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/librustc_metadata/decoder.rs b/src/librustc_metadata/decoder.rs
index dd5a643edc1..72fbbf80515 100644
--- a/src/librustc_metadata/decoder.rs
+++ b/src/librustc_metadata/decoder.rs
@@ -27,6 +27,7 @@ use rustc::hir::svh::Svh;
 use rustc::hir::map as hir_map;
 use rustc::util::nodemap::FnvHashMap;
 use rustc::hir;
+use rustc::session::config::PanicStrategy;
 
 use middle::cstore::{LOCAL_CRATE, FoundAst, InlinedItem, LinkagePreference};
 use middle::cstore::{DefLike, DlDef, DlField, DlImpl, tls};
@@ -1760,3 +1761,13 @@ pub fn def_path(cdata: Cmd, id: DefIndex) -> hir_map::DefPath {
     debug!("def_path(id={:?})", id);
     hir_map::DefPath::make(cdata.cnum, id, |parent| def_key(cdata, parent))
 }
+
+pub fn get_panic_strategy(data: &[u8]) -> PanicStrategy {
+    let crate_doc = rbml::Doc::new(data);
+    let strat_doc = reader::get_doc(crate_doc, tag_panic_strategy);
+    match reader::doc_as_u8(strat_doc) {
+        b'U' => PanicStrategy::Unwind,
+        b'A' => PanicStrategy::Abort,
+        b => panic!("unknown panic strategy in metadata: {}", b),
+    }
+}