docs: Integrate rustdoc generation into htmldocs

Change target `make htmldocs` to combine RST Sphinx and the generation of
Rust documentation, when support is available and .config exists.
Reviewed-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Carlos Bilbao <carlos.bilbao@amd.com>
Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230718151534.4067460-3-carlos.bilbao@amd.com

Documentation/Makefile

@@ -59,6 +59,12 @@ PAPEROPT_letter = -D latex_paper_size=letter
 KERNELDOC       = $(srctree)/scripts/kernel-doc
 KERNELDOC_CONF  = -D kerneldoc_srctree=$(srctree) -D kerneldoc_bin=$(KERNELDOC)
 ALLSPHINXOPTS   =  $(KERNELDOC_CONF) $(PAPEROPT_$(PAPER)) $(SPHINXOPTS)
+ifneq ($(wildcard $(srctree)/.config),)
+ifeq ($(CONFIG_RUST),y)
+	# Let Sphinx know we will include rustdoc
+	ALLSPHINXOPTS   +=  -t rustdoc
+endif
+endif
 # the i18n builder cannot share the environment and doctrees with the others
 I18NSPHINXOPTS  = $(PAPEROPT_$(PAPER)) $(SPHINXOPTS) .
 
@@ -95,6 +101,16 @@ htmldocs:
 	@$(srctree)/scripts/sphinx-pre-install --version-check
 	@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,html,$(var),,$(var)))
 
+# If Rust support is available and .config exists, add rustdoc generated contents.
+# If there are any, the errors from this make rustdoc will be displayed but
+# won't stop the execution of htmldocs
+
+ifneq ($(wildcard $(srctree)/.config),)
+ifeq ($(CONFIG_RUST),y)
+	$(Q)$(MAKE) rustdoc || true
+endif
+endif
+
 texinfodocs:
 	@$(srctree)/scripts/sphinx-pre-install --version-check
 	@+$(foreach var,$(SPHINXDIRS),$(call loop_cmd,sphinx,texinfo,$(var),texinfo,$(var)))