Support for MSVC -MTd and -MDd

[crumblingstatue]

The CMake documentation lists additional debug variants
for these MSVC flags, but as far as I can tell, the cc crate doesn't support the debug variants.

[retep998]

It doesn't support it because the rest of the rust ecosystem doesn't either. Fixing this would require a much more coordinated effort across multiple repos, and still would have an ugly transition period.

[NobodyXu]

Yeah I tried it and have to revert it, as it breaks rust compilation

[ChrisDenton]

The core issue is that rustc doesn't support this so there's no way for cc to know if this was wanted. cc could have a function for overriding the CRT libraries if someone really knows what they're doing but it is a bit of a footgun because it's super important that everything uses the same C runtime libraries.

To recap, this is the current state of affairs:

C/C++	rustc	Native libs
/MT	+crt-static	libcmt, libvcruntime, libucrt
/MTd	❌	libcmtd, libvcruntimed, libucrtd
/MD	-crt-static	msvcrt, vcruntime, ucrt
/MDd	❌	msvcrtd, vcruntimed, ucrtd

So compiling using rustc only has two options. You can override Rust's choices by using a linker argument. E.g. /NODEFAULTLIB:msvcrt /DEFAULTLIB:msvcrtd to use the debug libraries. But you need some way to coordinate that choice with everyone and everything else.

[NobodyXu]

So compiling using rustc only has two options. You can override Rust's choices by using a linker argument. E.g. /NODEFAULTLIB:msvcrt /DEFAULTLIB:msvcrtd to use the debug libraries.

If it is possible to override using a linker argument, then maybe we can add a new flag/env to use the debug version of msvc runtime?

[retep998]

Related issue: rust-lang/rust#39016

[ChrisDenton]

An environment variable might work to keep it consistent across the whole build.

We also need to keep in mind a future where rustc hopefully gains support for the debug CRTs. Then we'd just need to query it the same way we do crt-static.

[crumblingstatue]

Thank you for the information everyone!
I guess in my case I'll just have to force my CMake C++ dependency to build without debug support.

[MeFisto94]

Note: With the following contents in .cargo/config.toml and build.rs one can already opt into using MTd/MDd:

[build]
rustflags = ["-C", "target-feature=+crt-static"]

[env]
CFLAGS = "/MTd"
CXXFLAGS = "/MTd"

// Don't link the default CRT
    println!("cargo::rustc-link-arg=/nodefaultlib:msvcrt");
    println!("cargo::rustc-link-arg=/nodefaultlib:libcmtd");
    println!("cargo::rustc-link-arg=/nodefaultlib:libcmt");
    // Link the debug CRT instead
    println!("cargo::rustc-link-arg=/defaultlib:msvcrtd");

This will, however, issue a warning from MSVC because cc already passed /MT and /MTd takes precedence.

I wonder if it would make sense to emit a warning in cc, though, when .debug(true) is passed (though technically this would probably affect far too many builds even when using the debug MSCRT may not even be desired)

[NobodyXu]

Hmm yeah we could do that, but isn't there already warning from msvc?

Sorry maybe I'm missing something