Spurious failures on AppVeyor due to ar.exe errors

[alexcrichton]

I've previously been classifying these errors under #40240 but now I'm thinking that's incorrect, I suspect these are totally separate errors.

https://ci.appveyor.com/project/rust-lang/rust/build/1.0.2397

[ 13%] Linking CXX static library ../../libLLVMMCDisassembler.a
C:/projects/rust/mingw64/bin/ar.exe: ../../libLLVMMCDisassembler.a: Permission denied
make[2]: *** [lib/MC/MCDisassembler/CMakeFiles/LLVMMCDisassembler.dir/build.make:205: lib/libLLVMMCDisassembler.a] Error 1
make[2]: *** Deleting file 'lib/libLLVMMCDisassembler.a'
make[1]: *** [CMakeFiles/Makefile2:2539: lib/MC/MCDisassembler/CMakeFiles/LLVMMCDisassembler.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
[ 13%] Built target LLVMMCParser
make: *** [Makefile:150: all] Error 2
thread 'main' panicked at '
command did not execute successfully, got: exit code: 2

[alexcrichton]

The first hit on Google is quite ominous. My preference for an attempt to fix this would be to switch to ninja as it's likely just flat out more reliable than makefiles on MinGW

[alexcrichton]

Oh also we're going to want to switch the MSVC build to Ninja regardless, so we'll have it available anyway eventually on the bots.

[mati865]

I've met it few times with MSYS2 and (almost?) always it was caused by CL RF line endings or too long paths or file names.
When compiling Rust from git I found there are files that windows explorer cannot remove since they are too long. Unfortunately I don't have list as I used MSYS2 rm -rf * to clean those files.