Auto merge of #88558 - fee1-dead:const-drop, r=oli-obk

Const drop

The changes are pretty primitive at this point. But at least it works. ^-^

Problems with the current change that I can think of now:
 - [x] `~const Drop` shouldn't change anything in the non-const world.
 - [x] types that do not have drop glues shouldn't fail to satisfy `~const Drop` in const contexts. `struct S { a: u8, b: u16 }` This might not fail for `needs_non_const_drop`, but it will fail in `rustc_trait_selection`.
 - [x] The current change accepts types that have `const Drop` impls but have non-const `Drop` glue.

Fixes #88424.

Significant Changes:

- `~const Drop` is no longer treated as a normal trait bound. In non-const contexts, this bound has no effect, but in const contexts, this restricts the input type and all of its transitive fields to either a) have a `const Drop` impl or b) can be trivially dropped (i.e. no drop glue)
- `T: ~const Drop` will not be linted like `T: Drop`.
- Instead of recursing and iterating through the type in `rustc_mir::transform::check_consts`, we use the trait system to special case `~const Drop`. See [`rustc_trait_selection::...::candidate_assembly#assemble_const_drop_candidates`](https://github.com/fee1-dead/rust/blob/const-drop/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs#L817) and others.

Changes not related to `const Drop`ping and/or changes that are insignificant:

 - `Node.constness_for_typeck` no longer returns `hir::Constness::Const` for type aliases in traits. This was previously used to hack how we determine default bound constness for items. But because we now use an explicit opt-in, it is no longer needed.
 - Removed `is_const_impl_raw` query. We have `impl_constness`, and the only existing use of that query uses `HirId`, which means we can just operate it with hir.
 - `ty::Destructor` now has a field `constness`, which represents the constness of the destructor.

r? `@oli-obk`

4 files changed, 170 insertions, 37 deletions

diff --git a/compiler/rustc_trait_selection/src/traits/project.rs b/compiler/rustc_trait_selection/src/traits/project.rs
index 92db0ca2a7c..4f22543950c 100644
--- a/compiler/rustc_trait_selection/src/traits/project.rs
+++ b/compiler/rustc_trait_selection/src/traits/project.rs
@@ -1477,7 +1477,8 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
             }
             super::ImplSource::AutoImpl(..)
             | super::ImplSource::Builtin(..)
-            | super::ImplSource::TraitUpcasting(_) => {
+            | super::ImplSource::TraitUpcasting(_)
+            | super::ImplSource::ConstDrop(_) => {
                 // These traits have no associated types.
                 selcx.tcx().sess.delay_span_bug(
                     obligation.cause.span,
@@ -1549,7 +1550,8 @@ fn confirm_select_candidate<'cx, 'tcx>(
         | super::ImplSource::Param(..)
         | super::ImplSource::Builtin(..)
         | super::ImplSource::TraitUpcasting(_)
-        | super::ImplSource::TraitAlias(..) => {
+        | super::ImplSource::TraitAlias(..)
+        | super::ImplSource::ConstDrop(_) => {
             // we don't create Select candidates with this kind of resolution
             span_bug!(
                 obligation.cause.span,
diff --git a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
index e18828fec3f..6d64dc8254b 100644
--- a/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs
@@ -8,7 +8,7 @@
 use rustc_hir as hir;
 use rustc_infer::traits::{Obligation, SelectionError, TraitObligation};
 use rustc_middle::ty::print::with_no_trimmed_paths;
-use rustc_middle::ty::{self, TypeFoldable};
+use rustc_middle::ty::{self, Ty, TypeFoldable};
 use rustc_target::spec::abi::Abi;
 
 use crate::traits::coherence::Conflict;
@@ -277,6 +277,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             self.assemble_builtin_bound_candidates(sized_conditions, &mut candidates);
         } else if lang_items.unsize_trait() == Some(def_id) {
             self.assemble_candidates_for_unsizing(obligation, &mut candidates);
+        } else if lang_items.drop_trait() == Some(def_id)
+            && obligation.predicate.skip_binder().constness == ty::BoundConstness::ConstIfConst
+        {
+            if self.is_in_const_context {
+                self.assemble_const_drop_candidates(obligation, &mut candidates)?;
+            } else {
+                debug!("passing ~const Drop bound; in non-const context");
+                // `~const Drop` when we are not in a const context has no effect.
+                candidates.vec.push(ConstDropCandidate)
+            }
         } else {
             if lang_items.clone_trait() == Some(def_id) {
                 // Same builtin conditions as `Copy`, i.e., every type which has builtin support
@@ -803,4 +813,118 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             }
         }
     }
+
+    fn assemble_const_drop_candidates(
+        &mut self,
+        obligation: &TraitObligation<'tcx>,
+        candidates: &mut SelectionCandidateSet<'tcx>,
+    ) -> Result<(), SelectionError<'tcx>> {
+        let mut stack: Vec<(Ty<'tcx>, usize)> = vec![(obligation.self_ty().skip_binder(), 0)];
+
+        while let Some((ty, depth)) = stack.pop() {
+            let mut noreturn = false;
+
+            self.check_recursion_depth(depth, obligation)?;
+            let mut copy_candidates = SelectionCandidateSet { vec: Vec::new(), ambiguous: false };
+            let mut copy_obligation =
+                obligation.with(obligation.predicate.rebind(ty::TraitPredicate {
+                    trait_ref: ty::TraitRef {
+                        def_id: self.tcx().require_lang_item(hir::LangItem::Copy, None),
+                        substs: self.tcx().mk_substs_trait(ty, &[]),
+                    },
+                    constness: ty::BoundConstness::NotConst,
+                }));
+            copy_obligation.recursion_depth = depth + 1;
+            self.assemble_candidates_from_impls(&copy_obligation, &mut copy_candidates);
+            let copy_conditions = self.copy_clone_conditions(&copy_obligation);
+            self.assemble_builtin_bound_candidates(copy_conditions, &mut copy_candidates);
+            if !copy_candidates.vec.is_empty() {
+                noreturn = true;
+            }
+            debug!(?copy_candidates.vec, "assemble_const_drop_candidates - copy");
+
+            match ty.kind() {
+                ty::Int(_)
+                | ty::Uint(_)
+                | ty::Float(_)
+                | ty::Infer(ty::IntVar(_))
+                | ty::Infer(ty::FloatVar(_))
+                | ty::FnPtr(_)
+                | ty::Never
+                | ty::Ref(..)
+                | ty::FnDef(..)
+                | ty::RawPtr(_)
+                | ty::Bool
+                | ty::Char
+                | ty::Str
+                | ty::Foreign(_) => {} // Do nothing. These types satisfy `const Drop`.
+
+                ty::Adt(def, subst) => {
+                    let mut set = SelectionCandidateSet { vec: Vec::new(), ambiguous: false };
+                    self.assemble_candidates_from_impls(
+                        &obligation.with(obligation.predicate.map_bound(|mut pred| {
+                            pred.trait_ref.substs = self.tcx().mk_substs_trait(ty, &[]);
+                            pred
+                        })),
+                        &mut set,
+                    );
+                    stack.extend(def.all_fields().map(|f| (f.ty(self.tcx(), subst), depth + 1)));
+
+                    debug!(?set.vec, "assemble_const_drop_candidates - ty::Adt");
+                    if set.vec.into_iter().any(|candidate| {
+                        if let SelectionCandidate::ImplCandidate(did) = candidate {
+                            matches!(self.tcx().impl_constness(did), hir::Constness::NotConst)
+                        } else {
+                            false
+                        }
+                    }) {
+                        if !noreturn {
+                            // has non-const Drop
+                            return Ok(());
+                        }
+                        debug!("not returning");
+                    }
+                }
+
+                ty::Array(ty, _) => stack.push((ty, depth + 1)),
+
+                ty::Tuple(_) => stack.extend(ty.tuple_fields().map(|t| (t, depth + 1))),
+
+                ty::Closure(_, substs) => {
+                    stack.extend(substs.as_closure().upvar_tys().map(|t| (t, depth + 1)))
+                }
+
+                ty::Generator(_, substs, _) => {
+                    let substs = substs.as_generator();
+                    stack.extend(substs.upvar_tys().map(|t| (t, depth + 1)));
+                    stack.push((substs.witness(), depth + 1));
+                }
+
+                ty::GeneratorWitness(tys) => stack.extend(
+                    self.tcx().erase_late_bound_regions(*tys).iter().map(|t| (t, depth + 1)),
+                ),
+
+                ty::Slice(ty) => stack.push((ty, depth + 1)),
+
+                ty::Opaque(..)
+                | ty::Dynamic(..)
+                | ty::Error(_)
+                | ty::Bound(..)
+                | ty::Infer(_)
+                | ty::Placeholder(_)
+                | ty::Projection(..)
+                | ty::Param(..) => {
+                    if !noreturn {
+                        return Ok(());
+                    }
+                    debug!("not returning");
+                }
+            }
+            debug!(?stack, "assemble_const_drop_candidates - in loop");
+        }
+        // all types have passed.
+        candidates.vec.push(ConstDropCandidate);
+
+        Ok(())
+    }
 }
diff --git a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
index 6fae8173985..3b6555de912 100644
--- a/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/confirmation.rs
@@ -28,7 +28,7 @@ use crate::traits::TraitNotObjectSafe;
 use crate::traits::VtblSegment;
 use crate::traits::{BuiltinDerivedObligation, ImplDerivedObligation};
 use crate::traits::{
-    ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData,
+    ImplSourceAutoImplData, ImplSourceBuiltinData, ImplSourceClosureData, ImplSourceConstDropData,
     ImplSourceDiscriminantKindData, ImplSourceFnPointerData, ImplSourceGeneratorData,
     ImplSourceObjectData, ImplSourcePointeeData, ImplSourceTraitAliasData,
     ImplSourceTraitUpcastingData, ImplSourceUserDefinedData,
@@ -124,6 +124,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
                 Ok(ImplSource::TraitUpcasting(data))
             }
+
+            ConstDropCandidate => Ok(ImplSource::ConstDrop(ImplSourceConstDropData)),
         }
     }
 
diff --git a/compiler/rustc_trait_selection/src/traits/select/mod.rs b/compiler/rustc_trait_selection/src/traits/select/mod.rs
index 8adf9015933..7b948a0939f 100644
--- a/compiler/rustc_trait_selection/src/traits/select/mod.rs
+++ b/compiler/rustc_trait_selection/src/traits/select/mod.rs
@@ -1038,19 +1038,15 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         it.for_each(|o| o.recursion_depth = cmp::max(min_depth, o.recursion_depth) + 1);
     }
 
-    /// Checks that the recursion limit has not been exceeded.
-    ///
-    /// The weird return type of this function allows it to be used with the `try` (`?`)
-    /// operator within certain functions.
-    fn check_recursion_limit<T: Display + TypeFoldable<'tcx>, V: Display + TypeFoldable<'tcx>>(
+    fn check_recursion_depth<T: Display + TypeFoldable<'tcx>>(
         &self,
-        obligation: &Obligation<'tcx, T>,
-        error_obligation: &Obligation<'tcx, V>,
+        depth: usize,
+        error_obligation: &Obligation<'tcx, T>,
     ) -> Result<(), OverflowError> {
-        if !self.infcx.tcx.recursion_limit().value_within_limit(obligation.recursion_depth) {
+        if !self.infcx.tcx.recursion_limit().value_within_limit(depth) {
             match self.query_mode {
                 TraitQueryMode::Standard => {
-                    self.infcx().report_overflow_error(error_obligation, true);
+                    self.infcx.report_overflow_error(error_obligation, true);
                 }
                 TraitQueryMode::Canonical => {
                     return Err(OverflowError);
@@ -1060,6 +1056,18 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         Ok(())
     }
 
+    /// Checks that the recursion limit has not been exceeded.
+    ///
+    /// The weird return type of this function allows it to be used with the `try` (`?`)
+    /// operator within certain functions.
+    fn check_recursion_limit<T: Display + TypeFoldable<'tcx>, V: Display + TypeFoldable<'tcx>>(
+        &self,
+        obligation: &Obligation<'tcx, T>,
+        error_obligation: &Obligation<'tcx, V>,
+    ) -> Result<(), OverflowError> {
+        self.check_recursion_depth(obligation.recursion_depth, error_obligation)
+    }
+
     fn in_task<OP, R>(&mut self, op: OP) -> (R, DepNodeIndex)
     where
         OP: FnOnce(&mut Self) -> R,
@@ -1079,28 +1087,23 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let tcx = self.tcx();
         // Respect const trait obligations
         if self.is_trait_predicate_const(obligation.predicate.skip_binder()) {
-            if Some(obligation.predicate.skip_binder().trait_ref.def_id)
-                != tcx.lang_items().sized_trait()
-            // const Sized bounds are skipped
-            {
-                match candidate {
-                    // const impl
-                    ImplCandidate(def_id)
-                        if tcx.impl_constness(def_id) == hir::Constness::Const => {}
-                    // const param
-                    ParamCandidate(ty::ConstnessAnd {
-                        constness: ty::BoundConstness::ConstIfConst,
-                        ..
-                    }) => {}
-                    // auto trait impl
-                    AutoImplCandidate(..) => {}
-                    // generator, this will raise error in other places
-                    // or ignore error with const_async_blocks feature
-                    GeneratorCandidate => {}
-                    _ => {
-                        // reject all other types of candidates
-                        return Err(Unimplemented);
-                    }
+            match candidate {
+                // const impl
+                ImplCandidate(def_id) if tcx.impl_constness(def_id) == hir::Constness::Const => {}
+                // const param
+                ParamCandidate(ty::ConstnessAnd {
+                    constness: ty::BoundConstness::ConstIfConst,
+                    ..
+                }) => {}
+                // auto trait impl
+                AutoImplCandidate(..) => {}
+                // generator, this will raise error in other places
+                // or ignore error with const_async_blocks feature
+                GeneratorCandidate => {}
+                ConstDropCandidate => {}
+                _ => {
+                    // reject all other types of candidates
+                    return Err(Unimplemented);
                 }
             }
         }
@@ -1476,14 +1479,16 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             (
                 BuiltinCandidate { has_nested: false }
                 | DiscriminantKindCandidate
-                | PointeeCandidate,
+                | PointeeCandidate
+                | ConstDropCandidate,
                 _,
             ) => true,
             (
                 _,
                 BuiltinCandidate { has_nested: false }
                 | DiscriminantKindCandidate
-                | PointeeCandidate,
+                | PointeeCandidate
+                | ConstDropCandidate,
             ) => false,
 
             (ParamCandidate(other), ParamCandidate(victim)) => {