rustc: Implement custom panic runtimes

This commit is an implementation of [RFC 1513] which allows applications to
alter the behavior of panics at compile time. A new compiler flag, `-C panic`,
is added and accepts the values `unwind` or `panic`, with the default being
`unwind`. This model affects how code is generated for the local crate, skipping
generation of landing pads with `-C panic=abort`.

[RFC 1513]: https://github.com/rust-lang/rfcs/blob/master/text/1513-less-unwinding.md

Panic implementations are then provided by crates tagged with
`#![panic_runtime]` and lazily required by crates with
`#![needs_panic_runtime]`. The panic strategy (`-C panic` value) of the panic
runtime must match the final product, and if the panic strategy is not `abort`
then the entire DAG must have the same panic strategy.

With the `-C panic=abort` strategy, users can expect a stable method to disable
generation of landing pads, improving optimization in niche scenarios,
decreasing compile time, and decreasing output binary size. With the `-C
panic=unwind` strategy users can expect the existing ability to isolate failure
in Rust code from the outside world.

Organizationally, this commit dismantles the `sys_common::unwind` module in
favor of some bits moving part of it to `libpanic_unwind` and the rest into the
`panicking` module in libstd. The custom panic runtime support is pretty similar
to the custom allocator support with the only major difference being how the
panic runtime is injected (takes the `-C panic` flag into account).

1 files changed, 3 insertions, 4 deletions

diff --git a/src/liballoc_system/lib.rs b/src/liballoc_system/lib.rs
index ca4c9bfd954..a22299c5e1a 100644
--- a/src/liballoc_system/lib.rs
+++ b/src/liballoc_system/lib.rs
@@ -18,10 +18,8 @@
                       form or name",
             issue = "27783")]
 #![feature(allocator)]
-#![feature(libc)]
 #![feature(staged_api)]
-
-extern crate libc;
+#![cfg_attr(unix, feature(libc))]
 
 // The minimum alignment guaranteed by the architecture. This value is used to
 // add fast paths for low alignment values. In practice, the alignment is a
@@ -72,9 +70,10 @@ pub extern "C" fn __rust_usable_size(size: usize, align: usize) -> usize {
 
 #[cfg(unix)]
 mod imp {
+    extern crate libc;
+
     use core::cmp;
     use core::ptr;
-    use libc;
     use MIN_ALIGN;
 
     pub unsafe fn allocate(size: usize, align: usize) -> *mut u8 {