std: Mark allocation functions as nounwind

This commit flags all allocation-related functions in liballoc as "this can't
unwind" which should largely resolve the size-related issues found on #42808.
The documentation on the trait was updated with such a restriction (they can't
panic) as well as some other words about the relative instability about
implementing a bullet-proof allocator.

Closes #42808

1 files changed, 32 insertions, 0 deletions

diff --git a/src/test/codegen/dealloc-no-unwind.rs b/src/test/codegen/dealloc-no-unwind.rs
new file mode 100644
index 00000000000..551b66e103a
--- /dev/null
+++ b/src/test/codegen/dealloc-no-unwind.rs
@@ -0,0 +1,32 @@
+// Copyright 2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+//
+// no-system-llvm
+// compile-flags: -O
+
+#![crate_type="lib"]
+
+struct A;
+
+impl Drop for A {
+    fn drop(&mut self) {
+        extern { fn foo(); }
+        unsafe { foo(); }
+    }
+}
+
+#[no_mangle]
+pub fn a(a: Box<i32>) {
+    // CHECK-LABEL: define void @a
+    // CHECK: call void @__rust_dealloc
+    // CHECK-NEXT: call void @foo
+    let _a = A;
+    drop(a);
+}