Add workaround for regular CSS imports in SCSS

Fixes #7059

resources/resource_transformers/tocss/scss/client.go

@@ -14,6 +14,8 @@
 package scss
 
 import (
+	"regexp"
+
 	"github.com/gohugoio/hugo/helpers"
 	"github.com/gohugoio/hugo/hugolib/filesystems"
 	"github.com/gohugoio/hugo/resources"
@@ -72,3 +74,17 @@ func DecodeOptions(m map[string]interface{}) (opts Options, err error) {
 
 	return
 }
+
+var (
+	regularCSSImportTo   = regexp.MustCompile(`.*(@import "(.*.css)";).*`)
+	regularCSSImportFrom = regexp.MustCompile(`.*(\/\* HUGO_IMPORT_START (.*) HUGO_IMPORT_END \*\/).*`)
+)
+
+func replaceRegularImportsIn(s string) (string, bool) {
+	replaced := regularCSSImportTo.ReplaceAllString(s, "/* HUGO_IMPORT_START $2 HUGO_IMPORT_END */")
+	return replaced, s != replaced
+}
+
+func replaceRegularImportsOut(s string) string {
+	return regularCSSImportFrom.ReplaceAllString(s, "@import \"$2\";")
+}

resources/resource_transformers/tocss/scss/client_test.go

@@ -0,0 +1,49 @@
+// Copyright 2020 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package scss
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestReplaceRegularCSSImports(t *testing.T) {
+	c := qt.New(t)
+
+	scssWithImport := `
+	
+@import "moo";
+@import "regular.css";
+@import "moo";
+@import "another.css";
+
+/* foo */`
+
+	scssWithoutImport := `
+@import "moo";
+/* foo */`
+
+	res, replaced := replaceRegularImportsIn(scssWithImport)
+	c.Assert(replaced, qt.Equals, true)
+	c.Assert(res, qt.Equals, "\n\t\n@import \"moo\";\n/* HUGO_IMPORT_START regular.css HUGO_IMPORT_END */\n@import \"moo\";\n/* HUGO_IMPORT_START another.css HUGO_IMPORT_END */\n\n/* foo */")
+
+	res2, replaced2 := replaceRegularImportsIn(scssWithoutImport)
+	c.Assert(replaced2, qt.Equals, false)
+	c.Assert(res2, qt.Equals, scssWithoutImport)
+
+	reverted := replaceRegularImportsOut(res)
+	c.Assert(reverted, qt.Equals, scssWithImport)
+
+}

resources/resource_transformers/tocss/scss/tocss.go

@@ -65,7 +65,6 @@ func (t *toCSSTransformation) Transform(ctx *resources.ResourceTransformationCtx
 	// We add the entry directories for both project and themes to the include paths list, but
 	// that only work for overrides on the top level.
 	options.to.ImportResolver = func(url string, prev string) (newUrl string, body string, resolved bool) {
-
 		// We get URL paths from LibSASS, but we need file paths.
 		url = filepath.FromSlash(url)
 		prev = filepath.FromSlash(prev)
@@ -170,12 +169,26 @@ func (c *Client) toCSS(options libsass.Options, dst io.Writer, src io.Reader) (l
 	}
 
 	in := helpers.ReaderToString(src)
+
+	// See https://github.com/gohugoio/hugo/issues/7059
+	// We need to preserver the regular CSS imports. This is by far
+	// a perfect solution, and only works for the main entry file, but
+	// that should cover many use cases, e.g. using SCSS as a preprocessor
+	// for Tailwind.
+	var importsReplaced bool
+	in, importsReplaced = replaceRegularImportsIn(in)
+
 	res, err = transpiler.Execute(in)
 	if err != nil {
 		return res, errors.Wrap(err, "SCSS processing failed")
 	}
 
-	_, err = io.WriteString(dst, res.CSS)
+	out := res.CSS
+	if importsReplaced {
+		out = replaceRegularImportsOut(out)
+	}
+
+	_, err = io.WriteString(dst, out)
 
 	return res, err
 }