Commits on Dec 3, 2021

1. code-cov: generate dead functions with private/default linkage …

   As discovered in rust-lang#85461, the MSVC linker treats weak symbols slightly
   differently than unix-y linkers do. This causes link.exe to fail with
   LNK1227 "conflicting weak extern definition" where as other targets are
   able to link successfully.
   
   This changes the dead functions from being generated as weak/hidden to
   private/default which, as the LLVM reference says:
   
   > Global values with “private” linkage are only directly accessible by
   objects in the current module. In particular, linking code into a module
   with a private global value may cause the private to be renamed as
   necessary to avoid collisions. Because the symbol is private to the
   module, all references can be updated. This doesn’t show up in any
   symbol table in the object file.
   
   This fixes the conflicting weak symbols but doesn't address the reason
   *why* we have conflicting symbols for these dead functions. The test
   cases added in this commit contain a minimal repro of the fundamental
   issue which is that the logic used to decide what dead code functions
   should be codegen'd in the current CGU doesn't take into account that
   functions can be duplicated across multiple CGUs (for instance, in the
   case of `#[inline(always)]` functions).
   
   Fixing that is likely to be a more complex change (see
   rust-lang#85461 (comment)).
   
   Fixes rust-lang#85461

   

   wesleywiser committed Dec 3, 2021
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