Auto merge of #26569 - alexcrichton:msvc-llvm-update, r=brson

Now that LLVM has been updated, the only remaining roadblock to implementing
unwinding for MSVC is to fill out the runtime support in `std::rt::unwind::seh`.
This commit does precisely that, fixing up some other bits and pieces along the
way:

* The `seh` unwinding module now uses `RaiseException` to initiate a panic.
* The `rust_try.ll` file was rewritten for MSVC (as it's quite different) and is
  located at `rust_try_msvc_64.ll`, only included on MSVC builds for now.
* The personality function for all landing pads generated by LLVM is hard-wired
  to `__C_specific_handler` instead of the standard `rust_eh_personality` lang
  item. This is required to get LLVM to emit SEH unwinding information instead
  of DWARF unwinding information. This also means that on MSVC the
  `rust_eh_personality` function is entirely unused (but is defined as it's a
  lang item).

More details about how panicking works on SEH can be found in the
`rust_try_msvc_64.ll` or `seh.rs` files, but I'm always open to adding more
comments!

A key aspect of this PR is missing, however, which is that **unwinding is still
turned off by default for MSVC**. There is a [bug in llvm][llvm-bug] which
causes optimizations to inline enough landing pads that LLVM chokes. If the
compiler is optimized at `-O1` (where inlining isn't enabled) then it can
bootstrap with unwinding enabled, but when optimized at `-O2` (inlining is
enabled) then it hits a fatal LLVM error.

[llvm-bug]: https://llvm.org/bugs/show_bug.cgi?id=23884

2 files changed, 8 insertions, 57 deletions

diff --git a/src/test/auxiliary/lang-item-public.rs b/src/test/auxiliary/lang-item-public.rs
index d195bd7e77b..4b60a370187 100644
--- a/src/test/auxiliary/lang-item-public.rs
+++ b/src/test/auxiliary/lang-item-public.rs
@@ -8,15 +8,12 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![feature(no_std)]
+#![feature(no_std, core, libc)]
 #![no_std]
 #![feature(lang_items)]
 
-#[lang="sized"]
-pub trait Sized { }
-
-#[lang="panic"]
-fn panic(_: &(&'static str, &'static str, usize)) -> ! { loop {} }
+extern crate core;
+extern crate libc;
 
 #[lang = "stack_exhausted"]
 extern fn stack_exhausted() {}
@@ -24,26 +21,8 @@ extern fn stack_exhausted() {}
 #[lang = "eh_personality"]
 extern fn eh_personality() {}
 
-#[lang="copy"]
-pub trait Copy {
-    // Empty.
-}
-
-#[lang="rem"]
-pub trait Rem<RHS=Self> {
-    type Output = Self;
-    fn rem(self, rhs: RHS) -> Self::Output;
-}
-
-impl Rem for isize {
-    type Output = isize;
-
-    #[inline]
-    fn rem(self, other: isize) -> isize {
-        // if you use `self % other` here, as one would expect, you
-        // get back an error because of potential failure/overflow,
-        // which tries to invoke error fns that don't have the
-        // appropriate signatures anymore. So...just return 0.
-        0
-    }
+#[lang = "panic_fmt"]
+extern fn rust_begin_unwind(msg: core::fmt::Arguments, file: &'static str,
+                            line: u32) -> ! {
+    loop {}
 }
diff --git a/src/test/run-pass/lang-item-public.rs b/src/test/run-pass/lang-item-public.rs
index f5b9bd4fbaa..57a32ba599f 100644
--- a/src/test/run-pass/lang-item-public.rs
+++ b/src/test/run-pass/lang-item-public.rs
@@ -11,39 +11,11 @@
 // aux-build:lang-item-public.rs
 // ignore-android
 
-#![feature(lang_items, start, no_std)]
+#![feature(start, no_std)]
 #![no_std]
 
 extern crate lang_item_public as lang_lib;
 
-#[cfg(target_os = "linux")]
-#[link(name = "c")]
-extern {}
-
-#[cfg(target_os = "android")]
-#[link(name = "c")]
-extern {}
-
-#[cfg(target_os = "freebsd")]
-#[link(name = "execinfo")]
-extern {}
-
-#[cfg(target_os = "freebsd")]
-#[link(name = "c")]
-extern {}
-
-#[cfg(target_os = "dragonfly")]
-#[link(name = "c")]
-extern {}
-
-#[cfg(any(target_os = "bitrig", target_os = "openbsd"))]
-#[link(name = "c")]
-extern {}
-
-#[cfg(target_os = "macos")]
-#[link(name = "System")]
-extern {}
-
 #[start]
 fn main(_: isize, _: *const *const u8) -> isize {
     1_isize % 1_isize