Auto merge of #23452 - nikomatsakis:unsafety-subtyping, r=nrc

Safe fns are no longer subtypes of unsafe fns, but you can coerce from one to the other.

This is a [breaking-change] in that impl fns must now be declared `unsafe` if the trait is declared `unsafe`. In some rare cases, the subtyping change may also direct affect you, but no such cases were encountered in practice.

Fixes #23449.

r? @nrc

src/test/compile-fail/unsafe-subtyping.rs

@@ -0,0 +1,21 @@
+// Copyright 2013-2014 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.
+
+// Check that safe fns are not a subtype of unsafe fns.
+
+fn foo(x: Option<fn(i32)>) -> Option<unsafe fn(i32)> {
+    x //~ ERROR mismatched types
+}
+
+fn bar(x: fn(i32)) -> unsafe fn(i32) {
+    x // OK, coercion!
+}
+
+fn main() { }

src/test/compile-fail/unsafe-trait-impl.rs

@@ -0,0 +1,22 @@
+// Copyright 2013-2014 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.
+
+// Check that safe fns are not a subtype of unsafe fns.
+
+trait Foo {
+    unsafe fn len(&self) -> u32;
+}
+
+impl Foo for u32 {
+    fn len(&self) -> u32 { *self }
+    //~^ ERROR incompatible type for trait: expected unsafe fn, found normal fn
+}
+
+fn main() { }

src/test/compile-fail/variadic-ffi.rs

@@ -30,9 +30,9 @@ fn main() {
         //~| expected non-variadic fn
         //~| found variadic function
 
-        let y: unsafe extern "C" fn(f: isize, x: u8, ...) = bar;
+        let y: extern "C" fn(f: isize, x: u8, ...) = bar;
         //~^ ERROR: mismatched types
-        //~| expected `unsafe extern "C" fn(isize, u8, ...)`
+        //~| expected `extern "C" fn(isize, u8, ...)`
         //~| found `extern "C" fn(isize, u8) {bar}`
         //~| expected variadic fn
         //~| found non-variadic function

src/test/run-pass/unsafe-coercion.rs

@@ -0,0 +1,25 @@
+// Copyright 2013-2014 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.
+
+// Check that safe fns are not a subtype of unsafe fns.
+
+fn foo(x: i32) -> i32 {
+    x * 22
+}
+
+fn bar(x: fn(i32) -> i32) -> unsafe fn(i32) -> i32 {
+    x // OK, coercion!
+}
+
+fn main() {
+    let f = bar(foo);
+    let x = unsafe { f(2) };
+    assert_eq!(x, 44);
+}