About execution order in makefile - makefile

Question

in this Makefile:
ifeq ($(shell uname),Darwin)
LDFLAGS := -Wl,-dead_strip
else
LDFLAGS := -Wl,--gc-sections -lpthread -ldl
endif
all: target/double
target/double
target:
mkdir -p $#
target/double: target/main.o target/debug/libdouble_input.a
$(CC) -o $# $^ $(LDFLAGS)
target/debug/libdouble_input.a: src/lib.rs Cargo.toml
cargo build
target/main.o: src/main.c | target
$(CC) -o $# -c $<
clean:
rm -rf target
when i excute make all, get this output:
hello_c_use_rust [master] ⚡ make all
mkdir -p target
cc -o target/main.o -c src/main.c
cargo build
Compiling hello_c_use_rust v0.1.0 (/Users/jelly/code/own/hello_rust/hello_c_use_rust)
Finished dev [unoptimized + debuginfo] target(s) in 0.20s
cc -o target/double target/main.o target/debug/libdouble_input.a -Wl,-dead_strip
target/double
4 * 2 = 8
Please tell me why this is the execution order ? txs ^_^.
What puzzled me was why the first step was mkdir -p target;

Answer 1

Your goal is all. all depends on target/double that must thus be done first. In turn target/double depends on target/main.o and target/debug/libdouble_input.a. So target/main.o and target/debug/libdouble_input.a must be done first. Here, you are lucky (we'll see why later): make tries to build target/main.o first. As target/main.o has target as a prerequisite, target must be done first and it is. Qed.
Note: target is an order-only prerequisite of target/main.o, not a regular prerequisite (the | sign starts the list of order-only prerequisites). It means that make pays only attention to its existence. It ignores its last modification time, which is good as last modification times of directories are usually not relevant in a build process.
Why is target a prerequisite of target/main.o? Because you cannot build target/main.o if the target directory does not exist yet. The build would simply fail. So the order-only prerequisite tells make that the directory must exist first.
Why are you lucky? Because if make had tried to build target/debug/libdouble_input.a first and if cargo build does not create the destination directory, it would have failed. Even if you know that target/main.o is built first because it is the first prerequisite of target/double, you should not count on this. One day or another somebody could try to use parallel make (make -j) and things could go wrong. Moreover target could exist but not target/debug...
Note: even if you know that cargo build takes care of creating the destination directory it is probably wise to add one more order-only prerquisite to your Makefile. Just in case something changes one day or another. And also to show readers of your Makefile that you know what you are doing here:
target target/debug:
mkdir -p $#
target/debug/libdouble_input.a: src/lib.rs Cargo.toml | target/debug
cargo build
It is not a big deal and could save you some errors.