ARM/AArch64 cross-compilation targets aren't respected in an MSVC environment on Windows.

[just-harry]

In a fully set-up MSVC environment (e.g. an environment set-up by Visual Studio's vcvarsall.bat) with a target of AArch64 ($Env:VSCMD_ARG_TGT_ARCH == arm64) or ARM ($Env:VSCMD_ARG_TGT_ARCH == arm), LDC will set-up an MSVC environment for x86-64 or x86, even if the -mtriple supplied to LDC is aarch64-windows or arm-windows (for example).

This results in the wrong libraries being supplied to the linker, and C files being preprocessed for the wrong target when cl is used for preprocessing.

This seems to be because setupMsvcEnvironmentImpl expects only x64 or x86 for the VSCMD_ARG_TGT_ARCH environment variable:

ldc/driver/tool.cpp

Lines 295 to 300 in b75fc4c

	// Assume a fully set up environment (e.g., VS native tools command prompt).
	// Skip the MSVC setup unless the environment is set up for a different
	// target architecture.
	const auto tgtArch = env::get("VSCMD_ARG_TGT_ARCH"); // VS 2017+
	if (tgtArch.empty() || tgtArch == (x64 ? "x64" : "x86"))
	return true;

[just-harry]

A proposed fix: #4625

[kinke]

#4625 fixed this partially - an already set-up MSVC environment for ARM targets should work now, but setting it up manually would require more work, e.g., in function https://github.com/dlang/dmd/blob/748fab1e02b9f9e8980833b79e9448937bfecfab/compiler/src/dmd/vsoptions.d#L380.