Implement XML data support

Example:

```
{{ with resources.Get "https://example.com/rss.xml" | transform.Unmarshal }}
    {{ range .channel.item }}
        <strong>{{ .title | plainify | htmlUnescape }}</strong><br />
        <p>{{ .description | plainify | htmlUnescape }}</p>
        {{ $link := .link | plainify | htmlUnescape }}
        <a href="{{ $link }}">{{ $link }}</a><br />
        <hr>
    {{ end }}
{{ end }}
```

Closes #4470

12 files changed, 167 insertions, 12 deletions

diff --git a/docs/content/en/functions/transform.Unmarshal.md b/docs/content/en/functions/transform.Unmarshal.md
index 973779c4c..9b380dc57 100644
--- a/docs/content/en/functions/transform.Unmarshal.md
+++ b/docs/content/en/functions/transform.Unmarshal.md
@@ -1,6 +1,6 @@
 ---
 title: "transform.Unmarshal"
-description: "`transform.Unmarshal` (alias `unmarshal`) parses the input and converts it into a map or an array. Supported formats are JSON, TOML, YAML and CSV."
+description: "`transform.Unmarshal` (alias `unmarshal`) parses the input and converts it into a map or an array. Supported formats are JSON, TOML, YAML, XML and CSV."
 date: 2018-12-23
 categories: [functions]
 menu:
@@ -45,3 +45,32 @@ Example:
 ```go-html-template
 {{ $csv := "a;b;c" | transform.Unmarshal (dict "delimiter" ";") }}
 ```
+
+## XML data
+
+As a convenience, Hugo allows you to access XML data in the same way that you access JSON, TOML, and YAML: you do not need to specify the root node when accessing the data.
+
+To get the contents of `<title>` in the document below, you use `{{ .message.title }}`:
+
+```
+<root>
+    <message>
+        <title>Hugo rocks!</title>
+        <description>Thanks for using Hugo</description>
+    </message>
+</root>
+```
+
+The following example lists the items of an RSS feed:
+
+```
+{{ with resources.Get "https://example.com/rss.xml" | transform.Unmarshal }}
+    {{ range .channel.item }}
+        <strong>{{ .title | plainify | htmlUnescape }}</strong><br />
+        <p>{{ .description | plainify | htmlUnescape }}</p>
+        {{ $link := .link | plainify | htmlUnescape }}
+        <a href="{{ $link }}">{{ $link }}</a><br />
+        <hr>
+    {{ end }}
+{{ end }}
+```
diff --git a/docs/content/en/templates/data-templates.md b/docs/content/en/templates/data-templates.md
index c36344776..441cc2f10 100644
--- a/docs/content/en/templates/data-templates.md
+++ b/docs/content/en/templates/data-templates.md
@@ -6,7 +6,7 @@ date: 2017-02-01
 publishdate: 2017-02-01
 lastmod: 2017-03-12
 categories: [templates]
-keywords: [data,dynamic,csv,json,toml,yaml]
+keywords: [data,dynamic,csv,json,toml,yaml,xml]
 menu:
   docs:
     parent: "templates"
@@ -20,7 +20,7 @@ toc: true
 
 <!-- begin data files -->
 
-Hugo supports loading data from YAML, JSON, and TOML files located in the `data` directory in the root of your Hugo project.
+Hugo supports loading data from YAML, JSON, XML, and TOML files located in the `data` directory in the root of your Hugo project.
 
 {{< youtube FyPgSuwIMWQ >}}
 
@@ -28,7 +28,7 @@ Hugo supports loading data from YAML, JSON, and TOML files located in the `data`
 
 The `data` folder is where you can store additional data for Hugo to use when generating your site. Data files aren't used to generate standalone pages; rather, they're meant to be supplemental to content files. This feature can extend the content in case your front matter fields grow out of control. Or perhaps you want to show a larger dataset in a template (see example below). In both cases, it's a good idea to outsource the data in their own files.
 
-These files must be YAML, JSON, or TOML files (using the `.yml`, `.yaml`, `.json`, or `.toml` extension). The data will be accessible as a `map` in the `.Site.Data` variable.
+These files must be YAML, JSON, XML, or TOML files (using the `.yml`, `.yaml`, `.json`, `.xml`, or `.toml` extension). The data will be accessible as a `map` in the `.Site.Data` variable.
 
 ## Data Files in Themes
 
@@ -95,7 +95,7 @@ Discover a new favorite bass player? Just add another `.toml` file in the same d
 
 ## Example: Accessing Named Values in a Data File
 
-Assume you have the following data structure in your `User0123.[yml|toml|json]` data file located directly in `data/`:
+Assume you have the following data structure in your `User0123.[yml|toml|xml|json]` data file located directly in `data/`:
 
 {{< code-toggle file="User0123" >}}
 Name: User0123
@@ -232,6 +232,7 @@ If you change any local file and the LiveReload is triggered, Hugo will read the
 * [YAML Spec][yaml]
 * [JSON Spec][json]
 * [CSV Spec][csv]
+* [XML Spec][xml]
 
 [config]: /getting-started/configuration/
 [csv]: https://tools.ietf.org/html/rfc4180
@@ -247,3 +248,4 @@ If you change any local file and the LiveReload is triggered, Hugo will read the
 [variadic]: https://en.wikipedia.org/wiki/Variadic_function
 [vars]: /variables/
 [yaml]: https://yaml.org/spec/
+[xml]: https://www.w3.org/XML/
diff --git a/go.mod b/go.mod
index 0fbade221..db14b0100 100644
--- a/go.mod
+++ b/go.mod
@@ -13,6 +13,7 @@ require (
 	github.com/bep/golibsass v1.0.0
 	github.com/bep/gowebp v0.1.0
 	github.com/bep/tmc v0.5.1
+	github.com/clbanning/mxj/v2 v2.5.5
 	github.com/cli/safeexec v1.0.0
 	github.com/disintegration/gift v1.2.1
 	github.com/dustin/go-humanize v1.0.0
diff --git a/go.sum b/go.sum
index 4093bf695..952d02793 100644
--- a/go.sum
+++ b/go.sum
@@ -144,6 +144,8 @@ github.com/cheekybits/is v0.0.0-20150225183255-68e9c0620927/go.mod h1:h/aW8ynjgk
 github.com/chzyer/logex v1.1.10/go.mod h1:+Ywpsq7O8HXn0nuIou7OrIPyXbp3wmkHB+jjWRnGsAI=
 github.com/chzyer/readline v0.0.0-20180603132655-2972be24d48e/go.mod h1:nSuG5e5PlCu98SY8svDHJxuZscDgtXS6KTTbou5AhLI=
 github.com/chzyer/test v0.0.0-20180213035817-a1ea475d72b1/go.mod h1:Q3SI9o4m/ZMnBNeIyt5eFwwo7qiLfzFZmjNmxjkiQlU=
+github.com/clbanning/mxj/v2 v2.5.5 h1:oT81vUeEiQQ/DcHbzSytRngP6Ky9O+L+0Bw0zSJag9E=
+github.com/clbanning/mxj/v2 v2.5.5/go.mod h1:hNiWqW14h+kc+MdF9C6/YoRfjEJoR3ou6tn/Qo+ve2s=
 github.com/cli/safeexec v1.0.0 h1:0VngyaIyqACHdcMNWfo6+KdUYnqEr2Sg+bSP1pdF+dI=
 github.com/cli/safeexec v1.0.0/go.mod h1:Z/D4tTN8Vs5gXYHDCbaM1S/anmEDnJb1iW0+EJ5zx3Q=
 github.com/client9/misspell v0.3.4/go.mod h1:qj6jICC3Q7zFZvVWo7KLAzC3yx5G7kyvSDkc90ppPyw=
diff --git a/hugolib/resource_chain_test.go b/hugolib/resource_chain_test.go
index 0a997cda3..05e8a9d00 100644
--- a/hugolib/resource_chain_test.go
+++ b/hugolib/resource_chain_test.go
@@ -591,9 +591,9 @@ func TestResourceChains(t *testing.T) {
 
 		case "/mydata/xml1.xml":
 			w.Write([]byte(`
-				<hello>
-					<world>Hugo Rocks!</<world>
-				</hello>`))
+					<hello>
+						<world>Hugo Rocks!</<world>
+					</hello>`))
 			return
 
 		case "/mydata/svg1.svg":
@@ -872,16 +872,19 @@ Publish 2: {{ $cssPublish2.Permalink }}
 {{ $toml := "slogan = \"Hugo Rocks!\"" | resources.FromString "slogan.toml" | transform.Unmarshal }}
 {{ $csv1 := "\"Hugo Rocks\",\"Hugo is Fast!\"" | resources.FromString "slogans.csv" | transform.Unmarshal }}
 {{ $csv2 := "a;b;c" | transform.Unmarshal (dict "delimiter" ";") }}
+{{ $xml := "<?xml version=\"1.0\" encoding=\"UTF-8\"?><note><to>You</to><from>Me</from><heading>Reminder</heading><body>Do not forget XML</body></note>" | transform.Unmarshal }}
 
 Slogan: {{ $toml.slogan }}
 CSV1: {{ $csv1 }} {{ len (index $csv1 0)  }}
 CSV2: {{ $csv2 }}		
+XML: {{ $xml.body }}
 `)
 		}, func(b *sitesBuilder) {
 			b.AssertFileContent("public/index.html",
 				`Slogan: Hugo Rocks!`,
 				`[[Hugo Rocks Hugo is Fast!]] 2`,
 				`CSV2: [[a b c]]`,
+				`XML: Do not forget XML`,
 			)
 		}},
 		{"resources.Get", func() bool { return true }, func(b *sitesBuilder) {
diff --git a/parser/frontmatter.go b/parser/frontmatter.go
index 79701a0fc..a99829521 100644
--- a/parser/frontmatter.go
+++ b/parser/frontmatter.go
@@ -23,6 +23,8 @@ import (
 	toml "github.com/pelletier/go-toml/v2"
 
 	yaml "gopkg.in/yaml.v2"
+
+	xml "github.com/clbanning/mxj/v2"
 )
 
 const (
@@ -62,7 +64,14 @@ func InterfaceToConfig(in interface{}, format metadecoders.Format, w io.Writer)
 
 		_, err = w.Write([]byte{'\n'})
 		return err
+	case metadecoders.XML:
+		b, err := xml.AnyXmlIndent(in, "", "\t", "root")
+		if err != nil {
+			return err
+		}
 
+		_, err = w.Write(b)
+		return err
 	default:
 		return errors.New("unsupported Format provided")
 	}
diff --git a/parser/metadecoders/decoder.go b/parser/metadecoders/decoder.go
index 168c130ed..f0dcb0856 100644
--- a/parser/metadecoders/decoder.go
+++ b/parser/metadecoders/decoder.go
@@ -24,6 +24,7 @@ import (
 	"github.com/gohugoio/hugo/common/herrors"
 	"github.com/niklasfasching/go-org/org"
 
+	xml "github.com/clbanning/mxj/v2"
 	toml "github.com/pelletier/go-toml/v2"
 	"github.com/pkg/errors"
 	"github.com/spf13/afero"
@@ -135,6 +136,25 @@ func (d Decoder) UnmarshalTo(data []byte, f Format, v interface{}) error {
 		err = d.unmarshalORG(data, v)
 	case JSON:
 		err = json.Unmarshal(data, v)
+	case XML:
+		var xmlRoot xml.Map
+		xmlRoot, err = xml.NewMapXml(data)
+
+		var xmlValue map[string]interface{}
+		if err == nil {
+			xmlRootName, err := xmlRoot.Root()
+			if err != nil {
+				return toFileError(f, errors.Wrap(err, "failed to unmarshal XML"))
+			}
+			xmlValue = xmlRoot[xmlRootName].(map[string]interface{})
+		}
+
+		switch v := v.(type) {
+		case *map[string]interface{}:
+			*v = xmlValue
+		case *interface{}:
+			*v = xmlValue
+		}
 	case TOML:
 		err = toml.Unmarshal(data, v)
 	case YAML:
diff --git a/parser/metadecoders/decoder_test.go b/parser/metadecoders/decoder_test.go
index e0990a5f7..8cd5513b2 100644
--- a/parser/metadecoders/decoder_test.go
+++ b/parser/metadecoders/decoder_test.go
@@ -20,6 +20,59 @@ import (
 	qt "github.com/frankban/quicktest"
 )
 
+func TestUnmarshalXML(t *testing.T) {
+	c := qt.New(t)
+
+	xmlDoc := `<?xml version="1.0" encoding="utf-8" standalone="yes"?>
+	<rss version="2.0"
+		xmlns:atom="http://www.w3.org/2005/Atom">
+		<channel>
+			<title>Example feed</title>
+			<link>https://example.com/</link>
+			<description>Example feed</description>
+			<generator>Hugo -- gohugo.io</generator>
+			<language>en-us</language>
+			<copyright>Example</copyright>
+			<lastBuildDate>Fri, 08 Jan 2021 14:44:10 +0000</lastBuildDate>
+			<atom:link href="https://example.com/feed.xml" rel="self" type="application/rss+xml"/>
+			<item>
+				<title>Example title</title>
+				<link>https://example.com/2021/11/30/example-title/</link>
+				<pubDate>Tue, 30 Nov 2021 15:00:00 +0000</pubDate>
+				<guid>https://example.com/2021/11/30/example-title/</guid>
+				<description>Example description</description>
+			</item>
+		</channel>
+	</rss>`
+
+	expect := map[string]interface{}{
+		"-atom": "http://www.w3.org/2005/Atom", "-version": "2.0",
+		"channel": map[string]interface{}{
+			"copyright":   "Example",
+			"description": "Example feed",
+			"generator":   "Hugo -- gohugo.io",
+			"item": map[string]interface{}{
+				"description": "Example description",
+				"guid":        "https://example.com/2021/11/30/example-title/",
+				"link":        "https://example.com/2021/11/30/example-title/",
+				"pubDate":     "Tue, 30 Nov 2021 15:00:00 +0000",
+				"title":       "Example title"},
+			"language":      "en-us",
+			"lastBuildDate": "Fri, 08 Jan 2021 14:44:10 +0000",
+			"link": []interface{}{"https://example.com/", map[string]interface{}{
+				"-href": "https://example.com/feed.xml",
+				"-rel":  "self",
+				"-type": "application/rss+xml"}},
+			"title": "Example feed",
+		}}
+
+	d := Default
+
+	m, err := d.Unmarshal([]byte(xmlDoc), XML)
+	c.Assert(err, qt.IsNil)
+	c.Assert(m, qt.DeepEquals, expect)
+
+}
 func TestUnmarshalToMap(t *testing.T) {
 	c := qt.New(t)
 
@@ -38,6 +91,7 @@ func TestUnmarshalToMap(t *testing.T) {
 		{"a: Easy!\nb:\n  c: 2\n  d: [3, 4]", YAML, map[string]interface{}{"a": "Easy!", "b": map[string]interface{}{"c": 2, "d": []interface{}{3, 4}}}},
 		{"a:\n  true: 1\n  false: 2", YAML, map[string]interface{}{"a": map[string]interface{}{"true": 1, "false": 2}}},
 		{`{ "a": "b" }`, JSON, expect},
+		{`<root><a>b</a></root>`, XML, expect},
 		{`#+a: b`, ORG, expect},
 		// errors
 		{`a = b`, TOML, false},
@@ -72,6 +126,7 @@ func TestUnmarshalToInterface(t *testing.T) {
 		{`#+DATE: <2020-06-26 Fri>`, ORG, map[string]interface{}{"date": "2020-06-26"}},
 		{`a = "b"`, TOML, expect},
 		{`a: "b"`, YAML, expect},
+		{`<root><a>b</a></root>`, XML, expect},
 		{`a,b,c`, CSV, [][]string{{"a", "b", "c"}}},
 		{"a: Easy!\nb:\n  c: 2\n  d: [3, 4]", YAML, map[string]interface{}{"a": "Easy!", "b": map[string]interface{}{"c": 2, "d": []interface{}{3, 4}}}},
 		// errors
diff --git a/parser/metadecoders/format.go b/parser/metadecoders/format.go
index bba89dbea..d34a261bf 100644
--- a/parser/metadecoders/format.go
+++ b/parser/metadecoders/format.go
@@ -30,6 +30,7 @@ const (
 	TOML Format = "toml"
 	YAML Format = "yaml"
 	CSV  Format = "csv"
+	XML  Format = "xml"
 )
 
 // FormatFromString turns formatStr, typically a file extension without any ".",
@@ -51,6 +52,8 @@ func FormatFromString(formatStr string) Format {
 		return ORG
 	case "csv":
 		return CSV
+	case "xml":
+		return XML
 	}
 
 	return ""
@@ -68,27 +71,32 @@ func FormatFromMediaType(m media.Type) Format {
 	return ""
 }
 
-// FormatFromContentString tries to detect the format (JSON, YAML or TOML)
+// FormatFromContentString tries to detect the format (JSON, YAML, TOML or XML)
 // in the given string.
 // It return an empty string if no format could be detected.
 func (d Decoder) FormatFromContentString(data string) Format {
 	csvIdx := strings.IndexRune(data, d.Delimiter)
 	jsonIdx := strings.Index(data, "{")
 	yamlIdx := strings.Index(data, ":")
+	xmlIdx := strings.Index(data, "<")
 	tomlIdx := strings.Index(data, "=")
 
-	if isLowerIndexThan(csvIdx, jsonIdx, yamlIdx, tomlIdx) {
+	if isLowerIndexThan(csvIdx, jsonIdx, yamlIdx, xmlIdx, tomlIdx) {
 		return CSV
 	}
 
-	if isLowerIndexThan(jsonIdx, yamlIdx, tomlIdx) {
+	if isLowerIndexThan(jsonIdx, yamlIdx, xmlIdx, tomlIdx) {
 		return JSON
 	}
 
-	if isLowerIndexThan(yamlIdx, tomlIdx) {
+	if isLowerIndexThan(yamlIdx, xmlIdx, tomlIdx) {
 		return YAML
 	}
 
+	if isLowerIndexThan(xmlIdx, tomlIdx) {
+		return XML
+	}
+
 	if tomlIdx != -1 {
 		return TOML
 	}
diff --git a/parser/metadecoders/format_test.go b/parser/metadecoders/format_test.go
index 2f625935e..0d94cf67e 100644
--- a/parser/metadecoders/format_test.go
+++ b/parser/metadecoders/format_test.go
@@ -30,6 +30,7 @@ func TestFormatFromString(t *testing.T) {
 		{"json", JSON},
 		{"yaml", YAML},
 		{"yml", YAML},
+		{"xml", XML},
 		{"toml", TOML},
 		{"config.toml", TOML},
 		{"tOMl", TOML},
@@ -48,6 +49,7 @@ func TestFormatFromMediaType(t *testing.T) {
 	}{
 		{media.JSONType, JSON},
 		{media.YAMLType, YAML},
+		{media.XMLType, XML},
 		{media.TOMLType, TOML},
 		{media.CalendarType, ""},
 	} {
@@ -70,6 +72,7 @@ func TestFormatFromContentString(t *testing.T) {
 		{`foo:"bar"`, YAML},
 		{`{ "foo": "bar"`, JSON},
 		{`a,b,c"`, CSV},
+		{`<foo>bar</foo>"`, XML},
 		{`asdfasdf`, Format("")},
 		{``, Format("")},
 	} {
diff --git a/tpl/transform/remarshal_test.go b/tpl/transform/remarshal_test.go
index 2cb4c3a2f..8e94ef6bf 100644
--- a/tpl/transform/remarshal_test.go
+++ b/tpl/transform/remarshal_test.go
@@ -82,6 +82,25 @@ title: Test Metadata
    "title": "Test Metadata"
 }
 `
+		xmlExample := `<root>
+		  <resources>
+			<params>
+			  <byline>picasso</byline>
+			</params>
+			<src>**image-4.png</src>
+			<title>The Fourth Image!</title>
+		  </resources>
+		  <resources>
+			<name>my-cool-image-:counter</name>
+			<params>
+			  <byline>bep</byline>
+			</params>
+			<src>**.png</src>
+			<title>TOML: The Image #:counter</title>
+		  </resources>
+		  <title>Test Metadata</title>
+		</root>
+		`
 
 		variants := []struct {
 			format string
@@ -93,6 +112,7 @@ title: Test Metadata
 			{"TOML", tomlExample},
 			{"Toml", tomlExample},
 			{" TOML ", tomlExample},
+			{"XML", xmlExample},
 		}
 
 		for _, v1 := range variants {
diff --git a/tpl/transform/unmarshal_test.go b/tpl/transform/unmarshal_test.go
index 85e3610d1..fb0e446c3 100644
--- a/tpl/transform/unmarshal_test.go
+++ b/tpl/transform/unmarshal_test.go
@@ -111,6 +111,9 @@ func TestUnmarshal(t *testing.T) {
 		{testContentResource{key: "r1", content: `slogan = "Hugo Rocks!"`, mime: media.TOMLType}, nil, func(m map[string]interface{}) {
 			assertSlogan(m)
 		}},
+		{testContentResource{key: "r1", content: `<root><slogan>Hugo Rocks!</slogan></root>"`, mime: media.XMLType}, nil, func(m map[string]interface{}) {
+			assertSlogan(m)
+		}},
 		{testContentResource{key: "r1", content: `1997,Ford,E350,"ac, abs, moon",3000.00
 1999,Chevy,"Venture ""Extended Edition""","",4900.00`, mime: media.CSVType}, nil, func(r [][]string) {
 			c.Assert(len(r), qt.Equals, 2)