From e3aa9809b85f199083133760781ce0cb5de46d9a Mon Sep 17 00:00:00 2001
From: mu001999 <mu001999@outlook.com>
Date: Tue, 18 Jun 2024 22:06:08 +0800
Subject: [PATCH] Detect unused structs which derived Default

---
 compiler/rustc_passes/src/dead.rs             | 25 +++++++++++++++++++
 library/alloc/src/sync/tests.rs               |  2 +-
 library/core/src/default.rs                   |  1 +
 tests/ui/deriving/deriving-default-enum.rs    |  2 +-
 .../dead-code/unused-struct-derive-default.rs | 25 +++++++++++++++++++
 .../unused-struct-derive-default.stderr       | 24 ++++++++++++++++++
 6 files changed, 77 insertions(+), 2 deletions(-)
 create mode 100644 tests/ui/lint/dead-code/unused-struct-derive-default.rs
 create mode 100644 tests/ui/lint/dead-code/unused-struct-derive-default.stderr

diff --git a/compiler/rustc_passes/src/dead.rs b/compiler/rustc_passes/src/dead.rs
index 2cb3c5d8965c8..a188a1c256331 100644
--- a/compiler/rustc_passes/src/dead.rs
+++ b/compiler/rustc_passes/src/dead.rs
@@ -400,6 +400,31 @@ 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)
             {
diff --git a/library/alloc/src/sync/tests.rs b/library/alloc/src/sync/tests.rs
index 49eae718c1690..1b123aa58f205 100644
--- a/library/alloc/src/sync/tests.rs
+++ b/library/alloc/src/sync/tests.rs
@@ -396,7 +396,7 @@ fn show_arc() {
 
 // Make sure deriving works with Arc<T>
 #[derive(Eq, Ord, PartialEq, PartialOrd, Clone, Debug, Default)]
-struct Foo {
+struct _Foo {
     inner: Arc<i32>,
 }
 
diff --git a/library/core/src/default.rs b/library/core/src/default.rs
index 4524b352ec817..5cacedcb241a5 100644
--- a/library/core/src/default.rs
+++ b/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")]
+#[cfg_attr(not(bootstrap), rustc_trivial_field_reads)]
 pub trait Default: Sized {
     /// Returns the "default value" for a type.
     ///
diff --git a/tests/ui/deriving/deriving-default-enum.rs b/tests/ui/deriving/deriving-default-enum.rs
index 96eba258c97e8..6b59f39a67d48 100644
--- a/tests/ui/deriving/deriving-default-enum.rs
+++ b/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));
 }
diff --git a/tests/ui/lint/dead-code/unused-struct-derive-default.rs b/tests/ui/lint/dead-code/unused-struct-derive-default.rs
new file mode 100644
index 0000000000000..330ad32dd5709
--- /dev/null
+++ b/tests/ui/lint/dead-code/unused-struct-derive-default.rs
@@ -0,0 +1,25 @@
+#![deny(dead_code)]
+
+#[derive(Default)]
+struct T; //~ ERROR struct `T` is never constructed
+
+#[derive(Default)]
+struct Used;
+
+#[derive(Default)]
+enum E {
+    #[default]
+    A,
+    B, //~ ERROR variant `B` is never constructed
+}
+
+// external crate can call T2::default() to construct T2,
+// so that no warnings for pub adts
+#[derive(Default)]
+pub struct T2 {
+    _unread: i32,
+}
+
+fn main() {
+    let _x: Used = Default::default();
+}
diff --git a/tests/ui/lint/dead-code/unused-struct-derive-default.stderr b/tests/ui/lint/dead-code/unused-struct-derive-default.stderr
new file mode 100644
index 0000000000000..bbb0bd7be7064
--- /dev/null
+++ b/tests/ui/lint/dead-code/unused-struct-derive-default.stderr
@@ -0,0 +1,24 @@
+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: variant `B` is never constructed
+  --> $DIR/unused-struct-derive-default.rs:13:5
+   |
+LL | enum E {
+   |      - variant in this enum
+...
+LL |     B,
+   |     ^
+
+error: aborting due to 2 previous errors
+