feat: add source file metadata to output when applicable

lib/index.js

@@ -50,6 +50,8 @@ function initSass(options) {
         let compiled
 
         if (isInSrcDir) {
+          // if the sass file is inside src, it could contain front-matter or templating
+          // therefore it is unsafe/error-prone to (re-)read it from disk
           compiled = sassLib.compileString(files[srcRelPath].contents.toString(), options)
         } else {
           compiled = sassLib.compile(metalsmith.path(src), options)
@@ -67,20 +69,24 @@ function initSass(options) {
           css += `${EOL}/*# sourceMappingURL=${sourceMapRelPath} */`
         }
 
-        files[dest] = {
-          contents: Buffer.from(css)
-        }
-
-        // if the sass entry is inside Metalsmith.source, delete it
-        if (files[srcRelPath]) {
+        // if the sass entry is inside Metalsmith.source...
+        if (isInSrcDir) {
+          // ...reuse its file metadata
+          files[dest] = files[srcRelPath]
+          // ...then delete the source
           delete files[srcRelPath]
           // log a useful warning to avoid RAM overhead
           debug(
             'Removed %s from the build. For better performance, move your SASS source files out of Metalsmith.source',
             srcRelPath
           )
+        } else {
+          files[dest] = {}
         }
 
+        // finally (over)write its contents
+        files[dest].contents = Buffer.from(css)
+
         debug('Finished compiling %s to %s', src, dest)
       } catch (err) {
         // we don't call done(err) so we can log multiple sass errors at once for multiple entrypoints