Cargo shouldn't set /WX (-Werror for MSVC linker) outside of CI

[tbu-]

cargo/build.rs

Lines 99 to 100 in 8bcecfe

	// Turn linker warnings into errors.
	println!("cargo:rustc-link-arg-bin=cargo=/WX");

Setting /WX might make versions of Cargo harder to build in the future. If a new version of MSVC gets released that throws a warning here, users wanting to build an old version of Cargo will need to patch it to even make them build. See https://embeddedartistry.com/blog/2017/05/22/werror-is-not-your-friend/ ("-Werror Introduces a Toolchain Version Dependency"). It's better to only enable /WX in CI, this way you can always change it, but don't leave users who want to compile Cargo with errors.

(from #13131 (comment)).

[ChrisDenton]

Linker warnings (unlike compiler warnings) are very rare and almost always indicate an actual problem with the build (e.g. arguments being ignored). This is as relevant for local builds as for CI. Note that /WX:NO can be used to override it if you really do want to ignore warnings.

The better solution here would be for Rust to show linker warnings instead of hiding them.

[ChrisDenton]

That first sentence should more accurately be "linker warnings that are not immediately followed by a fatal error are very rare".