClassCache.set(
+        candidate,
+        context.classCache.get(candidate).map(([meta, rule]) => [meta, rule.clone()])
+      )
       continue
     }
 
@@ -38,7 +41,7 @@ function extractApplyCandidates(params) {
 }
 
 function expandApplyAtRules(context) {
-  return (root) => {
+  return function processApply(root) {
     let applyCandidates = new Set()
 
     // Collect all @apply rules and candidates
@@ -79,12 +82,16 @@ function expandApplyAtRules(context) {
        */
       // TODO: Should we use postcss-selector-parser for this instead?
       function replaceSelector(selector, utilitySelectors, candidate) {
+        let needle = `.${escapeClassName(candidate)}`
+        let utilitySelectorsList = utilitySelectors.split(/\s*,\s*/g)
+
         return selector
           .split(/\s*,\s*/g)
           .map((s) => {
             let replaced = []
-            for (let utilitySelector of utilitySelectors.split(/\s*,\s*/g)) {
-              let replacedSelector = utilitySelector.replace(`.${escapeClassName(candidate)}`, s)
+
+            for (let utilitySelector of utilitySelectorsList) {
+              let replacedSelector = utilitySelector.replace(needle, s)
               if (replacedSelector === utilitySelector) {
                 continue
               }
@@ -136,6 +143,9 @@ function expandApplyAtRules(context) {
           apply.parent.remove()
         }
       }
+
+      // Do it again, in case we have other `@apply` rules
+      processApply(root)
     }
   }
 }