fix #20: implement composes from css modules

CHANGELOG.md

@@ -2,6 +2,84 @@
 
 ## Unreleased
 
+* Implement `composes` from CSS modules ([#20](https://github.com/evanw/esbuild/issues/20))
+
+    This release implements the `composes` annotation from the [CSS modules specification](https://github.com/css-modules/css-modules#composition). It provides a way for class selectors to reference other class selectors (assuming you are using the `local-css` loader). And with the `from` syntax, this can even work with local names across CSS files. For example:
+
+    ```js
+    // app.js
+    import { submit } from './style.css'
+    const div = document.createElement('div')
+    div.className = submit
+    document.body.appendChild(div)
+    ```
+
+    ```css
+    /* style.css */
+    .button {
+      composes: pulse from "anim.css";
+      display: inline-block;
+    }
+    .submit {
+      composes: button;
+      font-weight: bold;
+    }
+    ```
+
+    ```css
+    /* anim.css */
+    @keyframes pulse {
+      from, to { opacity: 1 }
+      50% { opacity: 0.5 }
+    }
+    .pulse {
+      animation: 2s ease-in-out infinite pulse;
+    }
+    ```
+
+    Bundling this with esbuild using `--bundle --outdir=dist --loader:.css=local-css` now gives the following:
+
+    ```js
+    (() => {
+      // style.css
+      var submit = "anim_pulse style_button style_submit";
+
+      // app.js
+      var div = document.createElement("div");
+      div.className = submit;
+      document.body.appendChild(div);
+    })();
+    ```
+
+    ```css
+    /* anim.css */
+    @keyframes anim_pulse {
+      from, to {
+        opacity: 1;
+      }
+      50% {
+        opacity: 0.5;
+      }
+    }
+    .anim_pulse {
+      animation: 2s ease-in-out infinite anim_pulse;
+    }
+
+    /* style.css */
+    .style_button {
+      display: inline-block;
+    }
+    .style_submit {
+      font-weight: bold;
+    }
+    ```
+
+    Import paths in the `composes: ... from` syntax are resolved using the new `composes-from` import kind, which can be intercepted by plugins during import path resolution when bundling is enabled.
+
+    Note that the order in which composed CSS classes from separate files appear in the bundled output file is deliberately _**undefined**_ by design (see [the specification](https://github.com/css-modules/css-modules#composing-from-other-files) for details). You are not supposed to declare the same CSS property in two separate class selectors and then compose them together. You are only supposed to compose CSS class selectors that declare non-overlapping CSS properties.
+
+    Issue [#20](https://github.com/evanw/esbuild/issues/20) (the issue tracking CSS modules) is esbuild's most-upvoted issue! With this change, I now consider esbuild's implementation of CSS modules to be complete. There are still improvements to make and there may also be bugs with the current implementation, but these can be tracked in separate issues.
+
 * Fix non-determinism with `tsconfig.json` and symlinks ([#3284](https://github.com/evanw/esbuild/issues/3284))
 
     This release fixes an issue that could cause esbuild to sometimes emit incorrect build output in cases where a file under the effect of `tsconfig.json` is inconsistently referenced through a symlink. It can happen when using `npm link` to create a symlink within `node_modules` to an unpublished package. The build result was non-deterministic because esbuild runs module resolution in parallel and the result of the `tsconfig.json` lookup depended on whether the import through the symlink or not through the symlink was resolved first. This problem was fixed by moving the `realpath` operation before the `tsconfig.json` lookup.

cmd/esbuild/service.go

@@ -789,6 +789,8 @@ func resolveKindToString(kind api.ResolveKind) string {
 	// CSS
 	case api.ResolveCSSImportRule:
 		return "import-rule"
+	case api.ResolveCSSComposesFrom:
+		return "composes-from"
 	case api.ResolveCSSURLToken:
 		return "url-token"
 
@@ -815,6 +817,8 @@ func stringToResolveKind(kind string) (api.ResolveKind, bool) {
 	// CSS
 	case "import-rule":
 		return api.ResolveCSSImportRule, true
+	case "composes-from":
+		return api.ResolveCSSComposesFrom, true
 	case "url-token":
 		return api.ResolveCSSURLToken, true
 	}

internal/ast/ast.go

@@ -35,6 +35,9 @@ const (
 	// A CSS "@import" rule with import conditions
 	ImportAtConditional
 
+	// A CSS "composes" declaration
+	ImportComposesFrom
+
 	// A CSS "url(...)" token
 	ImportURL
 )
@@ -51,6 +54,8 @@ func (kind ImportKind) StringForMetafile() string {
 		return "require-resolve"
 	case ImportAt, ImportAtConditional:
 		return "import-rule"
+	case ImportComposesFrom:
+		return "composes-from"
 	case ImportURL:
 		return "url-token"
 	case ImportEntryPoint:
@@ -61,7 +66,19 @@ func (kind ImportKind) StringForMetafile() string {
 }
 
 func (kind ImportKind) IsFromCSS() bool {
-	return kind == ImportAt || kind == ImportURL
+	switch kind {
+	case ImportAt, ImportAtConditional, ImportComposesFrom, ImportURL:
+		return true
+	}
+	return false
+}
+
+func (kind ImportKind) MustResolveToCSS() bool {
+	switch kind {
+	case ImportAt, ImportAtConditional, ImportComposesFrom:
+		return true
+	}
+	return false
 }
 
 type ImportRecordFlags uint16

internal/bundler/bundler.go

@@ -2009,13 +2009,23 @@ func (s *scanner) processScannedFiles(entryPointMeta []graph.EntryPoint) []scann
 				}
 
 				switch record.Kind {
+				case ast.ImportComposesFrom:
+					// Using a JavaScript file with CSS "composes" is not allowed
+					if _, ok := otherFile.inputFile.Repr.(*graph.JSRepr); ok && otherFile.inputFile.Loader != config.LoaderEmpty {
+						s.log.AddErrorWithNotes(&tracker, record.Range,
+							fmt.Sprintf("Cannot use \"composes\" with %q", otherFile.inputFile.Source.PrettyPath),
+							[]logger.MsgData{{Text: fmt.Sprintf(
+								"You can only use \"composes\" with CSS files and %q is not a CSS file (it was loaded with the %q loader).",
+								otherFile.inputFile.Source.PrettyPath, config.LoaderToString[otherFile.inputFile.Loader])}})
+					}
+
 				case ast.ImportAt, ast.ImportAtConditional:
 					// Using a JavaScript file with CSS "@import" is not allowed
 					if _, ok := otherFile.inputFile.Repr.(*graph.JSRepr); ok && otherFile.inputFile.Loader != config.LoaderEmpty {
 						s.log.AddErrorWithNotes(&tracker, record.Range,
 							fmt.Sprintf("Cannot import %q into a CSS file", otherFile.inputFile.Source.PrettyPath),
 							[]logger.MsgData{{Text: fmt.Sprintf(
-								"An \"@import\" rule can only be used to import another CSS file, and %q is not a CSS file (it was loaded with the %q loader).",
+								"An \"@import\" rule can only be used to import another CSS file and %q is not a CSS file (it was loaded with the %q loader).",
 								otherFile.inputFile.Source.PrettyPath, config.LoaderToString[otherFile.inputFile.Loader])}})
 					} else if record.Kind == ast.ImportAtConditional {
 						s.log.AddError(&tracker, record.Range,
@@ -2067,55 +2077,15 @@ func (s *scanner) processScannedFiles(entryPointMeta []graph.EntryPoint) []scann
 							sourceIndex := s.allocateSourceIndex(stubKey, cache.SourceIndexJSStubForCSS)
 							source := otherFile.inputFile.Source
 							source.Index = sourceIndex
-
-							// Export all local CSS names for JavaScript to use
-							exports := js_ast.EObject{}
-							cssSourceIndex := record.SourceIndex.GetIndex()
-							for innerIndex, symbol := range css.AST.Symbols {
-								if symbol.Kind == ast.SymbolLocalCSS {
-									ref := ast.Ref{SourceIndex: cssSourceIndex, InnerIndex: uint32(innerIndex)}
-									loc := css.AST.DefineLocs[ref]
-									value := js_ast.Expr{Loc: loc, Data: &js_ast.ENameOfSymbol{Ref: ref}}
-									visited := map[ast.Ref]bool{ref: true}
-									var parts []js_ast.TemplatePart
-									var visitComposes func(ast.Ref)
-									visitComposes = func(ref ast.Ref) {
-										if composes, ok := css.AST.Composes[ref]; ok {
-											for _, name := range composes.Names {
-												if !visited[name.Ref] {
-													visited[name.Ref] = true
-													visitComposes(name.Ref)
-													parts = append(parts, js_ast.TemplatePart{
-														Value:      js_ast.Expr{Loc: name.Loc, Data: &js_ast.ENameOfSymbol{Ref: name.Ref}},
-														TailCooked: []uint16{' '},
-														TailLoc:    name.Loc,
-													})
-												}
-											}
-										}
-									}
-									visitComposes(ref)
-									if len(parts) > 0 {
-										value.Data = &js_ast.ETemplate{Parts: append(parts, js_ast.TemplatePart{
-											Value:   value,
-											TailLoc: value.Loc,
-										})}
-									}
-									exports.Properties = append(exports.Properties, js_ast.Property{
-										Key:        js_ast.Expr{Loc: loc, Data: &js_ast.EString{Value: helpers.StringToUTF16(symbol.OriginalName)}},
-										ValueOrNil: value,
-									})
-								}
-							}
-
 							s.results[sourceIndex] = parseResult{
 								file: scannerFile{
 									inputFile: graph.InputFile{
 										Source: source,
 										Loader: otherFile.inputFile.Loader,
 										Repr: &graph.JSRepr{
+											// Note: The actual export object will be filled in by the linker
 											AST: js_parser.LazyExportAST(s.log, source,
-												js_parser.OptionsFromConfig(&s.options), js_ast.Expr{Data: &exports}, ""),
+												js_parser.OptionsFromConfig(&s.options), js_ast.Expr{Data: js_ast.ENullShared}, ""),
 											CSSSourceIndex: ast.MakeIndex32(record.SourceIndex.GetIndex()),
 										},
 									},