Reject implementing const Drop for types that are not const Destruct already

compiler/rustc_hir_analysis/src/check/always_applicable.rs

@@ -11,7 +11,7 @@ use rustc_infer::infer::{RegionResolutionError, TyCtxtInferExt};
 use rustc_infer::traits::{ObligationCause, ObligationCauseCode};
 use rustc_middle::span_bug;
 use rustc_middle::ty::util::CheckRegions;
-use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, TypingMode};
+use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, TypeVisitableExt, TypingMode};
 use rustc_trait_selection::regions::InferCtxtRegionExt;
 use rustc_trait_selection::traits::{self, ObligationCtxt};
 
@@ -65,6 +65,8 @@ pub(crate) fn check_drop_impl(
                 adt_to_impl_args,
             )?;
 
+            ensure_all_fields_are_const_destruct(tcx, drop_impl_did, adt_def.did())?;
+
             ensure_impl_predicates_are_implied_by_item_defn(
                 tcx,
                 drop_impl_did,
@@ -173,6 +175,64 @@ fn ensure_impl_params_and_item_params_correspond<'tcx>(
     Err(err.emit())
 }
 
+fn ensure_all_fields_are_const_destruct<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    impl_def_id: LocalDefId,
+    adt_def_id: DefId,
+) -> Result<(), ErrorGuaranteed> {
+    if !tcx.is_conditionally_const(impl_def_id) {
+        return Ok(());
+    }
+    let infcx = tcx.infer_ctxt().build(TypingMode::non_body_analysis());
+    let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
+
+    let impl_span = tcx.def_span(impl_def_id.to_def_id());
+    let env =
+        ty::EarlyBinder::bind(tcx.param_env(impl_def_id)).instantiate_identity().skip_norm_wip();
+    let args = ty::GenericArgs::identity_for_item(tcx, impl_def_id);
+    let destruct_trait = tcx.lang_items().destruct_trait().unwrap();
+    for field in tcx.adt_def(adt_def_id).all_fields() {
+        let field_ty = field.ty(tcx, args);
+        let cause = traits::ObligationCause::new(
+            tcx.def_span(field.did),
+            impl_def_id,
+            ObligationCauseCode::Misc,
+        );
+        ocx.register_obligation(traits::Obligation::new(
+            tcx,
+            cause,
+            env,
+            ty::ClauseKind::HostEffect(ty::HostEffectPredicate {
+                trait_ref: ty::TraitRef::new(tcx, destruct_trait, [field_ty]),
+                constness: ty::BoundConstness::Maybe,
+            }),
+        ));
+    }
+    ocx.evaluate_obligations_error_on_ambiguity()
+        .into_iter()
+        .map(|error| {
+            let ty::ClauseKind::HostEffect(eff) =
+                error.root_obligation.predicate.expect_clause().kind().no_bound_vars().unwrap()
+            else {
+                unreachable!()
+            };
+            let field_ty = eff.trait_ref.self_ty();
+            let diag = struct_span_code_err!(
+                tcx.dcx(),
+                error.root_obligation.cause.span,
+                E0367,
+                "`{field_ty}` does not implement `[const] Destruct`",
+            )
+            .with_span_note(impl_span, "required for this `Drop` impl");
+            if field_ty.has_param() {
+                // FIXME: suggest adding `[const] Destruct` by teaching
+                // `suggest_restricting_param_bound` about const traits.
+            }
+            Err(diag.emit())
+        })
+        .collect()
+}
+
 /// Confirms that all predicates defined on the `Drop` impl (`drop_impl_def_id`) are able to be
 /// proven from within `adt_def_id`'s environment. I.e. all the predicates on the impl are
 /// implied by the ADT being well formed.

tests/ui/consts/drop-impl-nonconst-drop-field.rs

@@ -0,0 +1,32 @@
+#![feature(const_trait_impl)]
+#![feature(const_destruct)]
+
+use std::marker::Destruct;
+
+struct NotConstDrop;
+
+impl Drop for NotConstDrop {
+    fn drop(&mut self) {}
+}
+
+struct ConstDrop(NotConstDrop);
+//~^ ERROR: `NotConstDrop` does not implement `[const] Destruct`
+
+impl const Drop for ConstDrop {
+    fn drop(&mut self) {}
+}
+
+struct ConstDrop2<T>(T);
+//~^ ERROR:  `T` does not implement `[const] Destruct`
+
+impl<T> const Drop for ConstDrop2<T> {
+    fn drop(&mut self) {}
+}
+
+struct ConstDrop3<T>(T);
+
+impl<T: [const] Destruct> const Drop for ConstDrop3<T> {
+    fn drop(&mut self) {}
+}
+
+fn main() {}

tests/ui/consts/drop-impl-nonconst-drop-field.stderr

@@ -0,0 +1,27 @@
+error[E0367]: `NotConstDrop` does not implement `[const] Destruct`
+  --> $DIR/drop-impl-nonconst-drop-field.rs:12:18
+   |
+LL | struct ConstDrop(NotConstDrop);
+   |                  ^^^^^^^^^^^^
+   |
+note: required for this `Drop` impl
+  --> $DIR/drop-impl-nonconst-drop-field.rs:15:1
+   |
+LL | impl const Drop for ConstDrop {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0367]: `T` does not implement `[const] Destruct`
+  --> $DIR/drop-impl-nonconst-drop-field.rs:19:22
+   |
+LL | struct ConstDrop2<T>(T);
+   |                      ^
+   |
+note: required for this `Drop` impl
+  --> $DIR/drop-impl-nonconst-drop-field.rs:22:1
+   |
+LL | impl<T> const Drop for ConstDrop2<T> {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0367`.

tests/ui/traits/const-traits/const-drop-fail.new_precise.stderr

@@ -1,5 +1,17 @@
+error[E0367]: `NonTrivialDrop` does not implement `[const] Destruct`
+  --> $DIR/const-drop-fail.rs:19:30
+   |
+LL | struct ConstImplWithDropGlue(NonTrivialDrop);
+   |                              ^^^^^^^^^^^^^^
+   |
+note: required for this `Drop` impl
+  --> $DIR/const-drop-fail.rs:22:1
+   |
+LL | impl const Drop for ConstImplWithDropGlue {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0277]: the trait bound `NonTrivialDrop: const Destruct` is not satisfied
-  --> $DIR/const-drop-fail.rs:34:5
+  --> $DIR/const-drop-fail.rs:35:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
@@ -8,13 +20,13 @@ LL |     NonTrivialDrop,
    |     ^^^^^^^^^^^^^^
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:25:19
+  --> $DIR/const-drop-fail.rs:26:19
    |
 LL | const fn check<T: [const] Destruct>(_: T) {}
    |                   ^^^^^^^^^^^^^^^^ required by this bound in `check`
 
 error[E0277]: the trait bound `NonTrivialDrop: const Destruct` is not satisfied
-  --> $DIR/const-drop-fail.rs:36:5
+  --> $DIR/const-drop-fail.rs:37:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
@@ -23,11 +35,12 @@ LL |     ConstImplWithDropGlue(NonTrivialDrop),
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:25:19
+  --> $DIR/const-drop-fail.rs:26:19
    |
 LL | const fn check<T: [const] Destruct>(_: T) {}
    |                   ^^^^^^^^^^^^^^^^ required by this bound in `check`
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0277`.
+Some errors have detailed explanations: E0277, E0367.
+For more information about an error, try `rustc --explain E0277`.

tests/ui/traits/const-traits/const-drop-fail.new_stock.stderr

@@ -1,5 +1,17 @@
+error[E0367]: `NonTrivialDrop` does not implement `[const] Destruct`
+  --> $DIR/const-drop-fail.rs:19:30
+   |
+LL | struct ConstImplWithDropGlue(NonTrivialDrop);
+   |                              ^^^^^^^^^^^^^^
+   |
+note: required for this `Drop` impl
+  --> $DIR/const-drop-fail.rs:22:1
+   |
+LL | impl const Drop for ConstImplWithDropGlue {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0277]: the trait bound `NonTrivialDrop: const Destruct` is not satisfied
-  --> $DIR/const-drop-fail.rs:34:5
+  --> $DIR/const-drop-fail.rs:35:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
@@ -8,13 +20,13 @@ LL |     NonTrivialDrop,
    |     ^^^^^^^^^^^^^^
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:25:19
+  --> $DIR/const-drop-fail.rs:26:19
    |
 LL | const fn check<T: [const] Destruct>(_: T) {}
    |                   ^^^^^^^^^^^^^^^^ required by this bound in `check`
 
 error[E0277]: the trait bound `NonTrivialDrop: const Destruct` is not satisfied
-  --> $DIR/const-drop-fail.rs:36:5
+  --> $DIR/const-drop-fail.rs:37:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
@@ -23,11 +35,12 @@ LL |     ConstImplWithDropGlue(NonTrivialDrop),
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:25:19
+  --> $DIR/const-drop-fail.rs:26:19
    |
 LL | const fn check<T: [const] Destruct>(_: T) {}
    |                   ^^^^^^^^^^^^^^^^ required by this bound in `check`
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0277`.
+Some errors have detailed explanations: E0277, E0367.
+For more information about an error, try `rustc --explain E0277`.

tests/ui/traits/const-traits/const-drop-fail.old_precise.stderr

@@ -1,5 +1,17 @@
+error[E0367]: `NonTrivialDrop` does not implement `[const] Destruct`
+  --> $DIR/const-drop-fail.rs:19:30
+   |
+LL | struct ConstImplWithDropGlue(NonTrivialDrop);
+   |                              ^^^^^^^^^^^^^^
+   |
+note: required for this `Drop` impl
+  --> $DIR/const-drop-fail.rs:22:1
+   |
+LL | impl const Drop for ConstImplWithDropGlue {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0277]: the trait bound `NonTrivialDrop: const Destruct` is not satisfied
-  --> $DIR/const-drop-fail.rs:34:5
+  --> $DIR/const-drop-fail.rs:35:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
@@ -8,13 +20,13 @@ LL |     NonTrivialDrop,
    |     ^^^^^^^^^^^^^^
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:25:19
+  --> $DIR/const-drop-fail.rs:26:19
    |
 LL | const fn check<T: [const] Destruct>(_: T) {}
    |                   ^^^^^^^^^^^^^^^^ required by this bound in `check`
 
 error[E0277]: the trait bound `NonTrivialDrop: const Destruct` is not satisfied
-  --> $DIR/const-drop-fail.rs:36:5
+  --> $DIR/const-drop-fail.rs:37:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
@@ -23,11 +35,12 @@ LL |     ConstImplWithDropGlue(NonTrivialDrop),
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:25:19
+  --> $DIR/const-drop-fail.rs:26:19
    |
 LL | const fn check<T: [const] Destruct>(_: T) {}
    |                   ^^^^^^^^^^^^^^^^ required by this bound in `check`
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0277`.
+Some errors have detailed explanations: E0277, E0367.
+For more information about an error, try `rustc --explain E0277`.

tests/ui/traits/const-traits/const-drop-fail.old_stock.stderr

@@ -1,5 +1,17 @@
+error[E0367]: `NonTrivialDrop` does not implement `[const] Destruct`
+  --> $DIR/const-drop-fail.rs:19:30
+   |
+LL | struct ConstImplWithDropGlue(NonTrivialDrop);
+   |                              ^^^^^^^^^^^^^^
+   |
+note: required for this `Drop` impl
+  --> $DIR/const-drop-fail.rs:22:1
+   |
+LL | impl const Drop for ConstImplWithDropGlue {
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
 error[E0277]: the trait bound `NonTrivialDrop: const Destruct` is not satisfied
-  --> $DIR/const-drop-fail.rs:34:5
+  --> $DIR/const-drop-fail.rs:35:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
@@ -8,13 +20,13 @@ LL |     NonTrivialDrop,
    |     ^^^^^^^^^^^^^^
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:25:19
+  --> $DIR/const-drop-fail.rs:26:19
    |
 LL | const fn check<T: [const] Destruct>(_: T) {}
    |                   ^^^^^^^^^^^^^^^^ required by this bound in `check`
 
 error[E0277]: the trait bound `NonTrivialDrop: const Destruct` is not satisfied
-  --> $DIR/const-drop-fail.rs:36:5
+  --> $DIR/const-drop-fail.rs:37:5
    |
 LL |         const _: () = check($exp);
    |                       ----- required by a bound introduced by this call
@@ -23,11 +35,12 @@ LL |     ConstImplWithDropGlue(NonTrivialDrop),
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
 note: required by a bound in `check`
-  --> $DIR/const-drop-fail.rs:25:19
+  --> $DIR/const-drop-fail.rs:26:19
    |
 LL | const fn check<T: [const] Destruct>(_: T) {}
    |                   ^^^^^^^^^^^^^^^^ required by this bound in `check`
 
-error: aborting due to 2 previous errors
+error: aborting due to 3 previous errors
 
-For more information about this error, try `rustc --explain E0277`.
+Some errors have detailed explanations: E0277, E0367.
+For more information about an error, try `rustc --explain E0277`.

tests/ui/traits/const-traits/const-drop-fail.rs

@@ -17,6 +17,7 @@ impl Drop for NonTrivialDrop {
 }
 
 struct ConstImplWithDropGlue(NonTrivialDrop);
+//~^ ERROR: `NonTrivialDrop` does not implement `[const] Destruct`
 
 impl const Drop for ConstImplWithDropGlue {
     fn drop(&mut self) {}

tests/ui/traits/const-traits/minicore-drop-fail.rs

@@ -19,7 +19,7 @@ const trait Foo {}
 impl Foo for () {}
 
 struct Conditional<T: Foo>(T);
-impl<T> const Drop for Conditional<T> where T: [const] Foo {
+impl<T> const Drop for Conditional<T> where T: [const] Foo + [const] Destruct {
     fn drop(&mut self) {}
 }