Auto merge of #32900 - alexcrichton:panic2abort, r=nikomatsakis

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).

Closes #32837

2 files changed, 12 insertions, 1 deletions

diff --git a/src/libsyntax/ext/build.rs b/src/libsyntax/ext/build.rs
index 67bce440d4d..7958162986c 100644
--- a/src/libsyntax/ext/build.rs
+++ b/src/libsyntax/ext/build.rs
@@ -755,7 +755,7 @@ impl<'a> AstBuilder for ExtCtxt<'a> {
         let expr_file_line_ptr = self.expr_addr_of(span, expr_file_line_tuple);
         self.expr_call_global(
             span,
-            self.std_path(&["rt", "begin_unwind"]),
+            self.std_path(&["rt", "begin_panic"]),
             vec!(
                 self.expr_str(span, msg),
                 expr_file_line_ptr))
diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index cf797b85f54..acef98f2afc 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -141,6 +141,8 @@ declare_features! (
     (active, simd_ffi, "1.0.0", Some(27731)),
     (active, start, "1.0.0", Some(29633)),
     (active, structural_match, "1.8.0", Some(31434)),
+    (active, panic_runtime, "1.10.0", Some(32837)),
+    (active, needs_panic_runtime, "1.10.0", Some(32837)),
 
     // OIBIT specific features
     (active, optin_builtin_traits, "1.0.0", Some(13231)),
@@ -438,6 +440,15 @@ pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeGat
                                        attribute is an experimental \
                                        feature",
                                       cfg_fn!(needs_allocator))),
+    ("panic_runtime", Whitelisted, Gated("panic_runtime",
+                                         "the `#[panic_runtime]` attribute is \
+                                          an experimental feature",
+                                         cfg_fn!(panic_runtime))),
+    ("needs_panic_runtime", Whitelisted, Gated("needs_panic_runtime",
+                                               "the `#[needs_panic_runtime]` \
+                                                attribute is an experimental \
+                                                feature",
+                                               cfg_fn!(needs_panic_runtime))),
     ("rustc_variance", Normal, Gated("rustc_attrs",
                                      "the `#[rustc_variance]` attribute \
                                       is just used for rustc unit tests \