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Re: Double colon rules with phony prerequisites

From: Kaz Kylheku (gmake)
Subject: Re: Double colon rules with phony prerequisites
Date: Tue, 27 Apr 2021 14:30:13 -0700
User-agent: Roundcube Webmail/0.9.2

On 2021-04-27 07:32, Fogle, Benjamin wrote:

I have a large project that includes libraries in Rust as
dependencies. I want to run “cargo build” unconditionally, so it
should use a phony target, but I only want to rebuild downstream
binaries if a library actually changed. I found that double-colon
rules seem to be an exception to “a phony target should not be a
prerequisite of a real target file; if it is, its recipe will be run
every time make goes to update that file” mentioned in the docs.
(Though make only seems to reconsider timestamps the double-colon rule
has a recipe.)

The following seems to work for me, but my question is: Am I missing
something, and can I rely on this behavior?

.PHONY: build-rust
                cd rust-lib && cargo build

rust-lib/target/debug/ build-rust

binary: binary.o rust-lib/target/debug/

How do you know that an exception is being made; i.e. that the recipe
for is not being run?

The recipe is @true, which prints nothing and succeeds, concealing
any evidence.

You have to put some echo's in there.

Given this Makefile:

.PHONY: phony
        echo phony

real:: phony
        echo real

targ: real
        touch real
        touch targ

        rm -f targ real

and an initial state that the files "real" and "targ" do not exist,
this happens:

$ make targ
echo phony
echo real
touch real
touch targ

Then real and targ exist, so this happens:

$ make targ
echo phony
echo real

The phony rule runs, and so does the double colon real one. But
the "targ: real" relative timestamps have not changed, so targ's
recipe is not executed.

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