Rollup merge of #138399 - Bryanskiy:delegation-extern-fn, r=petrochenkov

Delegation: allow foreign fns `reuse`

In example:
```rust
unsafe extern "C" {
    fn foo();
}

reuse foo as bar;
```

Desugaring before:

```rust
fn bar() {
    foo()
    //~^ ERROR call to unsafe function `foo` is unsafe and requires unsafe function or block
}
```

after:

```rust
unsafe extern "C" fn bar() {
    foo()
}
```

Fixes https://github.com/rust-lang/rust/issues/127412

r? `@petrochenkov`

compiler/rustc_ast_lowering/src/delegation.rs

@@ -190,14 +190,19 @@ impl<'hir> LoweringContext<'_, 'hir> {
     ) -> hir::FnSig<'hir> {
         let header = if let Some(local_sig_id) = sig_id.as_local() {
             match self.resolver.delegation_fn_sigs.get(&local_sig_id) {
-                Some(sig) => self.lower_fn_header(
-                    sig.header,
+                Some(sig) => {
+                    let parent = self.tcx.parent(sig_id);
                     // HACK: we override the default safety instead of generating attributes from the ether.
                     // We are not forwarding the attributes, as the delegation fn sigs are collected on the ast,
                     // and here we need the hir attributes.
-                    if sig.target_feature { hir::Safety::Unsafe } else { hir::Safety::Safe },
-                    &[],
-                ),
+                    let default_safety =
+                        if sig.target_feature || self.tcx.def_kind(parent) == DefKind::ForeignMod {
+                            hir::Safety::Unsafe
+                        } else {
+                            hir::Safety::Safe
+                        };
+                    self.lower_fn_header(sig.header, default_safety, &[])
+                }
                 None => self.generate_header_error(),
             }
         } else {