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, 6 deletions

diff --git a/src/libstd/panic.rs b/src/libstd/panic.rs
index bbd89af01a7..9a195d93afd 100644
--- a/src/libstd/panic.rs
+++ b/src/libstd/panic.rs
@@ -16,10 +16,10 @@ use any::Any;
 use boxed::Box;
 use cell::UnsafeCell;
 use ops::{Deref, DerefMut};
+use panicking;
 use ptr::{Unique, Shared};
 use rc::Rc;
 use sync::{Arc, Mutex, RwLock};
-use sys_common::unwind;
 use thread::Result;
 
 #[unstable(feature = "panic_handler", issue = "30449")]
@@ -383,12 +383,9 @@ impl<R, F: FnOnce() -> R> FnOnce<()> for AssertRecoverSafe<F> {
 /// ```
 #[stable(feature = "catch_unwind", since = "1.9.0")]
 pub fn catch_unwind<F: FnOnce() -> R + UnwindSafe, R>(f: F) -> Result<R> {
-    let mut result = None;
     unsafe {
-        let result = &mut result;
-        unwind::try(move || *result = Some(f()))?
+        panicking::try(f)
     }
-    Ok(result.unwrap())
 }
 
 /// Deprecated, renamed to `catch_unwind`
@@ -425,7 +422,7 @@ pub fn recover<F: FnOnce() -> R + UnwindSafe, R>(f: F) -> Result<R> {
 /// ```
 #[stable(feature = "resume_unwind", since = "1.9.0")]
 pub fn resume_unwind(payload: Box<Any + Send>) -> ! {
-    unwind::rust_panic(payload)
+    panicking::rust_panic(payload)
 }
 
 /// Deprecated, use resume_unwind instead