[rust] Consider dynamically linkage to system libs. #9755
Description
Description
Rust choose to statically link to libgcc_eh and libpthread when no -C prefer-dynamic is specified:
https://github.com/rust-lang/rust/blob/1ddd4e6d7ed446934abd428a08e18535faef5e03/compiler/rustc_target/src/spec/windows_gnu_base.rs#L50-L58
It is because that some software end users may not have msys2 or other MinGW64 runtimes installed, and it may be complicated for developers to bundle the MinGW64 runtime. However, when we are using msys2 for building and distribution, we may strongly assume that the users have msys2 installed, otherwise they couldn't use pacman. If the developer would like to distribute without MinGW64 runtime, he/she should use -C target-feature=+crt-static.
Pros
Smaller binary size: We could simply link with the installed MinGW64 runtime, and it will help reduce some of the binary size. The distributed binary will also benifit from newer MinGW64 runtime without recompiling.
No sidebacks when using crt-static: MinGW64 doesn't statically link to msvcrt due to license restrictions. Therefore the modification here won't make difference when using crt-static. It should perform as usual, and simply bundle the MinGW64 runtime.
Future exception safety: Rust doesn't guarantee cross-language exception safety. However, it is in design. Then it needs the rust binary to dynamically link to the MinGW64 runtime.
Behavior consistence: Most other targets only link to the unwind runtime statically when feature crt-static is on.
Cons
Different from upstream: The behavior should be properly documented, as it is different from the upstream one.
MISC breaking: Some crates may assume the offical behavior, although I haven't found one.
Other notes
Not only the windows_gnu_base.rs should be modified, library/unwind/src/lib.rs should also be modified to properly deal with crt-static feature. Also, it should be investigated that where to link libpthread statically.