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, 11 insertions, 0 deletions

diff --git a/src/libstd/lib.rs b/src/libstd/lib.rs
index d4b40b844fc..8f41bdf39e9 100644
--- a/src/libstd/lib.rs
+++ b/src/libstd/lib.rs
@@ -245,6 +245,7 @@
 #![feature(on_unimplemented)]
 #![feature(oom)]
 #![feature(optin_builtin_traits)]
+#![feature(panic_unwind)]
 #![feature(placement_in_syntax)]
 #![feature(rand)]
 #![feature(raw)]
@@ -283,6 +284,13 @@
 #![allow(unused_features)] // std may use features in a platform-specific way
 #![cfg_attr(not(stage0), deny(warnings))]
 
+// FIXME(stage0): after a snapshot, move needs_panic_runtime up above and remove
+//                this `extern crate` declaration and feature(panic_unwind)
+#![cfg_attr(not(stage0), needs_panic_runtime)]
+#![cfg_attr(not(stage0), feature(needs_panic_runtime))]
+#[cfg(stage0)]
+extern crate panic_unwind as __please_just_link_me_dont_reference_me;
+
 #[cfg(test)] extern crate test;
 
 // We want to reexport a few macros from core but libcore has already been
@@ -301,6 +309,9 @@ extern crate alloc;
 extern crate rustc_unicode;
 extern crate libc;
 
+// We always need an unwinder currently for backtraces
+extern crate unwind;
+
 #[cfg(stage0)]
 extern crate alloc_system;