Add accurate targeting for imperfect derives diagnostic in E0277

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -4520,7 +4520,13 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
                             }
                         }
                         err.span_note(spans, msg);
-                        if derived && trait_name != "Copy" {
+                        if derived
+                            && self.is_truly_imperfect_derive(
+                                parent_trait_pred,
+                                predicate,
+                                param_env,
+                            )
+                        {
                             err.help(format!(
                                 "consider manually implementing `{trait_name}` to avoid undesired bounds caused by \"imperfect derives\"",
                             ));
@@ -6472,6 +6478,90 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
             _ => {}
         }
     }
+
+    /// Checks whether the field independently satisfies the trait bound, ignoring
+    /// the specific generic parameter that caused the original E0277 error.
+    fn is_truly_imperfect_derive(
+        &self,
+        parent_trait_pred: ty::PolyTraitClause<'tcx>,
+        predicate: ty::Predicate<'tcx>,
+        param_env: ty::ParamEnv<'tcx>,
+    ) -> bool {
+        let tcx = self.tcx;
+        let ty::Adt(adt_def, parent_args) = parent_trait_pred.skip_binder().self_ty().kind() else {
+            return false;
+        };
+        let Some(trait_clause) = predicate.as_trait_clause() else {
+            return false;
+        };
+        let failing_ty = trait_clause.skip_binder().self_ty();
+        let trait_def_id = parent_trait_pred.def_id();
+
+        let failing_adt_param_indices: FxHashSet<u32> = parent_args
+            .iter()
+            .enumerate()
+            .filter_map(|(idx, arg)| {
+                if let Some(t) = arg.as_type()
+                    && t == failing_ty
+                {
+                    Some(idx as u32)
+                } else {
+                    None
+                }
+            })
+            .collect();
+
+        if failing_adt_param_indices.is_empty() {
+            return false;
+        }
+
+        adt_def.all_fields().all(|field| {
+            let raw_field_ty = tcx.type_of(field.did).skip_binder();
+            // If the field type is exactly one of the failing parameters, it is not an imperfect derive.
+            if let ty::Param(p) = raw_field_ty.kind()
+                && failing_adt_param_indices.contains(&p.index)
+            {
+                return false;
+            }
+            // If the field type doesn't mention the parameter at all, it's independent.
+            if !raw_field_ty.walk().any(|arg| {
+                matches!(arg.as_type().map(|t| t.kind()), Some(ty::Param(p)) if failing_adt_param_indices.contains(&p.index))
+            }) {
+                return true;
+            }
+
+            self.probe(|_| {
+                // We keep the generic parameters that didn't fail as-is from the parent args,
+                // but replace the ones that did fail with fresh inference variable placeholders.
+                let fresh_args = ty::GenericArgs::for_item(tcx, adt_def.did(), |param, _| {
+                    if failing_adt_param_indices.contains(&param.index) {
+                        self.var_for_def(DUMMY_SP, param)
+                    } else {
+                        parent_args[param.index as usize]
+                    }
+                });
+
+                let field_ty = field.ty(tcx, fresh_args).skip_norm_wip();
+                // Substitute the field's type for the bound's `Self` type to check whether
+                // the field alone would satisfy the trait, independent of other generics.
+                let parent_trait_args = parent_trait_pred.skip_binder().trait_ref.args;
+                let trait_args = parent_trait_args.iter().map(|arg| {
+                    if arg.as_type() == Some(parent_trait_pred.skip_binder().self_ty()) {
+                        field_ty.into()
+                    } else {
+                        arg
+                    }
+                });
+                let obligation = Obligation::new(
+                    tcx,
+                    ObligationCause::dummy(),
+                    param_env,
+                    ty::TraitRef::new(tcx, trait_def_id, trait_args),
+                );
+                self.predicate_may_hold(&obligation)
+            })
+        })
+    }
 }
 
 /// Add a hint to add a missing borrow or remove an unnecessary one.

tests/ui/associated-types/issue-38821.stderr

@@ -108,8 +108,6 @@ LL | pub enum ColumnInsertValue<Col, Expr> where
 ...
 LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>,
    |                      ------------------------------------------------ unsatisfied trait bound
-   = help: consider manually implementing `Debug` to avoid undesired bounds caused by "imperfect derives"
-   = note: to learn more, visit <https://github.com/rust-lang/rust/issues/26925>
 help: consider further restricting the associated type
    |
 LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>, <Col as Expression>::SqlType: NotNull,
@@ -239,8 +237,6 @@ LL | pub enum ColumnInsertValue<Col, Expr> where
 ...
 LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>,
    |                      ------------------------------------------------ unsatisfied trait bound
-   = help: consider manually implementing `Clone` to avoid undesired bounds caused by "imperfect derives"
-   = note: to learn more, visit <https://github.com/rust-lang/rust/issues/26925>
 help: consider further restricting the associated type
    |
 LL |     Expr: Expression<SqlType=<Col::SqlType as IntoNullable>::Nullable>, <Col as Expression>::SqlType: NotNull,

tests/ui/const-generics/adt_const_params/unsizing-wfcheck-issue-126272.stderr

@@ -67,8 +67,6 @@ LL | #[derive(Debug, PartialEq, Eq, ConstParamTy)]
    |          ----- in this derive macro expansion
 LL | struct Bar<T>(T);
    |        ^^^ - unsatisfied trait bound
-   = help: consider manually implementing `Debug` to avoid undesired bounds caused by "imperfect derives"
-   = note: to learn more, visit <https://github.com/rust-lang/rust/issues/26925>
    = note: 2 redundant requirements hidden
    = note: required for `&&'static Bar<(dyn Debug + 'static)>` to implement `Debug`
    = note: required for the cast from `&&&'static Bar<(dyn Debug + 'static)>` to `&dyn Debug`
@@ -103,8 +101,6 @@ LL | #[derive(Debug, PartialEq, Eq, ConstParamTy)]
    |                            -- in this derive macro expansion
 LL | struct Bar<T>(T);
    |        ^^^ - type parameter would need to implement `Eq`
-   = help: consider manually implementing `Eq` to avoid undesired bounds caused by "imperfect derives"
-   = note: to learn more, visit <https://github.com/rust-lang/rust/issues/26925>
    = note: 1 redundant requirement hidden
    = note: required for `&'static Bar<dyn Debug>` to implement `Eq`
 note: required by a bound in `std::cmp::AssertParamIsEq`

tests/ui/derives/clone-copy/deriving-copyclone.stderr

@@ -38,8 +38,6 @@ LL | #[derive(Copy, Clone)]
    |                ----- in this derive macro expansion
 LL | struct B<T> {
    |        ^ - type parameter would need to implement `Clone`
-   = help: consider manually implementing `Clone` to avoid undesired bounds caused by "imperfect derives"
-   = note: to learn more, visit <https://github.com/rust-lang/rust/issues/26925>
 note: required by a bound in `is_clone`
   --> $DIR/deriving-copyclone.rs:19:16
    |

tests/ui/derives/imperfect-derive-advanced.rs

@@ -0,0 +1,42 @@
+// Test that the "imperfect derives" note is emitted for associated types,
+// `Rc<T>`, and coinductive types.
+
+use std::rc::Rc;
+
+trait Trait {
+    type Assoc: Clone;
+}
+
+#[derive(Clone)]
+struct AssocStruct<T: Trait> {
+    field: T::Assoc,
+}
+
+struct NonClone;
+impl Trait for NonClone {
+    type Assoc = u32;
+}
+
+#[derive(Clone)]
+struct RcStruct<T> {
+    field: Rc<T>,
+}
+
+#[derive(Clone)]
+struct List<T> {
+    value: Rc<T>,
+    next: Option<Box<List<T>>>,
+}
+
+fn require_clone<T: Clone>() {}
+
+fn main() {
+    require_clone::<AssocStruct<NonClone>>();
+    //~^ ERROR the trait bound `NonClone: Clone` is not satisfied
+
+    require_clone::<RcStruct<NonClone>>();
+    //~^ ERROR the trait bound `NonClone: Clone` is not satisfied
+
+    require_clone::<List<NonClone>>();
+    //~^ ERROR the trait bound `NonClone: Clone` is not satisfied
+}

tests/ui/derives/imperfect-derive-advanced.stderr

@@ -0,0 +1,81 @@
+error[E0277]: the trait bound `NonClone: Clone` is not satisfied
+  --> $DIR/imperfect-derive-advanced.rs:34:21
+   |
+LL |     require_clone::<AssocStruct<NonClone>>();
+   |                     ^^^^^^^^^^^^^^^^^^^^^ the trait `Clone` is not implemented for `NonClone`
+   |
+note: required for `AssocStruct<NonClone>` to implement `Clone`
+  --> $DIR/imperfect-derive-advanced.rs:11:8
+   |
+LL | #[derive(Clone)]
+   |          ----- in this derive macro expansion
+LL | struct AssocStruct<T: Trait> {
+   |        ^^^^^^^^^^^ - type parameter would need to implement `Clone`
+   = help: consider manually implementing `Clone` to avoid undesired bounds caused by "imperfect derives"
+   = note: to learn more, visit <https://github.com/rust-lang/rust/issues/26925>
+note: required by a bound in `require_clone`
+  --> $DIR/imperfect-derive-advanced.rs:31:21
+   |
+LL | fn require_clone<T: Clone>() {}
+   |                     ^^^^^ required by this bound in `require_clone`
+help: consider annotating `NonClone` with `#[derive(Clone)]`
+   |
+LL + #[derive(Clone)]
+LL | struct NonClone;
+   |
+
+error[E0277]: the trait bound `NonClone: Clone` is not satisfied
+  --> $DIR/imperfect-derive-advanced.rs:37:21
+   |
+LL |     require_clone::<RcStruct<NonClone>>();
+   |                     ^^^^^^^^^^^^^^^^^^ the trait `Clone` is not implemented for `NonClone`
+   |
+note: required for `RcStruct<NonClone>` to implement `Clone`
+  --> $DIR/imperfect-derive-advanced.rs:21:8
+   |
+LL | #[derive(Clone)]
+   |          ----- in this derive macro expansion
+LL | struct RcStruct<T> {
+   |        ^^^^^^^^ - type parameter would need to implement `Clone`
+   = help: consider manually implementing `Clone` to avoid undesired bounds caused by "imperfect derives"
+   = note: to learn more, visit <https://github.com/rust-lang/rust/issues/26925>
+note: required by a bound in `require_clone`
+  --> $DIR/imperfect-derive-advanced.rs:31:21
+   |
+LL | fn require_clone<T: Clone>() {}
+   |                     ^^^^^ required by this bound in `require_clone`
+help: consider annotating `NonClone` with `#[derive(Clone)]`
+   |
+LL + #[derive(Clone)]
+LL | struct NonClone;
+   |
+
+error[E0277]: the trait bound `NonClone: Clone` is not satisfied
+  --> $DIR/imperfect-derive-advanced.rs:40:21
+   |
+LL |     require_clone::<List<NonClone>>();
+   |                     ^^^^^^^^^^^^^^ the trait `Clone` is not implemented for `NonClone`
+   |
+note: required for `List<NonClone>` to implement `Clone`
+  --> $DIR/imperfect-derive-advanced.rs:26:8
+   |
+LL | #[derive(Clone)]
+   |          ----- in this derive macro expansion
+LL | struct List<T> {
+   |        ^^^^ - type parameter would need to implement `Clone`
+   = help: consider manually implementing `Clone` to avoid undesired bounds caused by "imperfect derives"
+   = note: to learn more, visit <https://github.com/rust-lang/rust/issues/26925>
+note: required by a bound in `require_clone`
+  --> $DIR/imperfect-derive-advanced.rs:31:21
+   |
+LL | fn require_clone<T: Clone>() {}
+   |                     ^^^^^ required by this bound in `require_clone`
+help: consider annotating `NonClone` with `#[derive(Clone)]`
+   |
+LL + #[derive(Clone)]
+LL | struct NonClone;
+   |
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/derives/imperfect-derive-copy.rs

@@ -0,0 +1,16 @@
+// Test that the "imperfect derives" note is emitted for Copy when the derive
+// bound is genuinely unnecessary (e.g., `PhantomData<T>`).
+
+use std::marker::PhantomData;
+
+#[derive(Copy, Clone)]
+struct X<T>(PhantomData<T>);
+
+struct Y; // does not implement Copy
+
+fn require_copy<T: Copy>(_t: T) {}
+
+fn main() {
+    require_copy(X::<Y>(PhantomData));
+    //~^ ERROR the trait bound `X<Y>: Copy` is not satisfied
+}

tests/ui/derives/imperfect-derive-copy.stderr

@@ -0,0 +1,30 @@
+error[E0277]: the trait bound `X<Y>: Copy` is not satisfied
+  --> $DIR/imperfect-derive-copy.rs:14:25
+   |
+LL |     require_copy(X::<Y>(PhantomData));
+   |     ------------        ^^^^^^^^^^^ the trait `Copy` is not implemented for `X<Y>`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required for `X<Y>` to implement `Copy`
+  --> $DIR/imperfect-derive-copy.rs:7:8
+   |
+LL | #[derive(Copy, Clone)]
+   |          ---- in this derive macro expansion
+LL | struct X<T>(PhantomData<T>);
+   |        ^ - type parameter would need to implement `Copy`
+   = help: consider manually implementing `Copy` to avoid undesired bounds caused by "imperfect derives"
+   = note: to learn more, visit <https://github.com/rust-lang/rust/issues/26925>
+note: required by a bound in `require_copy`
+  --> $DIR/imperfect-derive-copy.rs:11:20
+   |
+LL | fn require_copy<T: Copy>(_t: T) {}
+   |                    ^^^^ required by this bound in `require_copy`
+help: consider borrowing here
+   |
+LL |     require_copy(X::<Y>(&PhantomData));
+   |                         +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/derives/imperfect-derive-phantom.rs

@@ -0,0 +1,16 @@
+// Test that the "imperfect derives" note is emitted when the derive bound
+// is genuinely unnecessary (e.g., `PhantomData<T>`).
+
+use std::marker::PhantomData;
+
+#[derive(Clone)]
+struct S<T>(PhantomData<T>);
+
+struct X;
+
+fn require_clone<T: Clone>(_t: T) {}
+
+fn main() {
+    require_clone(S::<X>(PhantomData));
+    //~^ ERROR the trait bound `S<X>: Clone` is not satisfied
+}

tests/ui/derives/imperfect-derive-phantom.stderr

@@ -0,0 +1,30 @@
+error[E0277]: the trait bound `S<X>: Clone` is not satisfied
+  --> $DIR/imperfect-derive-phantom.rs:14:26
+   |
+LL |     require_clone(S::<X>(PhantomData));
+   |     -------------        ^^^^^^^^^^^ the trait `Clone` is not implemented for `S<X>`
+   |     |
+   |     required by a bound introduced by this call
+   |
+note: required for `S<X>` to implement `Clone`
+  --> $DIR/imperfect-derive-phantom.rs:7:8
+   |
+LL | #[derive(Clone)]
+   |          ----- in this derive macro expansion
+LL | struct S<T>(PhantomData<T>);
+   |        ^ - type parameter would need to implement `Clone`
+   = help: consider manually implementing `Clone` to avoid undesired bounds caused by "imperfect derives"
+   = note: to learn more, visit <https://github.com/rust-lang/rust/issues/26925>
+note: required by a bound in `require_clone`
+  --> $DIR/imperfect-derive-phantom.rs:11:21
+   |
+LL | fn require_clone<T: Clone>(_t: T) {}
+   |                     ^^^^^ required by this bound in `require_clone`
+help: consider borrowing here
+   |
+LL |     require_clone(S::<X>(&PhantomData));
+   |                          +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.

tests/ui/derives/redundant-derive-note-on-unimplemented.stderr

@@ -21,8 +21,6 @@ LL | #[derive(Debug)]
    |          ----- in this derive macro expansion
 LL | struct S<T>(T);
    |        ^ - type parameter would need to implement `Debug`
-   = help: consider manually implementing `Debug` to avoid undesired bounds caused by "imperfect derives"
-   = note: to learn more, visit <https://github.com/rust-lang/rust/issues/26925>
 help: consider annotating `X` with `#[derive(Debug)]`
    |
 LL + #[derive(Debug)]

tests/ui/proc-macro/issue-104884-trait-impl-sugg-err.stderr

@@ -44,8 +44,6 @@ LL | #[derive(PartialOrd, AddImpl)]
 ...
 LL | struct PriorityQueue<T>(BinaryHeap<PriorityQueueEntry<T>>);
    |        ^^^^^^^^^^^^^ - type parameter would need to implement `PartialOrd`
-   = help: consider manually implementing `PartialOrd` to avoid undesired bounds caused by "imperfect derives"
-   = note: to learn more, visit <https://github.com/rust-lang/rust/issues/26925>
 note: required by a bound in `Ord`
   --> $SRC_DIR/core/src/cmp.rs:LL:COL