[macOS] CoreHaptics framework is not weakly linked, causing a hard dependency on CoreHaptics

[bigfarts]

On macOS <Catalina, CoreHaptics.framework is not present so running a rust-sdl2 program linked against CoreHaptics doesn't work. It looks like usually CoreHaptics.framework is weakly linked in the C build of SDL, but I don't know how to do this in Rust.

This only occurs with statically linked SDL, I think with dynamic linking it's probably fine.

[VyrCossont]

I can't even build a program with statically linked SDL since #1219 was merged, let alone run one, even on macOS 13 (Ventura):

error: linking with `cc` failed: exit status: 1
  |
  = note: "cc" "-arch" "x86_64" "-m64" "/var/folders/nx/2bm5r6ld5fs1qx39r9f45_f00000gn/T/rustcvaL55t/symbols.o" [long list of .rlib files] "-framework" "DiskArbitration" "-framework" "IOKit" "-framework" "Foundation" "-framework" "IOKit" "-framework" "CoreFoundation" "-framework" "IOKit" "-framework" "CoreFoundation" "-framework" "AppKit" "-framework" "Foundation" "-framework" "CoreBluetooth" "-framework" "AppKit" "-framework" "QuartzCore" "-framework" "Foundation" "-lobjc" "-framework" "CoreGraphics" "-lSystem" "-framework" "Security" "-framework" "CoreFoundation" "-framework" "Cocoa" "-framework" "IOKit" "-framework" "Carbon" "-framework" "ForceFeedback" "-framework" "GameController" "-framework" "CoreVideo" "-framework" "CoreAudio" "-framework" "AudioToolbox" "-framework" "Metal" "-liconv" "-lSystem" "-lresolv" "-lc" "-lm" "-liconv" "-L" "/Users/user/.rustup/toolchains/stable-x86_64-apple-darwin/lib/rustlib/x86_64-apple-darwin/lib" "-o" "/Users/user/Projects/Foo/rust/foo/target/release/deps/foo-b3f9e61ac6a009a5" "-Wl,-dead_strip" "-nodefaultlibs"
  = note: Undefined symbols for architecture x86_64:
            "_CHHapticDynamicParameterIDHapticIntensityControl", referenced from:
                -[SDL_RumbleMotor setIntensity:] in libsdl2_sys-fab2ce44955feef8.rlib(SDL_mfijoystick.m.o)
            "_CHHapticEventParameterIDHapticIntensity", referenced from:
                -[SDL_RumbleMotor setIntensity:] in libsdl2_sys-fab2ce44955feef8.rlib(SDL_mfijoystick.m.o)
            "_CHHapticEventTypeHapticContinuous", referenced from:
                -[SDL_RumbleMotor setIntensity:] in libsdl2_sys-fab2ce44955feef8.rlib(SDL_mfijoystick.m.o)
            "_OBJC_CLASS_$_CHHapticDynamicParameter", referenced from:
                objc-class-ref in libsdl2_sys-fab2ce44955feef8.rlib(SDL_mfijoystick.m.o)
            "_OBJC_CLASS_$_CHHapticEvent", referenced from:
                objc-class-ref in libsdl2_sys-fab2ce44955feef8.rlib(SDL_mfijoystick.m.o)
            "_OBJC_CLASS_$_CHHapticEventParameter", referenced from:
                objc-class-ref in libsdl2_sys-fab2ce44955feef8.rlib(SDL_mfijoystick.m.o)
            "_OBJC_CLASS_$_CHHapticPattern", referenced from:
                objc-class-ref in libsdl2_sys-fab2ce44955feef8.rlib(SDL_mfijoystick.m.o)
          ld: symbol(s) not found for architecture x86_64
          clang: error: linker command failed with exit code 1 (use -v to see invocation)
          

error: could not compile `foo` due to previous error

Additionally, Core Haptics isn't even a native macOS API, it's a Catalyst API, and probably won't work outside of Catalyst apps. Is there some way we could make linking against it conditional to the platform (iOS, iPadOS, Catalyst)?

[VyrCossont]

Weak framework linker flags as in https://users.rust-lang.org/t/how-to-use-weak-linking-on-macos-link-with-weak-framework/50732 don't seem to be supported. Weak linking as a compiler feature is tracked in rust-lang/rust#29603 but probably won't happen any time soon given that it's now apparently marked "perma-unstable"; I'm not sure if it's relevant to this case anyway.

There's a workaround described at https://internals.rust-lang.org/t/support-for-macos-weak-linking/13796/5 using .cargo/config.toml that got my project unblocked, but it isn't usable at the sdl2 library level:

[target.x86_64-apple-darwin]
rustflags = ["-C", "link-args=-weak_framework CoreHaptics"]