Re: [PATCH v3 2/2] Makefile: support universal macOS builds via RUST_TARGETS

[Junio C Hamano <gitster@pobox.com>]

"Shardul Natu via GitGitGadget" <gitgitgadget@gmail.com> writes:

> From: Shardul Natu <snatu@google.com>
>
> On macOS, Universal Binaries contain native executable code for
> multiple architectures (such as Intel x86_64 and Apple Silicon arm64)
> bundled into a single file. This is standard practice for macOS
> distribution and CI packaging (such as internal distribution packages
> or tooling like Burrito/Homebrew), allowing a single build artifact
> to run natively across all Macs without Rosetta emulation or
> maintaining separate packages.
>
> When building Git C code for multiple architectures on macOS, the
> Apple toolchain (clang) natively supports universal builds via
> CFLAGS/LDFLAGS. When "-arch x86_64 -arch arm64" is passed, clang
> automatically compiles and links universal binaries for all C object
> files and executables out of the box.
>
> Cargo and rustc, however, do not support multiple "-arch" flags or
> emitting universal binaries in a single invocation. Instead, Cargo
> requires invoking each target triple independently (e.g., passing
> "--target x86_64-apple-darwin" and "--target aarch64-apple-darwin").

This is much easier to understand for those of us unfamiliar with
the macOS ecosystem.  Very much appreciated.

> +$(RUST_LIB): $(RUST_MEMBER_LIBS)
> +	@$(call mkdir_p_parent_template)

The leading @ is a bit curious because among ~20 existing use of
this pattern, nobody adds it to squelch "mkdir -p".  In fact, the
macro uses the standard pattern to define $(QUIET_MKDIR_P_PARENT)
that does the squelching when $(V) is unset.

> +	$(QUIET_GEN)\
> +	if [ $(words $(RUST_TARGETS)) -gt 1 ]; then \

Recipe parts in our Makefile that are written in bourne shell, the
CodingGuidelines apply.

    $ git grep -n -e 'if \[' ':(glob)**/Makefile'

gives empty.  Probably,

	if test $(words $(RUST_TARGETS)) -gt 1; \
	then \

would fit better.

> +		lipo -create $^ -output $@; \
> +	else \
> +		cp $< $@; \
> +	fi
> +endif
>  
>  .PHONY: rust
>  rust: $(RUST_LIB)

Other than that, looking good.

Thanks.