Pass real filepath to transform plugins (#3483)

withastro/astro#481

plugins/plugin-postcss/plugin.js

@@ -23,7 +23,7 @@ module.exports = function postcssPlugin(snowpackConfig, options) {
 
   return {
     name: '@snowpack/postcss-transform',
-    async transform({id, fileExt, contents}) {
+    async transform({id, srcPath, fileExt, contents}) {
       let {input = ['.css'], config} = options;
 
       if (!input.includes(fileExt) || !contents) return;
@@ -37,7 +37,7 @@ module.exports = function postcssPlugin(snowpackConfig, options) {
 
       const encodedResult = await worker.transformAsync(contents, {
         config,
-        filepath: id,
+        filepath: srcPath,
         cwd: snowpackConfig.root || process.cwd(),
         map:
           snowpackConfig.buildOptions && snowpackConfig.buildOptions.sourceMaps

snowpack/src/build/build-pipeline.ts

@@ -182,6 +182,7 @@ async function runPipelineTransformStep(
           isPackage,
           fileExt: destExt,
           id: filePath,
+          srcPath,
           // @ts-ignore: Deprecated
           filePath: fileName,
           // @ts-ignore: Deprecated

snowpack/src/types.ts

@@ -137,8 +137,10 @@ export interface PluginLoadOptions {
 }
 
 export interface PluginTransformOptions {
-  /** The absolute file path of the source file, on disk. */
+  /** The final build file path (note: this may differ from the source, e.g. `.vue` will yield `.js` and `.css` IDs) */
   id: string;
+  /** The original source location on disk (it may differ from ID) */
+  srcPath: string;
   /** The extension of the file */
   fileExt: string;
   /** Contents of the file to transform */