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, 5 deletions

diff --git a/src/libstd/rt.rs b/src/libstd/rt.rs
index 83091c72c0d..6eee4ee9bbe 100644
--- a/src/libstd/rt.rs
+++ b/src/libstd/rt.rs
@@ -25,12 +25,10 @@
 
 
 // Reexport some of our utilities which are expected by other crates.
-pub use sys_common::unwind::{begin_unwind, begin_unwind_fmt};
+pub use panicking::{begin_panic, begin_panic_fmt};
 
-// Rust runtime's startup objects depend on these symbols, so they must be public.
-// Since sys_common isn't public, we have to re-export them here.
-#[cfg(all(target_os="windows", target_arch = "x86", target_env="gnu"))]
-pub use sys_common::unwind::imp::eh_frame_registry::*;
+#[cfg(stage0)]
+pub use panicking::begin_panic as begin_unwind;
 
 #[cfg(not(test))]
 #[lang = "start"]