Avoid deref/ref cycles for no-op coercions between unsafe pointers

src/librustc_typeck/check/coercion.rs

@@ -403,8 +403,9 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                a.repr(self.tcx()),
                b.repr(self.tcx()));
 
-        let mt_a = match a.sty {
-            ty::TyRef(_, mt) | ty::TyRawPtr(mt) => mt,
+        let (is_ref, mt_a) = match a.sty {
+            ty::TyRef(_, mt) => (true, mt),
+            ty::TyRawPtr(mt) => (false, mt),
             _ => {
                 return self.subtype(a, b);
             }
@@ -418,11 +419,15 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         // Although references and unsafe ptrs have the same
         // representation, we still register an AutoDerefRef so that
         // regionck knows that the region for `a` must be valid here.
-        Ok(Some(AdjustDerefRef(AutoDerefRef {
-            autoderefs: 1,
-            autoref: Some(ty::AutoUnsafe(mutbl_b)),
-            unsize: None
-        })))
+        if is_ref {
+            Ok(Some(AdjustDerefRef(AutoDerefRef {
+                autoderefs: 1,
+                autoref: Some(ty::AutoUnsafe(mutbl_b)),
+                unsize: None
+            })))
+        } else {
+            Ok(None)
+        }
     }
 }
 

src/test/codegen/coercions.rs

@@ -0,0 +1,27 @@
+// Copyright 2015 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.
+
+// compile-flags: -C no-prepopulate-passes
+
+static X: i32 = 5;
+
+// CHECK-LABEL: @raw_ptr_to_raw_ptr_noop
+// CHECK-NOT: alloca
+#[no_mangle]
+pub fn raw_ptr_to_raw_ptr_noop() -> *const i32{
+    &X as *const i32
+}
+
+// CHECK-LABEL: @reference_to_raw_ptr_noop
+// CHECK-NOT: alloca
+#[no_mangle]
+pub fn reference_to_raw_ptr_noop() -> *const i32 {
+    &X
+}

src/test/compile-fail/issue-16538.rs

@@ -20,6 +20,7 @@ mod Y {
 
 static foo: *const Y::X = Y::foo(Y::x as *const Y::X);
 //~^ ERROR the trait `core::marker::Sync` is not implemented for the type
+//~| ERROR cannot refer to other statics by value, use the address-of operator or a constant instead
 //~| ERROR E0015
 
 fn main() {}