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, 13 insertions, 0 deletions

diff --git a/src/libpanic_unwind/Cargo.toml b/src/libpanic_unwind/Cargo.toml
new file mode 100644
index 00000000000..27edecd6f96
--- /dev/null
+++ b/src/libpanic_unwind/Cargo.toml
@@ -0,0 +1,13 @@
+[package]
+authors = ["The Rust Project Developers"]
+name = "panic_unwind"
+version = "0.0.0"
+
+[lib]
+path = "lib.rs"
+
+[dependencies]
+alloc = { path = "../liballoc" }
+core = { path = "../libcore" }
+libc = { path = "../rustc/libc_shim" }
+unwind = { path = "../libunwind" }