Commits on Apr 29, 2024

1. MSVC: Add support for linking against the "spectre-mitigated" CRT …

   Issue Details:
   Since VS 2017, MSVC has shipped a set of "spectre-mitigated" CRT static libs: https://devblogs.microsoft.com/cppblog/spectre-mitigations-in-msvc/
   
   Typically these are used by opening a VS Command Prompt in "spectre mode" (https://docs.microsoft.com/en-us/cpp/build/building-on-the-command-line?view=msvc-170#vcvarsall-syntax) which then sets the `LIB` environment variable to point to the directory with the spectre-mitigated libs. However, since `cc` builds its own `LIB` environment variable, it uses the non-spectre-mitigated libs even when invoked from a VS Command Prompt in "spectre mode". This causes issues when trying to build a spectre-mitigated binary using Rust, as `rustc` uses `cc` for linking.
   
   Fix Details:
   When `cc` detects that the `VSCMD_ARG_VCVARS_SPECTRE` environment variable is set to `spectre` (either by being run from a VS Command Prompt in "spectre mode", or users may explicitly set this themselves), it will use the spectre-mitigated lib directory when building its `LIB` environment variable.

   

   dpaoliello committed Apr 29, 2024
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