Don't pass -stdlib=libc++ when building C files on macOS

[Zalathar]

When using Command Line Tools for Xcode version 15.0, clang will warn about argument unused during compilation: '-stdlib=libc++' if this flag is present when compiling C files only (i.e. no C++ files).

To avoid this warning, we can add the flag only to CXXFLAGS and not to CFLAGS.

---

Zulip thread

[rustbot]

r? @albertlarsan68

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[Zalathar]

It's possible that this flag is no longer needed at all (since some sources claim that libc++ should be the default anyway), but since my main goal here is to get rid of the warning while I'm building, I didn't do the extra work of fully tracking that down.

[Zalathar]

The warning started appearing after I accepted a system update of Command Line Tools for Xcode from version 14.3 to 15.0.

Presumably the older compiler was accepting the argument and silently doing nothing with it, whereas the newer compiler now correctly identifies that the flag has no effect on C code. But since I no longer have the older compiler on my system, I can't test that hypothesis directly.

[albertlarsan68]

@rustbot ping macos
Does this fix the problem / does anything break ?

[rustbot]

Hey MacOS Group! This issue or PR could use some MacOS-specific guidance. Could one
of you weigh in? Thanks <3

cc @hkratz @inflation @nvzqz @shepmaster @thomcc

[thomcc]

What's a configuration where I can reproduce the issue?

[Zalathar]

This is my config.toml:

profile = "codegen"
changelog-seen = 2

[build]
profiler = true
low-priority = true

[rust]
llvm-tools = true
deny-warnings = false

I'm on macOS Ventura 13.5.2, using Apple's Command Line Tools for Xcode version 15.0:

$ clang --version
Apple clang version 15.0.0 (clang-1500.0.40.1)
Target: arm64-apple-darwin22.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

If I just do x build library as normal, I see plenty of warnings in my terminal about -stdlib=libc++ being unused.

When I started investigating the warnings, I switched to x build library/profiler_builtins --stage=0, as it's very fast and has very little output other than the warnings.

[Zalathar]

This is what I see before applying this PR:

$ x build library/profiler_builtins --stage=0
Building bootstrap
    Finished dev [unoptimized] target(s) in 0.03s
Building stage0 library artifacts {profiler_builtins} (aarch64-apple-darwin)
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
warning: clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]
    Finished release [optimized] target(s) in 0.07s
Build completed successfully in 0:00:00

Each warning corresponds to one of the .c files listed in library/profiler_builtins/build.rs.

(The number of warnings doesn't quite match the number of files because one of the listed files is a .cpp file that doesn't warn, and one of the .c files doesn't actually exist. But if I comment out one of the other C files in the list, the number of warnings decreases by one.)

[Zalathar]

After applying this PR:

$ x build library/profiler_builtins --stage=0
Building bootstrap
    Finished dev [unoptimized] target(s) in 0.03s
Building stage0 library artifacts {profiler_builtins} (aarch64-apple-darwin)
    Finished release [optimized] target(s) in 0.07s
Build completed successfully in 0:00:00

[inflation]

Is there any difference if you have gcc and libstdc++ in search path?

[Zalathar]

I don't have GCC installed on my system, so I don't know what the difference would be.

[Zalathar]

This is what happens when I ask clang to compile simple C and C++ files, with and without the flag:

$ clang --version
Apple clang version 15.0.0 (clang-1500.0.40.1)
Target: arm64-apple-darwin22.6.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

$ echo 'int main(int argc, char** argv) { return 0; }' > foo.cpp

$ cp foo.cpp bar.c

$ clang foo.cpp

$ clang -stdlib=libc++ foo.cpp

$ clang bar.c

$ clang -stdlib=libc++ bar.c
clang: warning: argument unused during compilation: '-stdlib=libc++' [-Wunused-command-line-argument]

[bors]

☔ The latest upstream changes (presumably #116196) made this pull request unmergeable. Please resolve the merge conflicts.

[albertlarsan68]

Thanks for the PR!
@bors r+

[bors]

📌 Commit 9eb70d6 has been approved by albertlarsan68

It is now in the queue for this repository.

[bors]

⌛ Testing commit 9eb70d6 with merge 04817ff...