prelude_str.mk: Add a 'str.eq' function

prelude.mk

@@ -16,6 +16,7 @@
 #  String support:
 #  - Variable: export NL
 #  - Variable:        SPACE
+#  - Function: str.eq
 #
 #  Path support:
 #  - Function: path.trimprefix

prelude_str.mk

@@ -15,3 +15,7 @@ export NL
 define SPACE
 
 endef
+
+# Usage: $(call str.eq,STR1,STR2)
+# Evaluates to either $(TRUE) or $(FALSE)
+str.eq = $(if $(subst x$1,,x$2)$(subst x$2,,x$1),$(FALSE),$(TRUE))

tests/prelude_str.bats

@@ -11,3 +11,29 @@ load common
 	# Honestly, this checks `quote.shell` more than it does SPACE.
 	check_expr_eq strict '$(SPACE)' ' '
 }
+
+@test "prelude_str.mk: str.eq" {
+	cat >>Makefile <<-'__EOT__'
+		include build-aux/prelude.mk
+		test = $(info ("$1" == "$2") => $(if $(call str.eq,$1,$2),$$(TRUE),$$(FALSE)))
+		$(call test,foo,foo)
+		$(call test,foo,bar)
+		$(call test,bar,bar)
+		$(call test,f%,foo)
+		$(call test,foo,f%)
+		$(call test,f%,f%)
+		all: noop
+		noop: ; @true
+		.PHONY: noop
+	__EOT__
+
+	make >actual
+	printf '("%s" == "%s") => $(%s)\n' >expected \
+	       foo foo TRUE \
+	       foo bar FALSE \
+	       bar bar TRUE \
+	       f% foo FALSE \
+	       foo f% FALSE \
+	       f% f% TRUE
+	diff -u expected actual
+}