clean up ADT sized constraint computation

Author: Lukas Markeffsky

compiler/rustc_middle/src/query/mod.rs

@@ -703,8 +703,8 @@ rustc_queries! {
         separate_provide_extern
     }
 
-    query adt_sized_constraint(key: DefId) -> ty::EarlyBinder<&'tcx ty::List<Ty<'tcx>>> {
-        desc { |tcx| "computing `Sized` constraints for `{}`", tcx.def_path_str(key) }
+    query adt_sized_constraint(key: DefId) -> Option<ty::EarlyBinder<Ty<'tcx>>> {
+        desc { |tcx| "computing `Sized` constraint for `{}`", tcx.def_path_str(key) }
     }
 
     query adt_dtorck_constraint(

compiler/rustc_middle/src/ty/adt.rs

@@ -590,10 +590,10 @@ impl<'tcx> AdtDef<'tcx> {
         tcx.adt_destructor(self.did())
     }
 
-    /// Returns a list of types such that `Self: Sized` if and only if that
-    /// type is `Sized`, or `ty::Error` if this type has a recursive layout.
-    pub fn sized_constraint(self, tcx: TyCtxt<'tcx>) -> ty::EarlyBinder<&'tcx ty::List<Ty<'tcx>>> {
-        tcx.adt_sized_constraint(self.did())
+    /// Returns a type such that `Self: Sized` if and only if that type is `Sized`,
+    /// or `None` if the type is always sized.
+    pub fn sized_constraint(self, tcx: TyCtxt<'tcx>) -> Option<ty::EarlyBinder<Ty<'tcx>>> {
+        if self.is_struct() { tcx.adt_sized_constraint(self.did()) } else { None }
     }
 }
 

compiler/rustc_middle/src/ty/sty.rs

@@ -2490,7 +2490,7 @@ impl<'tcx> Ty<'tcx> {
 
             ty::Tuple(tys) => tys.iter().all(|ty| ty.is_trivially_sized(tcx)),
 
-            ty::Adt(def, _args) => def.sized_constraint(tcx).skip_binder().is_empty(),
+            ty::Adt(def, _args) => def.sized_constraint(tcx).is_none(),
 
             ty::Alias(..) | ty::Param(_) | ty::Placeholder(..) | ty::Bound(..) => false,
 

compiler/rustc_trait_selection/src/solve/assembly/structural_traits.rs

@@ -157,10 +157,20 @@ pub(in crate::solve) fn instantiate_constituent_tys_for_sized_trait<'tcx>(
         // impl Sized for (T1, T2, .., Tn) where T1: Sized, T2: Sized, .. Tn: Sized
         ty::Tuple(tys) => Ok(tys.iter().map(ty::Binder::dummy).collect()),
 
-        // impl Sized for Adt where T: Sized forall T in field types
+        // impl Sized for Adt<Args...> where sized_constraint(Adt)<Args...>: Sized
+        //   `sized_constraint(Adt)` is the deepest struct trail that can be determined
+        //   by the definition of `Adt`, independent of the generic args.
+        // impl Sized for Adt<Args...> if sized_constraint(Adt) == None
+        //   As a performance optimization, `sized_constraint(Adt)` can return `None`
+        //   if the ADTs definition implies that it is sized by for all possible args.
+        //   In this case, the builtin impl will have no nested subgoals. This is a
+        //   "best effort" optimization and `sized_constraint` may return `Some`, even
+        //   if the ADT is sized for all possible args.
         ty::Adt(def, args) => {
             let sized_crit = def.sized_constraint(ecx.tcx());
-            Ok(sized_crit.iter_instantiated(ecx.tcx(), args).map(ty::Binder::dummy).collect())
+            Ok(sized_crit.map_or_else(Vec::new, |ty| {
+                vec![ty::Binder::dummy(ty.instantiate(ecx.tcx(), args))]
+            }))
         }
     }
 }

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -2120,11 +2120,9 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             ty::Adt(def, args) => {
                 let sized_crit = def.sized_constraint(self.tcx());
                 // (*) binder moved here
-                Where(
-                    obligation
-                        .predicate
-                        .rebind(sized_crit.iter_instantiated(self.tcx(), args).collect()),
-                )
+                Where(obligation.predicate.rebind(
+                    sized_crit.map_or_else(Vec::new, |ty| vec![ty.instantiate(self.tcx(), args)]),
+                ))
             }
 
             ty::Alias(..) | ty::Param(_) | ty::Placeholder(..) => None,

compiler/rustc_ty_utils/src/ty.rs

@@ -1,78 +1,59 @@
 use rustc_data_structures::fx::FxHashSet;
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
+use rustc_hir::LangItem;
 use rustc_index::bit_set::BitSet;
 use rustc_middle::query::Providers;
-use rustc_middle::ty::{self, EarlyBinder, Ty, TyCtxt, TypeVisitor};
+use rustc_middle::ty::{self, EarlyBinder, Ty, TyCtxt, TypeVisitableExt, TypeVisitor};
 use rustc_middle::ty::{ToPredicate, TypeSuperVisitable, TypeVisitable};
 use rustc_span::def_id::{DefId, LocalDefId, CRATE_DEF_ID};
 use rustc_span::DUMMY_SP;
 use rustc_trait_selection::traits;
 
-fn sized_constraint_for_ty<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    adtdef: ty::AdtDef<'tcx>,
-    ty: Ty<'tcx>,
-) -> Vec<Ty<'tcx>> {
+#[instrument(level = "debug", skip(tcx), ret)]
+fn sized_constraint_for_ty<'tcx>(tcx: TyCtxt<'tcx>, ty: Ty<'tcx>) -> Option<Ty<'tcx>> {
     use rustc_type_ir::TyKind::*;
 
-    let result = match ty.kind() {
-        Bool | Char | Int(..) | Uint(..) | Float(..) | RawPtr(..) | Ref(..) | FnDef(..)
-        | FnPtr(_) | Array(..) | Closure(..) | CoroutineClosure(..) | Coroutine(..) | Never => {
-            vec![]
-        }
-
-        Str | Dynamic(..) | Slice(_) | Foreign(..) | Error(_) | CoroutineWitness(..) => {
-            // these are never sized - return the target type
-            vec![ty]
-        }
-
-        Tuple(tys) => match tys.last() {
-            None => vec![],
-            Some(&ty) => sized_constraint_for_ty(tcx, adtdef, ty),
-        },
-
+    match ty.kind() {
+        // these are always sized
+        Bool
+        | Char
+        | Int(..)
+        | Uint(..)
+        | Float(..)
+        | RawPtr(..)
+        | Ref(..)
+        | FnDef(..)
+        | FnPtr(..)
+        | Array(..)
+        | Closure(..)
+        | CoroutineClosure(..)
+        | Coroutine(..)
+        | CoroutineWitness(..)
+        | Never
+        | Dynamic(_, _, ty::DynStar) => None,
+
+        // these are never sized
+        Str | Slice(..) | Dynamic(_, _, ty::Dyn) | Foreign(..) => Some(ty),
+
+        Tuple(tys) => tys.last().and_then(|&ty| sized_constraint_for_ty(tcx, ty)),
+
+        // recursive case
         Adt(adt, args) => {
-            // recursive case
-            let adt_tys = adt.sized_constraint(tcx);
-            debug!("sized_constraint_for_ty({:?}) intermediate = {:?}", ty, adt_tys);
-            adt_tys
-                .iter_instantiated(tcx, args)
-                .flat_map(|ty| sized_constraint_for_ty(tcx, adtdef, ty))
-                .collect()
+            let intermediate = adt.sized_constraint(tcx);
+            intermediate.and_then(|intermediate| {
+                let ty = intermediate.instantiate(tcx, args);
+                sized_constraint_for_ty(tcx, ty)
+            })
         }
 
-        Alias(..) => {
-            // must calculate explicitly.
-            // FIXME: consider special-casing always-Sized projections
-            vec![ty]
-        }
-
-        Param(..) => {
-            // perf hack: if there is a `T: Sized` bound, then
-            // we know that `T` is Sized and do not need to check
-            // it on the impl.
-
-            let Some(sized_trait_def_id) = tcx.lang_items().sized_trait() else { return vec![ty] };
-            let predicates = tcx.predicates_of(adtdef.did()).predicates;
-            if predicates.iter().any(|(p, _)| {
-                p.as_trait_clause().is_some_and(|trait_pred| {
-                    trait_pred.def_id() == sized_trait_def_id
-                        && trait_pred.self_ty().skip_binder() == ty
-                })
-            }) {
-                vec![]
-            } else {
-                vec![ty]
-            }
-        }
+        // these can be sized or unsized
+        Param(..) | Alias(..) | Error(_) => Some(ty),
 
         Placeholder(..) | Bound(..) | Infer(..) => {
-            bug!("unexpected type `{:?}` in sized_constraint_for_ty", ty)
+            bug!("unexpected type `{ty:?}` in sized_constraint_for_ty")
         }
-    };
-    debug!("sized_constraint_for_ty({:?}) = {:?}", ty, result);
-    result
+    }
 }
 
 fn defaultness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Defaultness {
@@ -90,29 +71,45 @@ fn defaultness(tcx: TyCtxt<'_>, def_id: LocalDefId) -> hir::Defaultness {
 ///
 /// In fact, there are only a few options for the types in the constraint:
 ///     - an obviously-unsized type
-///     - a type parameter or projection whose Sizedness can't be known
-///     - a tuple of type parameters or projections, if there are multiple
-///       such.
-///     - an Error, if a type is infinitely sized
+///     - a type parameter or projection whose sizedness can't be known
+#[instrument(level = "debug", skip(tcx), ret)]
 fn adt_sized_constraint<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: DefId,
-) -> ty::EarlyBinder<&'tcx ty::List<Ty<'tcx>>> {
+) -> Option<ty::EarlyBinder<Ty<'tcx>>> {
     if let Some(def_id) = def_id.as_local() {
-        if let ty::Representability::Infinite(guar) = tcx.representability(def_id) {
-            return ty::EarlyBinder::bind(tcx.mk_type_list(&[Ty::new_error(tcx, guar)]));
+        if let ty::Representability::Infinite(_) = tcx.representability(def_id) {
+            return None;
         }
     }
     let def = tcx.adt_def(def_id);
 
-    let result =
-        tcx.mk_type_list_from_iter(def.variants().iter().filter_map(|v| v.tail_opt()).flat_map(
-            |f| sized_constraint_for_ty(tcx, def, tcx.type_of(f.did).instantiate_identity()),
-        ));
+    if !def.is_struct() {
+        bug!("`adt_sized_constraint` called on non-struct type: {def:?}");
+    }
+
+    let tail_def = def.non_enum_variant().tail_opt()?;
+    let tail_ty = tcx.type_of(tail_def.did).instantiate_identity();
 
-    debug!("adt_sized_constraint: {:?} => {:?}", def, result);
+    let constraint_ty = sized_constraint_for_ty(tcx, tail_ty)?;
+    if constraint_ty.references_error() {
+        return None;
+    }
+
+    // perf hack: if there is a `constraint_ty: Sized` bound, then we know
+    // that the type is sized and do not need to check it on the impl.
+    let sized_trait_def_id = tcx.require_lang_item(LangItem::Sized, None);
+    let predicates = tcx.predicates_of(def.did()).predicates;
+    if predicates.iter().any(|(p, _)| {
+        p.as_trait_clause().is_some_and(|trait_pred| {
+            trait_pred.def_id() == sized_trait_def_id
+                && trait_pred.self_ty().skip_binder() == constraint_ty
+        })
+    }) {
+        return None;
+    }
 
-    ty::EarlyBinder::bind(result)
+    Some(ty::EarlyBinder::bind(constraint_ty))
 }
 
 /// See `ParamEnv` struct definition for details.

tests/ui/generic-associated-types/issue-119942-unsatisified-gat-bound-during-assoc-ty-selection.rs

@@ -29,5 +29,5 @@ where
 
 fn main() {
     let mut list = RcNode::<i32>::new();
-    //~^ ERROR the size for values of type `Node<i32, RcFamily>` cannot be known at compilation time
+    //~^ ERROR trait bounds were not satisfied
 }

tests/ui/generic-associated-types/issue-119942-unsatisified-gat-bound-during-assoc-ty-selection.stderr

@@ -15,20 +15,15 @@ help: consider relaxing the implicit `Sized` restriction
 LL |     type Pointer<T>: Deref<Target = T> + ?Sized;
    |                                        ++++++++
 
-error[E0599]: the size for values of type `Node<i32, RcFamily>` cannot be known at compilation time
+error[E0599]: the variant or associated item `new` exists for enum `Node<i32, RcFamily>`, but its trait bounds were not satisfied
   --> $DIR/issue-119942-unsatisified-gat-bound-during-assoc-ty-selection.rs:31:35
    |
 LL | enum Node<T, P: PointerFamily> {
-   | ------------------------------ variant or associated item `new` not found for this enum because it doesn't satisfy `Node<i32, RcFamily>: Sized`
+   | ------------------------------ variant or associated item `new` not found for this enum
 ...
 LL |     let mut list = RcNode::<i32>::new();
-   |                                   ^^^ doesn't have a size known at compile-time
+   |                                   ^^^ variant or associated item cannot be called on `Node<i32, RcFamily>` due to unsatisfied trait bounds
    |
-note: trait bound `Node<i32, RcFamily>: Sized` was not satisfied
-  --> $DIR/issue-119942-unsatisified-gat-bound-during-assoc-ty-selection.rs:4:18
-   |
-LL |     type Pointer<T>: Deref<Target = T>;
-   |          ------- ^ unsatisfied trait bound introduced here
 note: trait bound `(dyn Deref<Target = Node<i32, RcFamily>> + 'static): Sized` was not satisfied
   --> $DIR/issue-119942-unsatisified-gat-bound-during-assoc-ty-selection.rs:23:29
    |
@@ -37,8 +32,6 @@ LL | impl<T, P: PointerFamily> Node<T, P>
 LL | where
 LL |     P::Pointer<Node<T, P>>: Sized,
    |                             ^^^^^ unsatisfied trait bound introduced here
-note: the trait `Sized` must be implemented
-  --> $SRC_DIR/core/src/marker.rs:LL:COL
 
 error: aborting due to 2 previous errors