Give a specific lint for unsafety not being inherited

compiler/rustc_mir_transform/src/check_unsafety.rs

@@ -1,3 +1,4 @@
+use hir::{BlockCheckMode, ExprKind, Node};
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::struct_span_err;
 use rustc_hir as hir;
@@ -517,24 +518,49 @@ pub fn check_unsafety(tcx: TyCtxt<'_>, def_id: LocalDefId) {
     for &UnsafetyViolation { source_info, lint_root, kind, details } in violations.iter() {
         let (description, note) = details.description_and_note();
 
-        // Report an error.
-        let unsafe_fn_msg =
-            if unsafe_op_in_unsafe_fn_allowed(tcx, lint_root) { " function or" } else { "" };
-
         match kind {
             UnsafetyViolationKind::General => {
                 // once
-                struct_span_err!(
+                // Mutable statics always require an unsafe block
+                let unsafe_fn_msg = if unsafe_op_in_unsafe_fn_allowed(tcx, lint_root)
+                    && details != UnsafetyViolationDetails::UseOfMutableStatic
+                {
+                    " function or"
+                } else {
+                    ""
+                };
+
+                let mut err = struct_span_err!(
                     tcx.sess,
                     source_info.span,
                     E0133,
                     "{} is unsafe and requires unsafe{} block",
                     description,
                     unsafe_fn_msg,
-                )
-                .span_label(source_info.span, description)
-                .note(note)
-                .emit();
+                );
+                err.span_label(source_info.span, description).note(note);
+                let note_non_inherited = tcx.hir().parent_iter(lint_root).find(|(id, node)| {
+                    if let Node::Expr(block) = node
+                        && let ExprKind::Block(block, _) = block.kind
+                        && let BlockCheckMode::UnsafeBlock(_) = block.rules {
+                            true
+                        }
+                    else if let Some(sig) = tcx.hir().fn_sig_by_hir_id(*id)
+                        && sig.header.is_unsafe() {
+                            true
+                    } else {
+                        false
+                    }
+                });
+                if let Some((id, _)) = note_non_inherited {
+                    let span = tcx.hir().span(id);
+                    err.span_label(
+                        tcx.sess.source_map().guess_head_span(span),
+                        "items do not inherit unsafety from separate enclosing items",
+                    );
+                }
+
+                err.emit();
             }
             UnsafetyViolationKind::UnsafeFn => tcx.struct_span_lint_hir(
                 UNSAFE_OP_IN_UNSAFE_FN,