Gate and validate #[rustc_safe_intrinsic]

compiler/rustc_error_codes/src/error_codes/E0094.md

@@ -3,7 +3,7 @@ An invalid number of generic parameters was passed to an intrinsic function.
 Erroneous code example:
 
 ```compile_fail,E0094
-#![feature(intrinsics)]
+#![feature(intrinsics, rustc_attrs)]
 #![allow(internal_features)]
 
 extern "rust-intrinsic" {
@@ -18,7 +18,7 @@ and verify with the function declaration in the Rust source code.
 Example:
 
 ```
-#![feature(intrinsics)]
+#![feature(intrinsics, rustc_attrs)]
 #![allow(internal_features)]
 
 extern "rust-intrinsic" {

compiler/rustc_error_codes/src/error_codes/E0211.md

@@ -4,7 +4,7 @@ You used a function or type which doesn't fit the requirements for where it was
 used. Erroneous code examples:
 
 ```compile_fail
-#![feature(intrinsics)]
+#![feature(intrinsics, rustc_attrs)]
 #![allow(internal_features)]
 
 extern "rust-intrinsic" {
@@ -41,7 +41,7 @@ impl Foo {
 For the first code example, please check the function definition. Example:
 
 ```
-#![feature(intrinsics)]
+#![feature(intrinsics, rustc_attrs)]
 #![allow(internal_features)]
 
 extern "rust-intrinsic" {

compiler/rustc_feature/src/builtin_attrs.rs

@@ -537,7 +537,6 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         allow_internal_unsafe, Normal, template!(Word), WarnFollowing,
         "allow_internal_unsafe side-steps the unsafe_code lint",
     ),
-    ungated!(rustc_safe_intrinsic, Normal, template!(Word), DuplicatesOk),
     rustc_attr!(rustc_allowed_through_unstable_modules, Normal, template!(Word), WarnFollowing,
     "rustc_allowed_through_unstable_modules special cases accidental stabilizations of stable items \
     through unstable paths"),
@@ -806,6 +805,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         rustc_doc_primitive, Normal, template!(NameValueStr: "primitive name"), ErrorFollowing,
         r#"`rustc_doc_primitive` is a rustc internal attribute"#,
     ),
+    rustc_attr!(
+        rustc_safe_intrinsic, Normal, template!(Word), WarnFollowing,
+        "the `#[rustc_safe_intrinsic]` attribute is used internally to mark intrinsics as safe"
+    ),
 
     // ==========================================================================
     // Internal attributes, Testing:

compiler/rustc_passes/messages.ftl

@@ -648,6 +648,10 @@ passes_rustc_lint_opt_ty =
     `#[rustc_lint_opt_ty]` should be applied to a struct
     .label = not a struct
 
+passes_rustc_safe_intrinsic =
+    attribute should be applied to intrinsic functions
+    .label = not an intrinsic function
+
 passes_rustc_std_internal_symbol =
     attribute should be applied to functions or statics
     .label = not a function or static

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 {