[Windows] [CI] Use Rtools to test pre-computed bindings

[Ilia-Kosenkov]

This is a reminder to update CI workflow

libR-sys/.github/workflows/test.yml

Lines 264 to 266 in ccdf5d5

	if ($env:RUST_TARGETS -like "*x86_64*") {
	<# Amend rtools libgcc_eh.a #>
	cp C:\rtools40\mingw64\lib\gcc\x86_64-w64-mingw32\8.3.0\libgcc.a C:\rtools40\mingw64\lib\gcc\x86_64-w64-mingw32\8.3.0\libgcc_eh.a

in light of the recent merging of this PR r-windows/rtools-packages#186.

The problem on Windows side is that MSYS2 (used for generating bindings), Rtools (custom MSYS2 build) and mingw toolchains in rust's *pc-windows-gnu ship different versions of gcc and friends. This creates issues that can be resolved using hacks (see code snippet above).
In particular, in PR r-windows/rtools-packages#186 we suggested implementing a hack that was present in 32-bit version of Rtools, but not in 64-bit version.
As soon as updated rtools gets released (I believe it will take significant time), we can update CI here. I hope with Rtools patched we may be able to move additional workflows to Rtools only, avoiding both MSYS2 and rust's own mingw tools.

[clauswilke]

Could you remind us what exactly will have to be changed, so this info doesn't get lost between now and when the next Rtools is available?

[Ilia-Kosenkov]

It is easy: in the linked code snippet this line should be removed:

cp C:\rtools40\mingw64\lib\gcc\x86_64-w64-mingw32\8.3.0\libgcc.a C:\rtools40\mingw64\lib\gcc\x86_64-w64-mingw32\8.3.0\libgcc_eh.a  

Right now it creates a fake copy of libgcc_eh.a, because Rtools ships none for x64 version (but this lib is required for compilation). With the new Rtools release, this file will be a part of Rtools so there is no need to manually create a copy.