libcxxrt: add static lib to output

[ghost]

Description of changes

allow static linking libc++. libcxxrt.a is 220K when build for x64 linux, 198K x64 freebsd
tested with #305876

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 24.05 Release Notes (or backporting 23.05 and 23.11 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[ghost]

@ofborg eval

[rrbutani]

Apologies, what I said in #305876 wasn't quite right; stdenv.hostPlatform.extensions.library will yield .a when targeting a static platform:

nixpkgs/lib/systems/default.nix

Lines 126 to 134 in 65f3d71

	staticLibrary =
	/**/ if final.isWindows then ".lib"
	else ".a";
	library =
	/**/ if final.isStatic then final.extensions.staticLibrary
	else final.extensions.sharedLibrary;
	executable =
	/**/ if final.isWindows then ".exe"
	else "";

This means that nix-build --arg crossSystem '{ system = "x86_64-freebsd13"; useLLVM = true; isStatic = true; }' -A libcxxrt would¹ yield a libcxxrt that has lib/libcxxrt.a.

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Given that the staticlib is relatively small, providing it even when !hostPlatform.isStatic seems reasonable to me (especially since libc++.a isn't gated on isStatic) but I'll defer to @alyssais on this.

If we do want libcxxrt to expose both the shared and static versions of the library I think the above should become something like:

Suggested change

	cp lib/libcxxrt${stdenv.hostPlatform.extensions.library} $out/lib
	cp lib/libcxxrt.a $out/lib
	cp lib/libcxxrt${stdenv.hostPlatform.extensions.staticLibrary} $out/lib
	'' + lib.optionalString stdenv.hostPlatform.hasSharedLibraries ''
	cp lib/libcxxrt${stdenv.hostPlatform.extensions.sharedLibrary} $out/lib

Footnotes

1. currently this fails with an unrelated error while building libc; pkgsStatic.libcxxrt for x86_64-linux also fails because ld cannot produce lib/libcxxrt.so (shared object) with musl's crtbeginT.o ↩

[ghost]

this seems slightly overly pedantic to me tho i suppose what i have breaks in the windows case. but this is more work then i care to do so closing.

[rrbutani]

I'm sorry. I appreciate the work you're doing and did not mean to discourage you.

Agreed that the above isn't an important change; if it's okay with the libcxxrt package maintainer, I'm happy with this PR as is.