Detect unused structs which derived Default

compiler/rustc_passes/src/dead.rs

@@ -400,6 +400,21 @@ impl<'tcx> MarkSymbolVisitor<'tcx> {
                 return false;
             }
 
+            // don't ignore the impl Default for Enum,
+            // cause we don't know which variant is constructed
+            if let Some(local_impl_of) = impl_of.as_local()
+                && let Some(local_def_id) = def_id.as_local()
+                && let TyKind::Path(hir::QPath::Resolved(_, path)) =
+                    self.tcx.hir().expect_item(local_impl_of).expect_impl().self_ty.kind
+                && let Res::Def(DefKind::Enum, did) = path.res
+                && did.is_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)
+            {
+                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)
             {

library/core/src/default.rs

@@ -103,6 +103,7 @@ use crate::ascii::Char as AsciiChar;
 /// ```
 #[cfg_attr(not(test), rustc_diagnostic_item = "Default")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[rustc_trivial_field_reads]
 pub trait Default: Sized {
     /// Returns the "default value" for a type.
     ///

tests/ui/deriving/deriving-default-enum.rs

@@ -22,6 +22,6 @@ enum MyOption<T> {
 }
 
 fn main() {
-    assert_eq!(Foo::default(), Foo::Alpha);
+    assert!(matches!(Foo::default(), Foo::Alpha));
     assert!(matches!(MyOption::<NotDefault>::default(), MyOption::None));
 }

tests/ui/lint/dead-code/unused-struct-derive-default.rs

@@ -0,0 +1,11 @@
+#![deny(dead_code)]
+
+#[derive(Default)]
+struct T; //~ ERROR struct `T` is never constructed
+
+#[derive(Default)]
+struct Used;
+
+fn main() {
+    let _x: Used = Default::default();
+}

tests/ui/lint/dead-code/unused-struct-derive-default.stderr

@@ -0,0 +1,15 @@
+error: struct `T` is never constructed
+  --> $DIR/unused-struct-derive-default.rs:4:8
+   |
+LL | struct T;
+   |        ^
+   |
+   = note: `T` has a derived impl for the trait `Default`, but this is intentionally ignored during dead code analysis
+note: the lint level is defined here
+  --> $DIR/unused-struct-derive-default.rs:1:9
+   |
+LL | #![deny(dead_code)]
+   |         ^^^^^^^^^
+
+error: aborting due to 1 previous error
+