iOS (and other Apple platforms): System.Private.CoreLib.dll has P/Invokes to a libhostpolicy library that isn't shipped

[rolfbjarne]

Description

The System.Private.CoreLib.dll has P/Invokes to libhostpolicy:

$ monodis microsoft.netcore.app.runtime.ios-x64/5.0.0-preview.7.20317.2/runtimes/ios-x64/native/System.Private.CoreLib.dll | grep libhostpolicy
.module extern 'libhostpolicy'
    .method assembly static hidebysig pinvokeimpl ("libhostpolicy" as "corehost_resolve_component_dependencies" autochar cdecl )
    .method assembly static hidebysig pinvokeimpl ("libhostpolicy" as "corehost_set_error_writer" autochar cdecl )

But there's no libhostpolicy library in the Microsoft.NETCore.Runtime.ios-x64 nuget (nor are these native functions provided by any other library that I can find).

Other information

It's highly preferable to not have any P/Invokes to native functions that don't exist, because it allows the AOT compiler to generate more efficient code. It also makes other performance improvements possible.

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[AaronRobinsonMSFT]

/cc @vitek-karas

[marek-safar]

@rolfbjarne this should not matter a lot as they will be removed by illinker

@CoffeeFlux would be nice to clean this up

[rolfbjarne]

@rolfbjarne this should not matter a lot as they will be removed by illinker

The linker won't always be enabled, in particular for simulator builds.

There's another complication here, because sometimes we have to ask the native linker to keep the native symbols P/Invokes point to (because otherwise the native linker may remove them, if those symbols come from static libraries). The problem is that if there are P/Invokes to native functions that don't exist, we can't ask the native linker to keep them, because then the native linker will fail. This means we'll have to add code to inspect all the static libraries we link with, compute a list of all the native symbols, and match with the symbols P/Invokes point to, to avoid asking the native linker to keep native symbols that don't exist.