Rollup merge of #103964 - clubby789:lint-enclosing-unsafe, r=compiler-errors

Give a specific lint for unsafety not being inherited

In cases like
```rs
static mut FOO: u64 = 0;
fn main() {
    unsafe {static BAR: u64 = FOO;}
}
```
and
```rs
fn foo() {
  unsafe {
    fn bar() {
      unsafe_call();
    }
  }
}
```
Specifically inform the user that the unsafety is not inherited for the seperate enclosing items
Fixes #94077
r? compiler-errors
`@rustbot` label +A-diagnostics

compiler/rustc_mir_transform/src/check_unsafety.rs

@@ -4,6 +4,7 @@ use rustc_hir as hir;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_hir::hir_id::HirId;
 use rustc_hir::intravisit;
+use rustc_hir::{BlockCheckMode, ExprKind, Node};
 use rustc_middle::mir::visit::{MutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
 use rustc_middle::ty::query::Providers;
@@ -517,24 +518,48 @@ 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!(
+                let unsafe_fn_msg = if unsafe_op_in_unsafe_fn_allowed(tcx, lint_root) {
+                    " 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,

src/test/ui/unsafe/unsafe-not-inherited.rs

@@ -0,0 +1,26 @@
+#![allow(unused, dead_code)]
+
+static mut FOO: u64 = 0;
+
+fn static_mod() {
+    unsafe {static BAR: u64 = FOO;}
+    //~^ ERROR: use of mutable static is unsafe
+    //~| NOTE: use of mutable static
+    //~| NOTE: mutable statics can be mutated by multiple threads
+    //~| NOTE: items do not inherit unsafety
+}
+
+unsafe fn unsafe_call() {}
+fn foo() {
+    unsafe {
+    //~^ NOTE: items do not inherit unsafety
+        fn bar() {
+            unsafe_call();
+            //~^ ERROR: call to unsafe function
+            //~| NOTE: call to unsafe function
+            //~| NOTE: consult the function's documentation
+        }
+    }
+}
+
+fn main() {}

src/test/ui/unsafe/unsafe-not-inherited.stderr

@@ -0,0 +1,24 @@
+error[E0133]: use of mutable static is unsafe and requires unsafe function or block
+  --> $DIR/unsafe-not-inherited.rs:6:31
+   |
+LL |     unsafe {static BAR: u64 = FOO;}
+   |     ------                    ^^^ use of mutable static
+   |     |
+   |     items do not inherit unsafety from separate enclosing items
+   |
+   = note: mutable statics can be mutated by multiple threads: aliasing violations or data races will cause undefined behavior
+
+error[E0133]: call to unsafe function is unsafe and requires unsafe function or block
+  --> $DIR/unsafe-not-inherited.rs:18:13
+   |
+LL |     unsafe {
+   |     ------ items do not inherit unsafety from separate enclosing items
+...
+LL |             unsafe_call();
+   |             ^^^^^^^^^^^^^ call to unsafe function
+   |
+   = note: consult the function's documentation for information on how to avoid undefined behavior
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0133`.