rustc_pub_transparent: validation

compiler/rustc_passes/messages.ftl

@@ -657,6 +657,10 @@ passes_rustc_lint_opt_ty =
     `#[rustc_lint_opt_ty]` should be applied to a struct
     .label = not a struct
 
+passes_rustc_pub_transparent =
+    attribute should be applied to `#[repr(transparent)]` types
+    .label = not a `#[repr(transparent)]` type
+
 passes_rustc_safe_intrinsic =
     attribute should be applied to intrinsic functions
     .label = not an intrinsic function

compiler/rustc_passes/src/check_attr.rs

@@ -245,6 +245,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                     self.check_coroutine(attr, target);
                 }
                 [sym::linkage, ..] => self.check_linkage(attr, span, target),
+                [sym::rustc_pub_transparent, ..] => self.check_rustc_pub_transparent( attr.span, span, attrs),
                 [
                     // ok
                     sym::allow
@@ -2381,6 +2382,18 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             }
         }
     }
+
+    fn check_rustc_pub_transparent(&self, attr_span: Span, span: Span, attrs: &[Attribute]) {
+        if !attrs
+            .iter()
+            .filter(|attr| attr.has_name(sym::repr))
+            .filter_map(|attr| attr.meta_item_list())
+            .flatten()
+            .any(|nmi| nmi.has_name(sym::transparent))
+        {
+            self.dcx().emit_err(errors::RustcPubTransparent { span, attr_span });
+        }
+    }
 }
 
 impl<'tcx> Visitor<'tcx> for CheckAttrVisitor<'tcx> {

compiler/rustc_passes/src/errors.rs

@@ -622,6 +622,15 @@ pub struct RustcStdInternalSymbol {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(passes_rustc_pub_transparent)]
+pub struct RustcPubTransparent {
+    #[primary_span]
+    pub attr_span: Span,
+    #[label]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(passes_link_ordinal)]
 pub struct LinkOrdinal {

tests/ui/attributes/rustc_pub_transparent.rs

@@ -0,0 +1,25 @@
+#![feature(rustc_attrs, transparent_unions)]
+
+#[rustc_pub_transparent]
+#[repr(transparent)]
+union E<T: Copy> {
+    value: T,
+    uninit: (),
+}
+
+#[repr(transparent)]
+#[rustc_pub_transparent]
+struct S<T>(T);
+
+#[rustc_pub_transparent] //~ ERROR attribute should be applied to `#[repr(transparent)]` types
+#[repr(C)]
+struct S1 {
+    A: u8,
+}
+
+#[rustc_pub_transparent] //~ ERROR attribute should be applied to `#[repr(transparent)]` types
+struct S2<T> {
+    value: T,
+}
+
+fn main() {}

tests/ui/attributes/rustc_pub_transparent.stderr

@@ -0,0 +1,23 @@
+error: attribute should be applied to `#[repr(transparent)]` types
+  --> $DIR/rustc_pub_transparent.rs:14:1
+   |
+LL |   #[rustc_pub_transparent]
+   |   ^^^^^^^^^^^^^^^^^^^^^^^^
+LL |   #[repr(C)]
+LL | / struct S1 {
+LL | |     A: u8,
+LL | | }
+   | |_- not a `#[repr(transparent)]` type
+
+error: attribute should be applied to `#[repr(transparent)]` types
+  --> $DIR/rustc_pub_transparent.rs:20:1
+   |
+LL |   #[rustc_pub_transparent]
+   |   ^^^^^^^^^^^^^^^^^^^^^^^^
+LL | / struct S2<T> {
+LL | |     value: T,
+LL | | }
+   | |_- not a `#[repr(transparent)]` type
+
+error: aborting due to 2 previous errors
+