Fix unused_unsafe in THIR unsafeck

Author: LeSeulArtichaut

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -42,7 +42,7 @@ impl<'tcx> UnsafetyVisitor<'_, 'tcx> {
             self.warn_unused_unsafe(
                 hir_id,
                 block_span,
-                Some(self.tcx.sess.source_map().guess_head_span(enclosing_span)),
+                Some((self.tcx.sess.source_map().guess_head_span(enclosing_span), "block")),
             );
             f(self);
         } else {
@@ -52,7 +52,15 @@ impl<'tcx> UnsafetyVisitor<'_, 'tcx> {
             f(self);
 
             if let SafetyContext::UnsafeBlock { used: false, span, hir_id } = self.safety_context {
-                self.warn_unused_unsafe(hir_id, span, self.body_unsafety.unsafe_fn_sig_span());
+                self.warn_unused_unsafe(
+                    hir_id,
+                    span,
+                    if self.unsafe_op_in_unsafe_fn_allowed() {
+                        self.body_unsafety.unsafe_fn_sig_span().map(|span| (span, "fn"))
+                    } else {
+                        None
+                    },
+                );
             }
             self.safety_context = prev_context;
             return;
@@ -108,18 +116,15 @@ impl<'tcx> UnsafetyVisitor<'_, 'tcx> {
         &self,
         hir_id: hir::HirId,
         block_span: Span,
-        enclosing_span: Option<Span>,
+        enclosing_unsafe: Option<(Span, &'static str)>,
     ) {
         let block_span = self.tcx.sess.source_map().guess_head_span(block_span);
         self.tcx.struct_span_lint_hir(UNUSED_UNSAFE, hir_id, block_span, |lint| {
             let msg = "unnecessary `unsafe` block";
             let mut db = lint.build(msg);
             db.span_label(block_span, msg);
-            if let Some(enclosing_span) = enclosing_span {
-                db.span_label(
-                    enclosing_span,
-                    format!("because it's nested under this `unsafe` block"),
-                );
+            if let Some((span, kind)) = enclosing_unsafe {
+                db.span_label(span, format!("because it's nested under this `unsafe` {}", kind));
             }
             db.emit();
         });