build.rs: Use cc-rs's built-in logic for warnings-as-errors.

[briansmith]

Take a step towards supporting Clang as the compiler for -msvc targets.

[briansmith]

@awakecoding @Alovchin91 Please review this. This is one of many changes I'm making to officially support Clang for -msvc targets.

[awakecoding]

It looks good to me, I guess cc automatically detects the compiler and sets the correct flags. Is there a way to tell cc to use clang/MSVC and not the regular MSVC compiler? And if so, does it use clang-cl (same flags as cl.exe) or clang.exe (same flags as clang on all platforms)? Did you have a way to check which compiler was picked up by cc in the end?

[codecov]

Codecov Report

Merging #1343 (3adb0ca) into main (56561bc) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1343      +/-   ##
==========================================
- Coverage   92.83%   92.82%   -0.01%     
==========================================
  Files         117      117              
  Lines       17826    17826              
  Branches      195      195              
==========================================
- Hits        16548    16547       -1     
- Misses       1244     1245       +1     
  Partials       34       34              

Impacted Files	Coverage Δ
src/aead/less_safe_key.rs	95.68% <0.00%> (-0.87%)	⬇️

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[briansmith]

It looks good to me, I guess cc automatically detects the compiler and sets the correct flags. Is there a way to tell cc to use clang/MSVC and not the regular MSVC compiler?

$ which clang
/c/apps-x86/Microsoft Visual Studio/2019/BuildTools/VC/Tools/Llvm/bin/clang
$ clang --version
clang version 12.0.0
Target: i686-pc-windows-msvc
Thread model: posix
InstalledDir: C:\apps-x86\Microsoft Visual Studio\2019\BuildTools\VC\Tools\Llvm\bin
$ CC=clang cargo -vv test --target=x86_64-pc-windows-msvc
$ CC=clang cargo -vv test --release --target=x86_64-pc-windows-msvc

And if so, does it use clang-cl (same flags as cl.exe) or clang.exe (same flags as clang on all platforms)?

If you use CC=clang it uses traditional clang flags. If you use CC=clang-cl then it uses MSVC flags. This will become apparent in the next PR in this series.

Did you have a way to check which compiler was picked up by cc in the end?

cargo -vv will show you the compiler invocations and cc-rs's logging.

Surprisingly, on Windows I couldn't get TARGET_CC to be respected; I had to use CC instead. :(

[briansmith]

The codecov job's failure is caused by there being less covered code, not by there be any new uncovered code, so it's benign (good even).