Point to enclosing block/fn on nested unsafe

When declaring nested unsafe blocks (`unsafe {unsafe {}}`) that trigger
the "unnecessary `unsafe` block" error, point out the enclosing `unsafe
block` or `unsafe fn` that makes it unnecessary.

src/test/ui/span/lint-unused-unsafe.rs

@@ -0,0 +1,66 @@
+// Copyright 2013 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.
+
+// Exercise the unused_unsafe attribute in some positive and negative cases
+
+#![allow(dead_code)]
+#![deny(unused_unsafe)]
+
+
+mod foo {
+    extern {
+        pub fn bar();
+    }
+}
+
+fn callback<T, F>(_f: F) -> T where F: FnOnce() -> T { panic!() }
+unsafe fn unsf() {}
+
+fn bad1() { unsafe {} }                  //~ ERROR: unnecessary `unsafe` block
+fn bad2() { unsafe { bad1() } }          //~ ERROR: unnecessary `unsafe` block
+unsafe fn bad3() { unsafe {} }           //~ ERROR: unnecessary `unsafe` block
+fn bad4() { unsafe { callback(||{}) } }  //~ ERROR: unnecessary `unsafe` block
+unsafe fn bad5() { unsafe { unsf() } }   //~ ERROR: unnecessary `unsafe` block
+fn bad6() {
+    unsafe {                             // don't put the warning here
+        unsafe {                         //~ ERROR: unnecessary `unsafe` block
+            unsf()
+        }
+    }
+}
+unsafe fn bad7() {
+    unsafe {                             //~ ERROR: unnecessary `unsafe` block
+        unsafe {                         //~ ERROR: unnecessary `unsafe` block
+            unsf()
+        }
+    }
+}
+
+unsafe fn good0() { unsf() }
+fn good1() { unsafe { unsf() } }
+fn good2() {
+    /* bug uncovered when implementing warning about unused unsafe blocks. Be
+       sure that when purity is inherited that the source of the unsafe-ness
+       is tracked correctly */
+    unsafe {
+        unsafe fn what() -> Vec<String> { panic!() }
+
+        callback(|| {
+            what();
+        });
+    }
+}
+
+unsafe fn good3() { foo::bar() }
+fn good4() { unsafe { foo::bar() } }
+
+#[allow(unused_unsafe)] fn allowed() { unsafe {} }
+
+fn main() {}

src/test/ui/span/lint-unused-unsafe.stderr

@@ -0,0 +1,116 @@
+error: unnecessary `unsafe` block
+  --> $DIR/lint-unused-unsafe.rs:26:13
+   |
+26 | fn bad1() { unsafe {} }                  //~ ERROR: unnecessary `unsafe` block
+   |             ^^^^^^^^^ unnecessary `unsafe` block
+   |
+note: lint level defined here
+  --> $DIR/lint-unused-unsafe.rs:14:9
+   |
+14 | #![deny(unused_unsafe)]
+   |         ^^^^^^^^^^^^^
+
+error: unnecessary `unsafe` block
+  --> $DIR/lint-unused-unsafe.rs:27:13
+   |
+27 | fn bad2() { unsafe { bad1() } }          //~ ERROR: unnecessary `unsafe` block
+   |             ^^^^^^^^^^^^^^^^^ unnecessary `unsafe` block
+
+error: unnecessary `unsafe` block
+  --> $DIR/lint-unused-unsafe.rs:28:20
+   |
+28 | unsafe fn bad3() { unsafe {} }           //~ ERROR: unnecessary `unsafe` block
+   |                    ^^^^^^^^^ unnecessary `unsafe` block
+   |
+note: because it's nested under this `unsafe` fn
+  --> $DIR/lint-unused-unsafe.rs:28:1
+   |
+28 | unsafe fn bad3() { unsafe {} }           //~ ERROR: unnecessary `unsafe` block
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: unnecessary `unsafe` block
+  --> $DIR/lint-unused-unsafe.rs:29:13
+   |
+29 | fn bad4() { unsafe { callback(||{}) } }  //~ ERROR: unnecessary `unsafe` block
+   |             ^^^^^^^^^^^^^^^^^^^^^^^^^ unnecessary `unsafe` block
+
+error: unnecessary `unsafe` block
+  --> $DIR/lint-unused-unsafe.rs:30:20
+   |
+30 | unsafe fn bad5() { unsafe { unsf() } }   //~ ERROR: unnecessary `unsafe` block
+   |                    ^^^^^^^^^^^^^^^^^ unnecessary `unsafe` block
+   |
+note: because it's nested under this `unsafe` fn
+  --> $DIR/lint-unused-unsafe.rs:30:1
+   |
+30 | unsafe fn bad5() { unsafe { unsf() } }   //~ ERROR: unnecessary `unsafe` block
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: unnecessary `unsafe` block
+  --> $DIR/lint-unused-unsafe.rs:33:9
+   |
+33 |           unsafe {                         //~ ERROR: unnecessary `unsafe` block
+   |  _________^ starting here...
+34 | |             unsf()
+35 | |         }
+   | |_________^ ...ending here: unnecessary `unsafe` block
+   |
+note: because it's nested under this `unsafe` block
+  --> $DIR/lint-unused-unsafe.rs:32:5
+   |
+32 |       unsafe {                             // don't put the warning here
+   |  _____^ starting here...
+33 | |         unsafe {                         //~ ERROR: unnecessary `unsafe` block
+34 | |             unsf()
+35 | |         }
+36 | |     }
+   | |_____^ ...ending here
+
+error: unnecessary `unsafe` block
+  --> $DIR/lint-unused-unsafe.rs:39:5
+   |
+39 |       unsafe {                             //~ ERROR: unnecessary `unsafe` block
+   |  _____^ starting here...
+40 | |         unsafe {                         //~ ERROR: unnecessary `unsafe` block
+41 | |             unsf()
+42 | |         }
+43 | |     }
+   | |_____^ ...ending here: unnecessary `unsafe` block
+   |
+note: because it's nested under this `unsafe` fn
+  --> $DIR/lint-unused-unsafe.rs:38:1
+   |
+38 |   unsafe fn bad7() {
+   |  _^ starting here...
+39 | |     unsafe {                             //~ ERROR: unnecessary `unsafe` block
+40 | |         unsafe {                         //~ ERROR: unnecessary `unsafe` block
+41 | |             unsf()
+42 | |         }
+43 | |     }
+44 | | }
+   | |_^ ...ending here
+
+error: unnecessary `unsafe` block
+  --> $DIR/lint-unused-unsafe.rs:40:9
+   |
+40 |           unsafe {                         //~ ERROR: unnecessary `unsafe` block
+   |  _________^ starting here...
+41 | |             unsf()
+42 | |         }
+   | |_________^ ...ending here: unnecessary `unsafe` block
+   |
+note: because it's nested under this `unsafe` fn
+  --> $DIR/lint-unused-unsafe.rs:38:1
+   |
+38 |   unsafe fn bad7() {
+   |  _^ starting here...
+39 | |     unsafe {                             //~ ERROR: unnecessary `unsafe` block
+40 | |         unsafe {                         //~ ERROR: unnecessary `unsafe` block
+41 | |             unsf()
+42 | |         }
+43 | |     }
+44 | | }
+   | |_^ ...ending here
+
+error: aborting due to 8 previous errors
+