Rollup merge of #116162 - fmease:gate-n-validate-rustc_safe_intrinsic, r=Nilstrieb

Gate and validate `#[rustc_safe_intrinsic]`

Copied over from #116159:

> This was added as ungated in https://github.com/rust-lang/rust/pull/100719/files#diff-09c366d3ad3ec9a42125253b610ca83cad6b156aa2a723f6c7e83eddef7b1e8fR502, probably because the author looked at the surrounding attributes, which are ungated because they are gated specially behind the staged_api feature.
>
> I don't think we need to crater this, the attribute is entirely useless without the intrinsics feature, which is already unstable..

r? ``@Nilstrieb``

compiler/rustc_passes/src/check_attr.rs

@@ -195,6 +195,9 @@ impl CheckAttrVisitor<'_> {
                 | sym::rustc_promotable => self.check_stability_promotable(&attr, span, target),
                 sym::link_ordinal => self.check_link_ordinal(&attr, span, target),
                 sym::rustc_confusables => self.check_confusables(&attr, target),
+                sym::rustc_safe_intrinsic => {
+                    self.check_rustc_safe_intrinsic(hir_id, attr, span, target)
+                }
                 _ => true,
             };
 
@@ -2042,6 +2045,29 @@ impl CheckAttrVisitor<'_> {
         }
     }
 
+    fn check_rustc_safe_intrinsic(
+        &self,
+        hir_id: HirId,
+        attr: &Attribute,
+        span: Span,
+        target: Target,
+    ) -> bool {
+        let hir = self.tcx.hir();
+
+        if let Target::ForeignFn = target
+            && let Some(parent) = hir.opt_parent_id(hir_id)
+            && let hir::Node::Item(Item {
+                kind: ItemKind::ForeignMod { abi: Abi::RustIntrinsic | Abi::PlatformIntrinsic, .. },
+                ..
+            }) = hir.get(parent)
+        {
+            return true;
+        }
+
+        self.tcx.sess.emit_err(errors::RustcSafeIntrinsic { attr_span: attr.span, span });
+        false
+    }
+
     fn check_rustc_std_internal_symbol(
         &self,
         attr: &Attribute,

compiler/rustc_passes/src/errors.rs

@@ -620,6 +620,15 @@ pub struct RustcAllowConstFnUnstable {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(passes_rustc_safe_intrinsic)]
+pub struct RustcSafeIntrinsic {
+    #[primary_span]
+    pub attr_span: Span,
+    #[label]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(passes_rustc_std_internal_symbol)]
 pub struct RustcStdInternalSymbol {