Add a new attr indicates pub struct with private fields could be contructed externally

compiler/rustc_feature/src/builtin_attrs.rs

@@ -432,6 +432,10 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ungated!(unsafe no_mangle, Normal, template!(Word), WarnFollowing, EncodeCrossCrate::No),
     ungated!(used, Normal, template!(Word, List: "compiler|linker"), WarnFollowing, EncodeCrossCrate::No),
     ungated!(link_ordinal, Normal, template!(List: "ordinal"), ErrorPreceding, EncodeCrossCrate::Yes),
+    gated!(
+        externally_constructed, Normal, template!(Word), WarnFollowing,
+        EncodeCrossCrate::Yes, externally_constructed_attr, experimental!(mark_externally_constructed)
+    ),
 
     // Limits:
     ungated!(

compiler/rustc_feature/src/unstable.rs

@@ -463,6 +463,9 @@ declare_features! (
     (unstable, extended_varargs_abi_support, "1.65.0", Some(100189)),
     /// Allows defining `extern type`s.
     (unstable, extern_types, "1.23.0", Some(43467)),
+    /// Allows `#[externally_constructed]` on pub structs with private fields,
+    /// indicates they will be contructed externally during dead code analysis.
+    (unstable, externally_constructed_attr, "CURRENT_RUSTC_VERSION", Some(126634)),
     /// Allow using 128-bit (quad precision) floating point numbers.
     (unstable, f128, "1.78.0", Some(116909)),
     /// Allow using 16-bit (half precision) floating point numbers.

compiler/rustc_passes/src/dead.rs

@@ -930,8 +930,11 @@ fn create_and_seed_worklist(
                     // checks impls, impl-items and pub structs with all public fields later
                     match tcx.def_kind(id) {
                         DefKind::Impl { .. } => false,
-                        DefKind::AssocConst | DefKind::AssocTy | DefKind::AssocFn => !matches!(tcx.associated_item(id).container, AssocItemContainer::ImplContainer),
-                        DefKind::Struct => struct_all_fields_are_public(tcx, id.to_def_id()) || has_allow_dead_code_or_lang_attr(tcx, id).is_some(),
+                        DefKind::AssocConst | DefKind::AssocTy | DefKind::AssocFn
+                            => !matches!(tcx.associated_item(id).container, AssocItemContainer::ImplContainer),
+                        DefKind::Struct => struct_all_fields_are_public(tcx, id.to_def_id())
+                            || has_allow_dead_code_or_lang_attr(tcx, id).is_some()
+                            || tcx.has_attr(id, sym::externally_constructed),
                         _ => true
                     })
                 .map(|id| (id, ComesFromAllowExpect::No))

compiler/rustc_span/src/symbol.rs

@@ -809,6 +809,8 @@ symbols! {
         extern_types,
         external,
         external_doc,
+        externally_constructed,
+        externally_constructed_attr,
         f,
         f128,
         f128_nan,

tests/ui/externally_constructed_attr/not-lint-structs-constructed-externally.rs

@@ -0,0 +1,41 @@
+//@ check-pass
+#![feature(externally_constructed_attr)]
+
+#[repr(C)]
+#[externally_constructed]
+pub struct Foo {
+    pub i: i16,
+    align: i16
+}
+
+mod ffi {
+    use super::*;
+
+    extern "C" {
+        pub fn DomPromise_AddRef(promise: *const Promise);
+        pub fn DomPromise_Release(promise: *const Promise);
+    }
+}
+
+#[repr(C)]
+#[externally_constructed]
+pub struct Promise {
+    private: [u8; 0],
+    __nosync: ::std::marker::PhantomData<::std::rc::Rc<u8>>,
+}
+
+pub unsafe trait RefCounted {
+    unsafe fn addref(&self);
+    unsafe fn release(&self);
+}
+
+unsafe impl RefCounted for Promise {
+    unsafe fn addref(&self) {
+        ffi::DomPromise_AddRef(self)
+    }
+    unsafe fn release(&self) {
+        ffi::DomPromise_Release(self)
+    }
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-externally_constructed_attr.rs

@@ -0,0 +1,8 @@
+#[repr(C)]
+#[externally_constructed] //~ ERROR the `#[mark_externally_constructed]` attribute is an experimental feature
+pub struct Foo {
+    pub i: i16,
+    align: i16
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-externally_constructed_attr.stderr

@@ -0,0 +1,13 @@
+error[E0658]: the `#[mark_externally_constructed]` attribute is an experimental feature
+  --> $DIR/feature-gate-externally_constructed_attr.rs:2:1
+   |
+LL | #[externally_constructed]
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #126634 <https://github.com/rust-lang/rust/issues/126634> for more information
+   = help: add `#![feature(externally_constructed_attr)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.