improvement #2: skip work, when generated AST is the same

Currently incremental rebuilds are additive, which means that we are not
keeping track if we should remove CSS again in development.

We can exploit this information, because now we can quickly check the
amoutn of generated AST nodes.

- If they are the same then nothing new is generated — this means that
  we can re-use the previous compiled CSS. We don't even have to
  re-print the AST because we already did do that work in the past.
- If there are more AST nodes, something new is generated — this means
  that we should update the `@tailwind utilities` rule and re-print the
  CSS. We can store the result for future incremental rebuilds.

Author: RobinMalfait

packages/tailwindcss/src/index.ts

@@ -195,8 +195,16 @@ export function compile(
       }
 
       if (tailwindUtilitiesNode) {
+        let previousAstNodeCount = designSystem.getAstNodeSize()
         let newNodes = compileCandidates(allValidCandidates, designSystem).astNodes
 
+        // If no new ast nodes were generated, then we can return the original
+        // CSS. This currently assumes that we only add new ast nodes and never
+        // remove any.
+        if (previousAstNodeCount === designSystem.getAstNodeSize()) {
+          return compiledCss
+        }
+
         tailwindUtilitiesNode.nodes = newNodes
         compiledCss = toCss(ast)
       }