Rollup merge of #126302 - mu001999-contrib:ignore/default, r=michaelwoerister

Detect unused structs which derived Default

<!--
If this PR is related to an unstable feature or an otherwise tracked effort,
please link to the relevant tracking issue here. If you don't know of a related
tracking issue or there are none, feel free to ignore this.

This PR will get automatically assigned to a reviewer. In case you would like
a specific user to review your work, you can assign it to them by using

    r​? <reviewer name>
-->

Fixes #98871

1 files changed, 25 insertions, 0 deletions

diff --git a/compiler/rustc_passes/src/dead.rs b/compiler/rustc_passes/src/dead.rs
index 7c7700dd859..2c86949685b 100644
--- a/compiler/rustc_passes/src/dead.rs
+++ b/compiler/rustc_passes/src/dead.rs
@@ -399,6 +399,31 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
                 return false;
             }
 
+            // don't ignore impls for Enums and pub Structs whose methods don't have self receiver,
+            // cause external crate may call such methods to construct values of these types
+            if let Some(local_impl_of) = impl_of.as_local()
+                && let Some(local_def_id) = def_id.as_local()
+                && let Some(fn_sig) =
+                    self.tcx.hir().fn_sig_by_hir_id(self.tcx.local_def_id_to_hir_id(local_def_id))
+                && matches!(fn_sig.decl.implicit_self, hir::ImplicitSelfKind::None)
+                && let TyKind::Path(hir::QPath::Resolved(_, path)) =
+                    self.tcx.hir().expect_item(local_impl_of).expect_impl().self_ty.kind
+                && let Res::Def(def_kind, did) = path.res
+            {
+                match def_kind {
+                    // for example, #[derive(Default)] pub struct T(i32);
+                    // external crate can call T::default() to construct T,
+                    // so that don't ignore impl Default for pub Enum and Structs
+                    DefKind::Struct | DefKind::Union if self.tcx.visibility(did).is_public() => {
+                        return false;
+                    }
+                    // don't ignore impl Default for Enums,
+                    // cause we don't know which variant is constructed
+                    DefKind::Enum => return false,
+                    _ => (),
+                };
+            }
+
             if let Some(trait_of) = self.tcx.trait_id_of_impl(impl_of)
                 && self.tcx.has_attr(trait_of, sym::rustc_trivial_field_reads)
             {