overriding '/MT' with '/MD'

[steveklabnik]

Here's an excerpt from building rustc:

Building stage1 compiler artifacts (x86_64-pc-windows-msvc -> x86_64-pc-windows-msvc)
   Compiling graphviz v0.0.0 (file:///C:/Users/steve/src/rust/src/libgraphviz)
   Compiling log v0.0.0 (file:///C:/Users/steve/src/rust/src/liblog)
   Compiling arena v0.0.0 (file:///C:/Users/steve/src/rust/src/libarena)
   Compiling gcc v0.3.43
   Compiling rustc_platform_intrinsics v0.0.0 (file:///C:/Users/steve/src/rust/src/librustc_platform_intrinsics)
   Compiling rustc_bitflags v0.0.0 (file:///C:/Users/steve/src/rust/src/librustc_bitflags)
   Compiling serialize v0.0.0 (file:///C:/Users/steve/src/rust/src/libserialize)
   Compiling fmt_macros v0.0.0 (file:///C:/Users/steve/src/rust/src/libfmt_macros)
   Compiling libc v0.2.20
   Compiling filetime v0.1.10
   Compiling build_helper v0.1.0 (file:///C:/Users/steve/src/rust/src/build_helper)
   Compiling flate v0.0.0 (file:///C:/Users/steve/src/rust/src/libflate)
   Compiling rustdoc v0.0.0 (file:///C:/Users/steve/src/rust/src/librustdoc)
   Compiling rustc_llvm v0.0.0 (file:///C:/Users/steve/src/rust/src/librustc_llvm)
   Compiling syntax_pos v0.0.0 (file:///C:/Users/steve/src/rust/src/libsyntax_pos)
   Compiling rustc_data_structures v0.0.0 (file:///C:/Users/steve/src/rust/src/librustc_data_structures)
   Compiling rustc_errors v0.0.0 (file:///C:/Users/steve/src/rust/src/librustc_errors)
warning: cl : Command line warning D9025 : overriding '/MT' with '/MD'
warning: cl : Command line warning D9025 : overriding '/MD' with '/MT'
warning: cl : Command line warning D9025 : overriding '/MT' with '/MD'
warning: cl : Command line warning D9025 : overriding '/MD' with '/MT'
warning: cl : Command line warning D9025 : overriding '/MT' with '/MD'
warning: cl : Command line warning D9025 : overriding '/MD' with '/MT'
   Compiling syntax v0.0.0 (file:///C:/Users/steve/src/rust/src/libsyntax)

I don't know what these warnings are, and the build seems to work, but seems bad maybe?

[luser]

These switches select either the static or DLL version of the CRT. If you try to mix code that was compiled with different settings you'll usually get a linker error, so that must not be happening.

[retep998]

This is very likely due to #39837. Should be a simple matter of figuring out which compiler invocation is passing /MD and tracing it back into the build system and fixing the bit responsible.

cc @alexcrichton

[alexcrichton]

I believe the last time I checked this was because LLVM passes /MD unconditionally, we then configure it to pass /MT so it passes that later. We then ourselves also pass /MT ahead of things which ends up with:

• Our /MT is first
• LLVM's /MD is next, unsure how to remove this
• LLVM's /MT is last

The last one wins, which is what we care about, but it'd be great to fix this! I may also be wrong in this diagnosis, this is just from memory.

[liranringel]

@alexcrichton is right, llvm passed /MD.
I added the following lines to llvm-config/CMakeLists.txt, after LLVM_CFLAGS and LLVM_CXXFLAGS are set:

set(CompilerFlags
LLVM_CFLAGS
LLVM_CXXFLAGS
)
foreach(CompilerFlag ${CompilerFlags})
string(REPLACE "/MD" "/MT" ${CompilerFlag} "${${CompilerFlag}}")
endforeach()

That indeed resolved the issue, but it probably be better if we could achieve that by editing the cmake::Config in src/bootstrap/native.rs. I hope I'll have time to check that later today.

Edit: If I understand it correctly, the bootstrap project is not compiled with +static-crt, so here:
https://github.com/rust-lang/rust/blob/master/src/bootstrap/cc.rs#L77
it returns the flag /MD (see https://github.com/alexcrichton/gcc-rs/blob/master/src/lib.rs#L542), which is used later here: https://github.com/rust-lang/rust/blob/master/src/bootstrap/native.rs#L179
Perhaps gcc-rs should provide a mean to explicitly set the crt-static flag?

[retep998]

This should remain open until the version of gcc and cmake that Rust itself uses are updated.

[retep998]

This should remain open until #41959 lands.