markup: Inconsistent heading attribute behaviour #14227
Description
What version of Hugo are you using (hugo version)?
$ hugo version hugo v0.152.2-6abdacad3f3fe944ea42177844469139e81feda6+extended windows/amd64 BuildDate=2025-10-24T15:31:49Z VendorInfo=gohugoio
Does this issue reproduce with the latest release?
Yes
Description
I wanted to use slices as attributes (which goldmark supports) and found out they don't work as I expected. I did some tests on how goldmark attributes are handled in the headings and these are my finding:
render-heading.html
<h{{ .Level }}>{{ .Text }}</h{{ .Level }}>
<pre>{{ .Attributes | jsonify }}</pre>
_index.html
# Simple 1 {test=simple}
# Simple 1
{test=simple}
# Slice 1 {test=[1, 2, 3]}
# Slice 1
{test=[1, 2, 3]}
# Slice 2 {test=[1, 2, string]}
# Slice 2
{test=[1, 2, string]}
# Nested 1 {test={a=1}}
# Nested 2 {test=[a, simple, "attribute", { with="nested values" }]}
# Nested 2
{test=[a, simple, "attribute", { with="nested values" }]}
Output:
<h1>Simple 1</h1>
<pre>{"id":"simple-1","test":"simple"}</pre>
<h1>Simple 1</h1>
<pre>{"id":"simple-1-1","test":"simple"}</pre>
<h1>Slice 1</h1>
<pre>{"id":"slice-1","test":[[0,0],[1,1],[2,2]]}</pre>
<h1>Slice 1</h1>
<pre>{"id":"slice-1-1","test":[[0,0],[1,1],[2,2]]}</pre>
<h1>Slice 2</h1>
<pre>{"id":"slice-2","test":[[0,0],[1,1]]}</pre>
<h1>Slice 2</h1>
<pre>{"id":"slice-2-1","test":[[0,0],[1,1]]}</pre>
<h1>Nested 1 {test={a=1}}</h1>
<pre>{"id":"nested-1-testa1"}</pre>
<h1>Nested 2 {test=[a, simple, “attribute”, { with=“nested values” }]}</h1>
<pre>{"id":"nested-2-testa-simple-attribute--withnested-values-"}</pre>
<h1>Nested 2</h1>
<pre>{"id":"nested-2","test":null}</pre>
Also the next example fails with this error: Error: error building site: render: failed to render pages: render of ".../repro-hugo-goldmark-attributes/content/_index.md" failed: "...\repro-hugo-goldmark-attributes\layouts\baseof.html:13:13": execute of template failed: template: list.html:13:13: executing "main" at <.Content>: error calling Content: not implemented: parser.Attributes
# Nested 1
{test={a=1}}