ci: check for lack of byte-compilation warnings

As part of that we test with latest released Emacs version rather than
snapshot, because we don't want CI to start suddenly failing on
unrelated changes because upstream introduced a new warning/check.

.github/workflows/test.yml

@@ -12,7 +12,7 @@ jobs:
           - 25.3
           - 26.3
           - 27.1
-          - snapshot
+          - 29.2
     steps:
     - uses: purcell/setup-emacs@master
       with:
@@ -24,6 +24,10 @@ jobs:
       run: |
         emacs --version
 
+    - name: Test lack of byte-compile warnings
+      run: |
+        make compile-check
+
     - name: Run tests
       run: |
         make test

Makefile

@@ -9,12 +9,14 @@ DOC = doc
 TAG =
 
 ELCFILES = $(FILES:.el=.elc)
+ELCFILES_CHECK = $(FILES:.el=.elc)
 
-.PHONY: all compile compile-batch docstrings doc clean tests test emacs term terminal profiler indent elpa version
+.PHONY: all compile compile-check compile-batch docstrings doc clean tests test emacs term terminal profiler indent elpa version
 
 # Byte-compile Evil.
 all: compile
 compile: $(ELCFILES)
+compile-check: $(ELCFILES_CHECK)
 
 .depend: $(FILES)
 	@echo Compute dependencies
@@ -28,6 +30,9 @@ compile: $(ELCFILES)
 $(ELCFILES): %.elc: %.el
 	$(EMACS) --batch -Q -L . -f batch-byte-compile $<
 
+$(ELCFILES_CHECK): %.elc: %.el
+	$(EMACS) --batch -Q -L . --eval "(setq byte-compile-error-on-warn t)" -f batch-byte-compile $<
+
 # Byte-compile all files in one batch. This is faster than
 # compiling each file in isolation, but also less stringent.
 compile-batch: clean