..make-restat.md

see comment in Makefile

.envrc

use nix -p gnumake

Makefile

.PHONY: clean
clean:
	rm -rf *.out

top.out: c.out b.out
	touch top.out

c.out:
	touch c.out
b.out: a.out
	@if [[ ! -f b.out ]]; then echo "touch b.out"; touch b.out; elif [[ $${FORCE_REBUILD+x} ]]; then echo "touch b.out (forced)"; touch b.out; else echo "No rebuild b."; fi

a.out:
	touch a.out

.DEFAULT_GOAL := top.out

# you can update `a` and `b` will not be rebuilt. thus, changes to `a` do not trigger rebuilds of `top.out`.
#
# however note that until `b` is rebuilt, it will always be built because `a` is newer than `b`

# compare with ninja's `restat`: https://ninja-build.org/manual.html
#
# we get the "ECO"-esque functionality (i.e. build actions can say that dependents do not need to be rebuilt)
#
# however the "re stat" part is missing: we cannot tell Make that the changes in deps (i.e. the changes in A)
# have been "accounted for" already and thus don't need to trigger a rebuild of the target (b)
#
# this makes sense. `ninja` has it's `.ninja_log` file in which it records modification times; Make has no
# such machinery. it just checks the filesystem. in other words: there's no where for make to even _record_
# this information (i.e. that changes of A not newer than <some time> needn't retrigger B (for the artifact
# built at <some older time>))
#
# also see: https://fuchsia.dev/fuchsia-src/development/build/ninja_how#restat