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/librustc_lint/unused.rs

@@ -189,11 +189,38 @@ impl LintPass for UnusedUnsafe {
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for UnusedUnsafe {
     fn check_expr(&mut self, cx: &LateContext, e: &hir::Expr) {
+        /// Return the NodeId for an enclosing scope that is also `unsafe`
+        fn is_enclosed(cx: &LateContext, id: ast::NodeId) -> Option<(String, ast::NodeId)> {
+            let parent_id = cx.tcx.hir.get_parent_node(id);
+            if parent_id != id {
+                if cx.tcx.used_unsafe.borrow().contains(&parent_id) {
+                    Some(("block".to_string(), parent_id))
+                } else if let Some(hir::map::NodeItem(&hir::Item {
+                    node: hir::ItemFn(_, hir::Unsafety::Unsafe, _, _, _, _),
+                    ..
+                })) = cx.tcx.hir.find(parent_id) {
+                    Some(("fn".to_string(), parent_id))
+                } else {
+                    is_enclosed(cx, parent_id)
+                }
+            } else {
+                None
+            }
+        }
         if let hir::ExprBlock(ref blk) = e.node {
             // Don't warn about generated blocks, that'll just pollute the output.
             if blk.rules == hir::UnsafeBlock(hir::UserProvided) &&
                !cx.tcx.used_unsafe.borrow().contains(&blk.id) {
-                cx.span_lint(UNUSED_UNSAFE, blk.span, "unnecessary `unsafe` block");
+
+                let mut db = cx.struct_span_lint(UNUSED_UNSAFE, blk.span,
+                                                 "unnecessary `unsafe` block");
+
+                db.span_label(blk.span, &"unnecessary `unsafe` block");
+                if let Some((kind, id)) = is_enclosed(cx, blk.id) {
+                    db.span_note(cx.tcx.hir.span(id),
+                                 &format!("because it's nested under this `unsafe` {}", kind));
+                }
+                db.emit();
             }
         }
     }

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
+