1.49 seems to have bumped required macOS version

[pkgw]

After the release at 1.49, we at conda-forge are seeing some new build failures on macOS builds due to a missing _linkat symbol:

error: linking with `x86_64-apple-darwin13.4.0-clang` failed: exit code: 1
  |
  = note: "x86_64-apple-darwin13.4.0-clang" "-m64" "-arch" "x86_64" [... long command line ...]
  = note: Undefined symbols for architecture x86_64:
            "_linkat", referenced from:
                std::sys::unix::fs::link::had1f447ad1437b53 in libstd-518979da66993550.rlib(std-518979da66993550.std.byn0lf9c-cgu.0.rcgu.o)
          ld: symbol(s) not found for architecture x86_64
          clang-11: error: linker command failed with exit code 1 (use -v to see invocation)

linkat is only availabe starting in macOS 10.10, while on conda-forge our targeted minimum version is 10.9. It looks like this may have been introduced unintentionally in #78026.

I'm filing this as a regression because I don't see any documentation of a bump in the minimum macOS version for 1.49.

CC @isuruf @bollwyvl conda-forge/rust-feedstock#73.

[jyn514]

#78026 (comment)

What if an OS doesn't have support for this?

Good question. linkat is in POSIX.1-2008 and pretty widely supported, but it appears at least VxWorks and Redox don't support it, so I've now added a patch special-casing those two platforms.

Is there a way to tell if we're compiling on MacOS 10.9 or lower and special case it if so?

[ehuss]

Is there a way to tell if we're compiling on MacOS 10.9 or lower and special case it if so?

I think the weak macro is normally used for runtime detection of a symbol.

[pkgw]

@jyn514 In the interest of generating portable binaries, one could make the argument for not using the API even if one is compiling on a newer macOS. For precompiled stdlib in particular, I'd guess that somewhere there's a specification for the minimum supported OS version?

[Mark-Simulacrum]

We can't really compile on old macos feasibly in CI, but it should be fairly straightforward to use linkat via the weak macro (which performs runtime detection of availability). I think we'll likely want to land using weak on nightly for testing, and then follow up with a beta backport. We can consider a stable backport as well, since this seems like a major problem for (all?) users targeting older macs.

[apiraino]

Assigning P-high as discussed as part of the Prioritization Working Group procedure and removing I-prioritize.

[Mark-Simulacrum]

@sunfishcode would you be up for filing a PR to fix this?