Add the [params] concept to front matter

This is deliberately very simple, but should not break anything. We need to introduce this in baby steps, but this should allow us to introduce this in the documentation.

Note that the `params` section's key/values will be added to `.Params` last. This means that you can have different values for "Hugo's summary" and the custom ".Params.summary" if you want to.

Updates #11055

hugolib/page__meta.go

@@ -421,9 +421,20 @@ func (p *pageState) setMetaPostParams() error {
 	var sitemapSet bool
 
 	var draft, published, isCJKLanguage *bool
+	var userParams map[string]any
 	for k, v := range pm.params {
 		loki := strings.ToLower(k)
 
+		if loki == "params" {
+			vv, err := maps.ToStringMapE(v)
+			if err != nil {
+				return err
+			}
+			userParams = vv
+			delete(pm.params, k)
+			continue
+		}
+
 		if loki == "published" { // Intentionally undocumented
 			vv, err := cast.ToBoolE(v)
 			if err == nil {
@@ -589,6 +600,10 @@ func (p *pageState) setMetaPostParams() error {
 		}
 	}
 
+	for k, v := range userParams {
+		pm.params[strings.ToLower(k)] = v
+	}
+
 	if !sitemapSet {
 		pm.sitemap = p.s.conf.Sitemap
 	}

hugolib/params_test.go

@@ -0,0 +1,54 @@
+// Copyright 2024 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 hugolib
+
+import "testing"
+
+func TestFrontMatterParamsInItsOwnSection(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+baseURL = "https://example.org/"
+-- content/_index.md --
++++
+title = "Home"
+[[cascade]]
+background = 'yosemite.jpg'
+[cascade.params]
+a = "home-a"
+b = "home-b"
+[cascade._target]
+kind = 'page'
++++
+-- content/p1.md --
+---
+title: "P1"
+summary: "frontmatter.summary"
+params:
+   a: "p1-a"
+   summary: "params.summary"
+---	
+-- layouts/_default/single.html --
+Params: {{ range $k, $v := .Params }}{{ $k }}: {{ $v }}|{{ end }}$
+Summary: {{ .Summary }}|
+`
+
+	b := Test(t, files)
+
+	b.AssertFileContent("public/p1/index.html",
+		"Params: a: p1-a|b: home-b|background: yosemite.jpg|draft: false|iscjklanguage: false|summary: params.summary|title: P1|$",
+		"Summary: frontmatter.summary|",
+	)
+}

resources/page/page_matcher.go

@@ -144,7 +144,16 @@ func mapToPageMatcherParamsConfig(m map[string]any) (PageMatcherParamsConfig, er
 			// those values will now be moved to the top level.
 			// This should be very unlikely as it would lead to constructs like .Params.params.foo,
 			// and most people see params as an Hugo internal keyword.
-			pcfg.Params = maps.ToStringMap(v)
+			params := maps.ToStringMap(v)
+			if pcfg.Params == nil {
+				pcfg.Params = params
+			} else {
+				for k, v := range params {
+					if _, found := pcfg.Params[k]; !found {
+						pcfg.Params[k] = v
+					}
+				}
+			}
 		case "_target", "target":
 			var target PageMatcher
 			if err := decodePageMatcher(v, &target); err != nil {