expanding #[rustc_trivial_field_reads] to more targets, place on Derive and Clone

compiler/rustc_attr_parsing/src/attributes/rustc_internal.rs

@@ -401,7 +401,12 @@ pub(crate) struct RustcTrivialFieldReadsParser;
 
 impl NoArgsAttributeParser for RustcTrivialFieldReadsParser {
     const PATH: &[Symbol] = &[sym::rustc_trivial_field_reads];
-    const ALLOWED_TARGETS: AllowedTargets<'_> = AllowedTargets::AllowList(&[Allow(Target::Trait)]);
+    const ALLOWED_TARGETS: AllowedTargets<'_> = AllowedTargets::AllowList(&[
+        Allow(Target::Fn),
+        Allow(Target::Method(MethodKind::Inherent)),
+        Allow(Target::Method(MethodKind::Trait { body: true })),
+        Allow(Target::Method(MethodKind::TraitImpl)),
+    ]);
     const STABILITY: AttributeStability = unstable!(rustc_attrs);
     const CREATE: fn(Span) -> AttributeKind = |_| AttributeKind::RustcTrivialFieldReads;
 }

compiler/rustc_builtin_macros/src/deriving/clone.rs

@@ -98,7 +98,10 @@ pub(crate) fn expand_deriving_clone(
             explicit_self: true,
             nonself_args: SmallVec::new(),
             ret_ty: Self_,
-            attributes: thin_vec![cx.attr_word(sym::inline, span)],
+            attributes: thin_vec![
+                cx.attr_word(sym::inline, span),
+                cx.attr_word(sym::rustc_trivial_field_reads, span)
+            ],
             fieldless_variants_strategy: FieldlessVariantsStrategy::Default,
             combine_substructure: substructure,
         }],

compiler/rustc_builtin_macros/src/deriving/debug.rs

@@ -32,7 +32,10 @@ pub(crate) fn expand_deriving_debug(
             explicit_self: true,
             nonself_args: smallvec![(fmtr, sym::character('f'))],
             ret_ty: Path(path_std!(fmt::Result)),
-            attributes: thin_vec![cx.attr_word(sym::inline, span)],
+            attributes: thin_vec![
+                cx.attr_word(sym::inline, span),
+                cx.attr_word(sym::rustc_trivial_field_reads, span)
+            ],
             fieldless_variants_strategy:
                 FieldlessVariantsStrategy::SpecializeIfAllVariantsFieldless,
             combine_substructure: combine_substructure(show_substructure),

compiler/rustc_passes/src/dead.rs

@@ -59,7 +59,7 @@ fn should_explore(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
         | DefKind::ExternCrate
         | DefKind::Use
         | DefKind::Ctor(..)
-        | DefKind::ForeignMod => true,
+        | DefKind::ForeignMod => !find_attr!(tcx, def_id, RustcTrivialFieldReads),
 
         DefKind::TyParam
         | DefKind::ConstParam
@@ -444,17 +444,16 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
         ControlFlow::Continue(())
     }
 
-    /// Automatically generated items marked with `rustc_trivial_field_reads`
+    /// Items marked with `rustc_trivial_field_reads`
     /// will be ignored for the purposes of dead code analysis (see PR #85200
-    /// for discussion).
+    /// for discussion, and PR #160666).
     fn should_ignore_impl_item(&mut self, impl_item: &hir::ImplItem<'_>) -> bool {
         if let hir::ImplItemImplKind::Trait { .. } = impl_item.impl_kind
             && let impl_of = self.tcx.local_parent(impl_item.owner_id.def_id)
-            && self.tcx.is_automatically_derived(impl_of.to_def_id())
-            && let trait_ref =
-                self.tcx.impl_trait_ref(impl_of).instantiate_identity().skip_norm_wip()
-            && find_attr!(self.tcx, trait_ref.def_id, RustcTrivialFieldReads)
+            && find_attr!(self.tcx, impl_item.owner_id.def_id, RustcTrivialFieldReads)
         {
+            let trait_ref = self.tcx.impl_trait_ref(impl_of).instantiate_identity().skip_norm_wip();
+
             if let ty::Adt(adt_def, _) = trait_ref.self_ty().kind()
                 && let Some(adt_def_id) = adt_def.did().as_local()
             {
@@ -463,7 +462,7 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
             return true;
         }
 
-        false
+        return false;
     }
 
     fn visit_node(

library/core/src/clone.rs

@@ -189,7 +189,6 @@ mod uninit;
 #[stable(feature = "rust1", since = "1.0.0")]
 #[lang = "clone"]
 #[rustc_diagnostic_item = "Clone"]
-#[rustc_trivial_field_reads]
 #[rustc_const_unstable(feature = "const_clone", issue = "142757")]
 pub const trait Clone: Sized {
     /// Returns a duplicate of the value.
@@ -285,7 +284,7 @@ pub const unsafe trait TrivialClone: [const] Clone {}
 /// Derive macro generating an impl of the trait `Clone`.
 #[rustc_builtin_macro]
 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
-#[allow_internal_unstable(core_intrinsics, derive_clone_copy_internals, trivial_clone)]
+#[allow_internal_unstable(core_intrinsics, derive_clone_copy_internals, trivial_clone, rustc_attrs)]
 pub macro Clone($item:item) {
     /* compiler built-in */
 }

library/core/src/fmt/mod.rs

@@ -1049,7 +1049,6 @@ impl Display for Arguments<'_> {
 )]
 #[doc(alias = "{:?}")]
 #[rustc_diagnostic_item = "Debug"]
-#[rustc_trivial_field_reads]
 pub trait Debug: PointeeSized {
     #[doc = include_str!("fmt_trait_method_doc.md")]
     ///
@@ -1089,7 +1088,7 @@ pub(crate) mod macros {
     /// Derive macro generating an impl of the trait `Debug`.
     #[rustc_builtin_macro]
     #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
-    #[allow_internal_unstable(core_intrinsics, fmt_helpers_for_derive)]
+    #[allow_internal_unstable(core_intrinsics, fmt_helpers_for_derive, rustc_attrs)]
     pub macro Debug($item:item) {
         /* compiler built-in */
     }

tests/ui/attributes/trivial-field-reads-invalid-targets.rs

@@ -0,0 +1,23 @@
+//! Checks that `#[rustc_trivial_field_reads]` are not allowed due to invalid targets
+
+#![feature(rustc_attrs)]
+#![deny(dead_code)]
+
+trait Whatever {
+    #[rustc_trivial_field_reads] //~ERROR the `rustc_trivial_field_reads` attribute cannot be used on required trait methods
+    fn read(&self);
+}
+
+#[rustc_trivial_field_reads] //~ERROR the `rustc_trivial_field_reads` attribute cannot be used on traits
+trait Whatever1 {}
+
+#[rustc_trivial_field_reads] //~ERROR the `rustc_trivial_field_reads` attribute cannot be used on macro defs
+  macro_rules! number {
+      () => {
+          67
+      };
+  }
+
+fn main() {
+    let _ = number!();
+}

tests/ui/attributes/trivial-field-reads-invalid-targets.stderr

@@ -0,0 +1,26 @@
+error: the `rustc_trivial_field_reads` attribute cannot be used on required trait methods
+  --> $DIR/trivial-field-reads-invalid-targets.rs:7:7
+   |
+LL |     #[rustc_trivial_field_reads]
+   |       ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: the `rustc_trivial_field_reads` attribute can only be applied to functions with a body
+
+error: the `rustc_trivial_field_reads` attribute cannot be used on traits
+  --> $DIR/trivial-field-reads-invalid-targets.rs:11:3
+   |
+LL | #[rustc_trivial_field_reads]
+   |   ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: the `rustc_trivial_field_reads` attribute can only be applied to functions
+
+error: the `rustc_trivial_field_reads` attribute cannot be used on macro defs
+  --> $DIR/trivial-field-reads-invalid-targets.rs:14:3
+   |
+LL | #[rustc_trivial_field_reads]
+   |   ^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: the `rustc_trivial_field_reads` attribute can only be applied to functions
+
+error: aborting due to 3 previous errors
+