Add Dart Sass support

But note that the Dart Sass Embedded Protocol is still in beta (beta 5), a main release scheduled for Q1 2021.

Fixes #7380
Fixes #8102

resources/resource_transformers/tocss/dartsass/client.go

@@ -0,0 +1,115 @@
+// 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 godartsass integrates with the Dass Sass Embedded protocol to transpile
+// SCSS/SASS.
+package dartsass
+
+import (
+	"io"
+
+	"github.com/gohugoio/hugo/helpers"
+	"github.com/gohugoio/hugo/hugolib/filesystems"
+	"github.com/gohugoio/hugo/resources"
+	"github.com/gohugoio/hugo/resources/resource"
+	"github.com/spf13/afero"
+
+	"github.com/bep/godartsass"
+	"github.com/mitchellh/mapstructure"
+)
+
+// used as part of the cache key.
+const transformationName = "tocss-dart"
+
+func New(fs *filesystems.SourceFilesystem, rs *resources.Spec) (*Client, error) {
+	if !Supports() {
+		return &Client{dartSassNoAvailable: true}, nil
+	}
+	transpiler, err := godartsass.Start(godartsass.Options{})
+	if err != nil {
+		return nil, err
+	}
+	return &Client{sfs: fs, workFs: rs.BaseFs.Work, rs: rs, transpiler: transpiler}, nil
+}
+
+type Client struct {
+	dartSassNoAvailable bool
+	rs                  *resources.Spec
+	sfs                 *filesystems.SourceFilesystem
+	workFs              afero.Fs
+	transpiler          *godartsass.Transpiler
+}
+
+func (c *Client) ToCSS(res resources.ResourceTransformer, args map[string]interface{}) (resource.Resource, error) {
+	if c.dartSassNoAvailable {
+		return res.Transform(resources.NewFeatureNotAvailableTransformer(transformationName, args))
+	}
+	return res.Transform(&transform{c: c, optsm: args})
+}
+
+func (c *Client) Close() error {
+	if c.transpiler == nil {
+		return nil
+	}
+	return c.transpiler.Close()
+}
+
+func (c *Client) toCSS(args godartsass.Args, src io.Reader) (godartsass.Result, error) {
+	var res godartsass.Result
+
+	in := helpers.ReaderToString(src)
+	args.Source = in
+
+	res, err := c.transpiler.Execute(args)
+	if err != nil {
+		return res, err
+	}
+
+	return res, err
+}
+
+type Options struct {
+
+	// Hugo, will by default, just replace the extension of the source
+	// to .css, e.g. "scss/main.scss" becomes "scss/main.css". You can
+	// control this by setting this, e.g. "styles/main.css" will create
+	// a Resource with that as a base for RelPermalink etc.
+	TargetPath string
+
+	// Hugo automatically adds the entry directories (where the main.scss lives)
+	// for project and themes to the list of include paths sent to LibSASS.
+	// Any paths set in this setting will be appended. Note that these will be
+	// treated as relative to the working dir, i.e. no include paths outside the
+	// project/themes.
+	IncludePaths []string
+
+	// Default is nested.
+	// One of nested, expanded, compact, compressed.
+	OutputStyle string
+
+	// When enabled, Hugo will generate a source map.
+	EnableSourceMap bool
+}
+
+func decodeOptions(m map[string]interface{}) (opts Options, err error) {
+	if m == nil {
+		return
+	}
+	err = mapstructure.WeakDecode(m, &opts)
+
+	if opts.TargetPath != "" {
+		opts.TargetPath = helpers.ToSlashTrimLeading(opts.TargetPath)
+	}
+
+	return
+}