Commits on Feb 28, 2018

1. rustc: Tweak funclet cleanups of ffi functions …

   This commit is targeted at addressing rust-lang#48251 by specifically fixing a case where
   a longjmp over Rust frames on MSVC runs cleanups, accidentally running the
   "abort the program" cleanup as well. Added in rust-lang#46833 `extern` ABI functions in
   Rust will abort the process if Rust panics, and currently this is modeled as a
   normal cleanup like all other destructors.
   
   Unfortunately it turns out that `longjmp` on MSVC is implemented with SEH, the
   same mechanism used to implement panics in Rust. This means that `longjmp` over
   Rust frames will run Rust cleanups (even though we don't necessarily want it
   to). Notably this means that if you `longjmp` over a Rust stack frame then that
   probably means you'll abort the program because one of the cleanups will abort
   the process.
   
   After some discussion on IRC it turns out that `longjmp` doesn't run cleanups
   for *caught* exceptions, it only runs cleanups for cleanup pads. Using this
   information this commit tweaks the codegen for an `extern` function to
   a catch-all clause for exceptions instead of a cleanup block. This catch-all is
   equivalent to the C++ code:
   
       try {
           foo();
       } catch (...) {
           bar();
       }
   
   and in fact our codegen here is designed to match exactly what clang emits for
   that C++ code!
   
   With this tweak a longjmp over Rust code will no longer abort the process. A
   longjmp will continue to "accidentally" run Rust cleanups (destructors) on MSVC.
   Other non-MSVC platforms will not rust destructors with a longjmp, so we'll
   probably still recommend "don't have destructors on the stack", but in any case
   this is a more surgical fix than rust-lang#48567 and should help us stick to standard
   personality functions a bit longer.

   

   alexcrichton committed Feb 28, 2018
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