doc: Rename 2X.XX to "Upcoming release", and only generate if applicable

.gitignore

@@ -21,6 +21,7 @@ perl/Makefile.config
 /doc/manual/language.json
 /doc/manual/xp-features.json
 /doc/manual/src/SUMMARY.md
+/doc/manual/src/SUMMARY-rl-next.md
 /doc/manual/src/command-ref/new-cli
 /doc/manual/src/command-ref/conf-file.md
 /doc/manual/src/command-ref/experimental-features-shortlist.md

doc/manual/local.mk

@@ -92,7 +92,7 @@ $(d)/nix-profiles.5: $(d)/src/command-ref/files/profiles.md
 	$(trace-gen) lowdown -sT man --nroff-nolinks -M section=5 $^.tmp -o $@
 	@rm $^.tmp
 
-$(d)/src/SUMMARY.md: $(d)/src/SUMMARY.md.in $(d)/src/command-ref/new-cli $(d)/src/contributing/experimental-feature-descriptions.md
+$(d)/src/SUMMARY.md: $(d)/src/SUMMARY.md.in $(d)/src/SUMMARY-rl-next.md $(d)/src/command-ref/new-cli $(d)/src/contributing/experimental-feature-descriptions.md
 	@cp $< $@
 	@$(call process-includes,$@,$@)
 
@@ -144,9 +144,18 @@ $(d)/language.json: $(bindir)/nix
 	$(trace-gen) $(dummy-env) $(bindir)/nix __dump-language > $@.tmp
 	@mv $@.tmp $@
 
-$(d)/src/release-notes/rl-next.md: $(d)/rl-next/*
+# Generate "Upcoming release" notes (or clear it and remove from menu)
+$(d)/src/release-notes/rl-next.md: $(d)/rl-next $(d)/rl-next/*
 	$(trace-gen) changelog-d doc/manual/rl-next > $@
 
+$(d)/src/SUMMARY-rl-next.md: $(d)/src/release-notes/rl-next.md
+	$(trace-gen) true
+	@if [ -s $< ]; then \
+		echo '  - [Upcoming release](release-notes/rl-next.md)' > $@; \
+	else \
+	  true > $@; \
+	fi
+
 # Generate the HTML manual.
 .PHONY: manual-html
 manual-html: $(docdir)/manual/index.html

doc/manual/src/SUMMARY.md.in

@@ -114,7 +114,7 @@
   - [CLI guideline](contributing/cli-guideline.md)
   - [C++ style guide](contributing/cxx.md)
 - [Release Notes](release-notes/release-notes.md)
-  - [Release X.Y (202?-??-??)](release-notes/rl-next.md)
+{{#include ./SUMMARY-rl-next.md}}
   - [Release 2.19 (2023-11-17)](release-notes/rl-2.19.md)
   - [Release 2.18 (2023-09-20)](release-notes/rl-2.18.md)
   - [Release 2.17 (2023-07-24)](release-notes/rl-2.17.md)