auto merge of #12419 : huonw/rust/compiler-unsafe, r=alexcrichton

Previously an `unsafe` block created by the compiler (like those in the
formatting macros) would be "ignored" if surrounded by `unsafe`, that
is, the internal unsafety would be being legitimised by the external
block:

    unsafe { println!("...") } =(expansion)=> unsafe { ... unsafe { ... } }

And the code in the inner block would be using the outer block, making
it considered used (and the inner one considered unused).

This patch forces the compiler to create a new unsafe context for
compiler generated blocks, so that their internal unsafety doesn't
escape to external blocks.

Fixes #12418.

2 files changed, 13 insertions, 17 deletions

diff --git a/src/libstd/comm/shared.rs b/src/libstd/comm/shared.rs
index 0884c46ee84..444f2d14dba 100644
--- a/src/libstd/comm/shared.rs
+++ b/src/libstd/comm/shared.rs
@@ -486,14 +486,12 @@ impl<T: Send> Packet<T> {
 #[unsafe_destructor]
 impl<T: Send> Drop for Packet<T> {
     fn drop(&mut self) {
-        unsafe {
-            // Note that this load is not only an assert for correctness about
-            // disconnection, but also a proper fence before the read of
-            // `to_wake`, so this assert cannot be removed with also removing
-            // the `to_wake` assert.
-            assert_eq!(self.cnt.load(atomics::SeqCst), DISCONNECTED);
-            assert_eq!(self.to_wake.load(atomics::SeqCst), 0);
-            assert_eq!(self.channels.load(atomics::SeqCst), 0);
-        }
+        // Note that this load is not only an assert for correctness about
+        // disconnection, but also a proper fence before the read of
+        // `to_wake`, so this assert cannot be removed with also removing
+        // the `to_wake` assert.
+        assert_eq!(self.cnt.load(atomics::SeqCst), DISCONNECTED);
+        assert_eq!(self.to_wake.load(atomics::SeqCst), 0);
+        assert_eq!(self.channels.load(atomics::SeqCst), 0);
     }
 }
diff --git a/src/libstd/comm/stream.rs b/src/libstd/comm/stream.rs
index 0bacf1e1d28..4eac22b813d 100644
--- a/src/libstd/comm/stream.rs
+++ b/src/libstd/comm/stream.rs
@@ -471,13 +471,11 @@ impl<T: Send> Packet<T> {
 #[unsafe_destructor]
 impl<T: Send> Drop for Packet<T> {
     fn drop(&mut self) {
-        unsafe {
-            // Note that this load is not only an assert for correctness about
-            // disconnection, but also a proper fence before the read of
-            // `to_wake`, so this assert cannot be removed with also removing
-            // the `to_wake` assert.
-            assert_eq!(self.cnt.load(atomics::SeqCst), DISCONNECTED);
-            assert_eq!(self.to_wake.load(atomics::SeqCst), 0);
-        }
+        // Note that this load is not only an assert for correctness about
+        // disconnection, but also a proper fence before the read of
+        // `to_wake`, so this assert cannot be removed with also removing
+        // the `to_wake` assert.
+        assert_eq!(self.cnt.load(atomics::SeqCst), DISCONNECTED);
+        assert_eq!(self.to_wake.load(atomics::SeqCst), 0);
     }
 }