Auto merge of #126161 - Bryanskiy:delegation-generics-4, r=petrochenkov

Delegation: support generics in associated delegation items

This is a continuation of https://github.com/rust-lang/rust/pull/125929.

[design](https://github.com/Bryanskiy/posts/blob/master/delegation%20in%20generic%20contexts.md)

Generic parameters inheritance was implemented in all contexts. Generic arguments are not yet supported.

r? `@petrochenkov`

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -59,6 +59,12 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
     let hir_id = tcx.local_def_id_to_hir_id(def_id);
 
     let node = tcx.hir_node(hir_id);
+    if let Some(sig) = node.fn_sig()
+        && let Some(sig_id) = sig.decl.opt_delegation_sig_id()
+    {
+        return inherit_generics_for_delegation_item(tcx, def_id, sig_id);
+    }
+
     let parent_def_id = match node {
         Node::ImplItem(_)
         | Node::TraitItem(_)
@@ -229,16 +235,6 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                 // inherit the generics of the item.
                 Some(parent.to_def_id())
             }
-            ItemKind::Fn(sig, _, _) => {
-                // For a delegation item inherit generics from callee.
-                if let Some(sig_id) = sig.decl.opt_delegation_sig_id()
-                    && let Some(generics) =
-                        inherit_generics_for_delegation_item(tcx, def_id, sig_id)
-                {
-                    return generics;
-                }
-                None
-            }
             _ => None,
         },
         _ => None,

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -138,6 +138,12 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
     let hir_id = tcx.local_def_id_to_hir_id(def_id);
     let node = tcx.hir_node(hir_id);
 
+    if let Some(sig) = node.fn_sig()
+        && let Some(sig_id) = sig.decl.opt_delegation_sig_id()
+    {
+        return inherit_predicates_for_delegation_item(tcx, def_id, sig_id);
+    }
+
     let mut is_trait = None;
     let mut is_default_impl_trait = None;
 
@@ -164,16 +170,6 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
             ItemKind::Trait(_, _, _, self_bounds, ..) | ItemKind::TraitAlias(_, self_bounds) => {
                 is_trait = Some(self_bounds);
             }
-
-            ItemKind::Fn(sig, _, _) => {
-                // For a delegation item inherit predicates from callee.
-                if let Some(sig_id) = sig.decl.opt_delegation_sig_id()
-                    && let Some(predicates) =
-                        inherit_predicates_for_delegation_item(tcx, def_id, sig_id)
-                {
-                    return predicates;
-                }
-            }
             _ => {}
         }
     };

compiler/rustc_hir_analysis/src/delegation.rs

@@ -1,3 +1,7 @@
+//! Support inheriting generic parameters and predicates for function delegation.
+//!
+//! For more information about delegation design, see the tracking issue #118212.
+
 use std::assert_matches::debug_assert_matches;
 
 use rustc_data_structures::fx::FxHashMap;
@@ -5,7 +9,7 @@ use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LocalDefId};
 use rustc_middle::ty::fold::{TypeFoldable, TypeFolder, TypeSuperFoldable};
 use rustc_middle::ty::{self, Ty, TyCtxt};
-use rustc_span::ErrorGuaranteed;
+use rustc_span::{ErrorGuaranteed, Span};
 use rustc_type_ir::visit::TypeVisitableExt;
 
 type RemapTable = FxHashMap<u32, u32>;
@@ -76,127 +80,381 @@ fn fn_kind<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> FnKind {
     }
 }
 
+/// Given the current context(caller and callee `FnKind`), it specifies
+/// the policy of predicates and generic parameters inheritance.
+#[derive(Clone, Copy, Debug, PartialEq)]
+enum InheritanceKind {
+    /// Copying all predicates and parameters, including those of the parent
+    /// container.
+    ///
+    /// Boolean value defines whether the `Self` parameter or `Self: Trait`
+    /// predicate are copied. It's always equal to `false` except when
+    /// delegating from a free function to a trait method.
+    ///
+    /// FIXME(fn_delegation): This often leads to type inference
+    /// errors. Support providing generic arguments or restrict use sites.
+    WithParent(bool),
+    /// The trait implementation should be compatible with the original trait.
+    /// Therefore, for trait implementations only the method's own parameters
+    /// and predicates are copied.
+    Own,
+}
+
+struct GenericsBuilder<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    sig_id: DefId,
+    parent: Option<DefId>,
+    inh_kind: InheritanceKind,
+}
+
+impl<'tcx> GenericsBuilder<'tcx> {
+    fn new(tcx: TyCtxt<'tcx>, sig_id: DefId) -> GenericsBuilder<'tcx> {
+        GenericsBuilder { tcx, sig_id, parent: None, inh_kind: InheritanceKind::WithParent(false) }
+    }
+
+    fn with_parent(mut self, parent: DefId) -> Self {
+        self.parent = Some(parent);
+        self
+    }
+
+    fn with_inheritance_kind(mut self, inh_kind: InheritanceKind) -> Self {
+        self.inh_kind = inh_kind;
+        self
+    }
+
+    fn build(self) -> ty::Generics {
+        let mut own_params = vec![];
+
+        let sig_generics = self.tcx.generics_of(self.sig_id);
+        if let InheritanceKind::WithParent(has_self) = self.inh_kind
+            && let Some(parent_def_id) = sig_generics.parent
+        {
+            let sig_parent_generics = self.tcx.generics_of(parent_def_id);
+            own_params.append(&mut sig_parent_generics.own_params.clone());
+            if !has_self {
+                own_params.remove(0);
+            }
+        }
+        own_params.append(&mut sig_generics.own_params.clone());
+
+        // Lifetime parameters must be declared before type and const parameters.
+        // Therefore, When delegating from a free function to a associated function,
+        // generic parameters need to be reordered:
+        //
+        // trait Trait<'a, A> {
+        //     fn foo<'b, B>(...) {...}
+        // }
+        //
+        // reuse Trait::foo;
+        // desugaring:
+        // fn foo<'a, 'b, This: Trait<'a, A>, A, B>(...) {
+        //     Trait::foo(...)
+        // }
+        own_params.sort_by_key(|key| key.kind.is_ty_or_const());
+
+        let param_def_id_to_index =
+            own_params.iter().map(|param| (param.def_id, param.index)).collect();
+
+        let (parent_count, has_self) = if let Some(def_id) = self.parent {
+            let parent_generics = self.tcx.generics_of(def_id);
+            let parent_kind = self.tcx.def_kind(def_id);
+            (parent_generics.count(), parent_kind == DefKind::Trait)
+        } else {
+            (0, false)
+        };
+
+        for (idx, param) in own_params.iter_mut().enumerate() {
+            param.index = (idx + parent_count) as u32;
+            // FIXME(fn_delegation): Default parameters are not inherited, because they are
+            // not permitted in functions. Therefore, there are 2 options here:
+            //
+            // - We can create non-default generic parameters.
+            // - We can substitute default parameters into the signature.
+            //
+            // At the moment, first option has been selected as the most general.
+            if let ty::GenericParamDefKind::Type { has_default, .. }
+            | ty::GenericParamDefKind::Const { has_default, .. } = &mut param.kind
+            {
+                *has_default = false;
+            }
+        }
+
+        ty::Generics {
+            parent: self.parent,
+            parent_count,
+            own_params,
+            param_def_id_to_index,
+            has_self,
+            has_late_bound_regions: sig_generics.has_late_bound_regions,
+            host_effect_index: sig_generics.host_effect_index,
+        }
+    }
+}
+
+struct PredicatesBuilder<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    sig_id: DefId,
+    parent: Option<DefId>,
+    inh_kind: InheritanceKind,
+    args: ty::GenericArgsRef<'tcx>,
+}
+
+impl<'tcx> PredicatesBuilder<'tcx> {
+    fn new(
+        tcx: TyCtxt<'tcx>,
+        args: ty::GenericArgsRef<'tcx>,
+        sig_id: DefId,
+    ) -> PredicatesBuilder<'tcx> {
+        PredicatesBuilder {
+            tcx,
+            sig_id,
+            parent: None,
+            inh_kind: InheritanceKind::WithParent(false),
+            args,
+        }
+    }
+
+    fn with_parent(mut self, parent: DefId) -> Self {
+        self.parent = Some(parent);
+        self
+    }
+
+    fn with_inheritance_kind(mut self, inh_kind: InheritanceKind) -> Self {
+        self.inh_kind = inh_kind;
+        self
+    }
+
+    fn build(self) -> ty::GenericPredicates<'tcx> {
+        struct PredicatesCollector<'tcx> {
+            tcx: TyCtxt<'tcx>,
+            preds: Vec<(ty::Clause<'tcx>, Span)>,
+            args: ty::GenericArgsRef<'tcx>,
+        }
+
+        impl<'tcx> PredicatesCollector<'tcx> {
+            fn new(tcx: TyCtxt<'tcx>, args: ty::GenericArgsRef<'tcx>) -> PredicatesCollector<'tcx> {
+                PredicatesCollector { tcx, preds: vec![], args }
+            }
+
+            fn with_own_preds(
+                mut self,
+                f: impl Fn(DefId) -> ty::GenericPredicates<'tcx>,
+                def_id: DefId,
+            ) -> Self {
+                let preds = f(def_id).instantiate_own(self.tcx, self.args);
+                self.preds.extend(preds);
+                self
+            }
+
+            fn with_preds(
+                mut self,
+                f: impl Fn(DefId) -> ty::GenericPredicates<'tcx> + Copy,
+                def_id: DefId,
+            ) -> Self {
+                let preds = f(def_id);
+                if let Some(parent_def_id) = preds.parent {
+                    self = self.with_own_preds(f, parent_def_id);
+                }
+                self.with_own_preds(f, def_id)
+            }
+        }
+        let collector = PredicatesCollector::new(self.tcx, self.args);
+
+        // `explicit_predicates_of` is used here to avoid copying `Self: Trait` predicate.
+        // Note: `predicates_of` query can also add inferred outlives predicates, but that
+        // is not the case here as `sig_id` is either a trait or a function.
+        let preds = match self.inh_kind {
+            InheritanceKind::WithParent(false) => {
+                collector.with_preds(|def_id| self.tcx.explicit_predicates_of(def_id), self.sig_id)
+            }
+            InheritanceKind::WithParent(true) => {
+                collector.with_preds(|def_id| self.tcx.predicates_of(def_id), self.sig_id)
+            }
+            InheritanceKind::Own => {
+                collector.with_own_preds(|def_id| self.tcx.predicates_of(def_id), self.sig_id)
+            }
+        }
+        .preds;
+
+        ty::GenericPredicates {
+            parent: self.parent,
+            predicates: self.tcx.arena.alloc_from_iter(preds),
+            // FIXME(fn_delegation): Support effects.
+            effects_min_tys: ty::List::empty(),
+        }
+    }
+}
+
+struct GenericArgsBuilder<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    remap_table: RemapTable,
+    sig_id: DefId,
+    def_id: LocalDefId,
+}
+
+impl<'tcx> GenericArgsBuilder<'tcx> {
+    fn new(tcx: TyCtxt<'tcx>, sig_id: DefId, def_id: LocalDefId) -> GenericArgsBuilder<'tcx> {
+        GenericArgsBuilder { tcx, remap_table: FxHashMap::default(), sig_id, def_id }
+    }
+
+    fn build_from_args(mut self, args: ty::GenericArgsRef<'tcx>) -> ty::GenericArgsRef<'tcx> {
+        let caller_generics = self.tcx.generics_of(self.def_id);
+        let callee_generics = self.tcx.generics_of(self.sig_id);
+
+        for caller_param in &caller_generics.own_params {
+            let callee_index =
+                callee_generics.param_def_id_to_index(self.tcx, caller_param.def_id).unwrap();
+            self.remap_table.insert(callee_index, caller_param.index);
+        }
+
+        let mut folder = ParamIndexRemapper { tcx: self.tcx, remap_table: self.remap_table };
+        args.fold_with(&mut folder)
+    }
+}
+
 fn create_generic_args<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: LocalDefId,
     sig_id: DefId,
 ) -> ty::GenericArgsRef<'tcx> {
-    let caller_generics = tcx.generics_of(def_id);
-    let callee_generics = tcx.generics_of(sig_id);
+    let builder = GenericArgsBuilder::new(tcx, sig_id, def_id);
 
     let caller_kind = fn_kind(tcx, def_id.into());
     let callee_kind = fn_kind(tcx, sig_id);
-    // FIXME(fn_delegation): Support generics on associated delegation items.
-    // Error will be reported in `check_constraints`.
     match (caller_kind, callee_kind) {
-        (FnKind::Free, _) => {
-            // Lifetime parameters must be declared before type and const parameters.
-            // Therefore, When delegating from a free function to a associated function,
-            // generic parameters need to be reordered:
-            //
-            // trait Trait<'a, A> {
-            //     fn foo<'b, B>(...) {...}
-            // }
-            //
-            // reuse Trait::foo;
-            // desugaring:
-            // fn foo<'a, 'b, This: Trait<'a, A>, A, B>(...) {
-            //     Trait::foo(...)
-            // }
-            let mut remap_table = RemapTable::default();
-            for caller_param in &caller_generics.own_params {
-                let callee_index =
-                    callee_generics.param_def_id_to_index(tcx, caller_param.def_id).unwrap();
-                remap_table.insert(callee_index, caller_param.index);
-            }
-            let mut folder = ParamIndexRemapper { tcx, remap_table };
-            ty::GenericArgs::identity_for_item(tcx, sig_id).fold_with(&mut folder)
+        (FnKind::Free, FnKind::Free)
+        | (FnKind::Free, FnKind::AssocTrait)
+        | (FnKind::AssocInherentImpl, FnKind::Free)
+        | (FnKind::AssocTrait, FnKind::Free)
+        | (FnKind::AssocTrait, FnKind::AssocTrait) => {
+            let args = ty::GenericArgs::identity_for_item(tcx, sig_id);
+            builder.build_from_args(args)
         }
-        // FIXME(fn_delegation): Only `Self` param supported here.
-        (FnKind::AssocTraitImpl, FnKind::AssocTrait)
-        | (FnKind::AssocInherentImpl, FnKind::AssocTrait) => {
+
+        (FnKind::AssocTraitImpl, FnKind::AssocTrait) => {
+            let callee_generics = tcx.generics_of(sig_id);
+            let parent = tcx.parent(def_id.into());
+            let parent_args =
+                tcx.impl_trait_header(parent).unwrap().trait_ref.instantiate_identity().args;
+
+            let trait_args = ty::GenericArgs::identity_for_item(tcx, sig_id);
+            let method_args = tcx.mk_args_from_iter(trait_args.iter().skip(callee_generics.parent_count));
+            let method_args = builder.build_from_args(method_args);
+
+            tcx.mk_args_from_iter(parent_args.iter().chain(method_args))
+        }
+
+        (FnKind::AssocInherentImpl, FnKind::AssocTrait) => {
             let parent = tcx.parent(def_id.into());
             let self_ty = tcx.type_of(parent).instantiate_identity();
             let generic_self_ty = ty::GenericArg::from(self_ty);
-            tcx.mk_args_from_iter(std::iter::once(generic_self_ty))
+
+            let trait_args = ty::GenericArgs::identity_for_item(tcx, sig_id);
+            let trait_args = builder.build_from_args(trait_args);
+
+            let args = std::iter::once(generic_self_ty).chain(trait_args.iter().skip(1));
+            tcx.mk_args_from_iter(args)
         }
-        _ => ty::GenericArgs::identity_for_item(tcx, sig_id),
+
+        // For trait impl's `sig_id` is always equal to the corresponding trait method.
+        (FnKind::AssocTraitImpl, _)
+        | (_, FnKind::AssocTraitImpl)
+        // Delegation to inherent methods is not yet supported.
+        | (_, FnKind::AssocInherentImpl) => unreachable!(),
     }
 }
 
+// FIXME(fn_delegation): Move generics inheritance to the AST->HIR lowering.
+// For now, generic parameters are not propagated to the generated call,
+// which leads to inference errors:
+//
+// fn foo<T>(x: i32) {}
+//
+// reuse foo as bar;
+// desugaring:
+// fn bar<T>() {
+//   foo::<_>() // ERROR: type annotations needed
+// }
 pub(crate) fn inherit_generics_for_delegation_item<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: LocalDefId,
     sig_id: DefId,
-) -> Option<ty::Generics> {
-    // FIXME(fn_delegation): Support generics on associated delegation items.
-    // Error will be reported in `check_constraints`.
-    if fn_kind(tcx, def_id.into()) != FnKind::Free {
-        return None;
-    }
+) -> ty::Generics {
+    let builder = GenericsBuilder::new(tcx, sig_id);
 
-    let mut own_params = vec![];
+    let caller_kind = fn_kind(tcx, def_id.into());
+    let callee_kind = fn_kind(tcx, sig_id);
+    match (caller_kind, callee_kind) {
+        (FnKind::Free, FnKind::Free)
+        | (FnKind::Free, FnKind::AssocTrait) => builder.with_inheritance_kind(InheritanceKind::WithParent(true)).build(),
 
-    let callee_generics = tcx.generics_of(sig_id);
-    if let Some(parent_sig_id) = callee_generics.parent {
-        let parent_sig_generics = tcx.generics_of(parent_sig_id);
-        own_params.append(&mut parent_sig_generics.own_params.clone());
-    }
-    own_params.append(&mut callee_generics.own_params.clone());
-
-    // Lifetimes go first.
-    own_params.sort_by_key(|key| key.kind.is_ty_or_const());
-
-    for (idx, param) in own_params.iter_mut().enumerate() {
-        param.index = idx as u32;
-        // Default parameters are not inherited: they are not allowed
-        // in fn's.
-        if let ty::GenericParamDefKind::Type { has_default, .. }
-        | ty::GenericParamDefKind::Const { has_default, .. } = &mut param.kind
-        {
-            *has_default = false;
+        (FnKind::AssocTraitImpl, FnKind::AssocTrait) => {
+            builder
+            .with_parent(tcx.parent(def_id.into()))
+            .with_inheritance_kind(InheritanceKind::Own)
+            .build()
         }
+
+        (FnKind::AssocInherentImpl, FnKind::AssocTrait)
+        | (FnKind::AssocTrait, FnKind::AssocTrait) => {
+            builder
+            .with_parent(tcx.parent(def_id.into()))
+            .build()
+        }
+
+        (FnKind::AssocInherentImpl, FnKind::Free)
+        | (FnKind::AssocTrait, FnKind::Free) => {
+            builder
+            .with_parent(tcx.parent(def_id.into()))
+            .build()
+        }
+
+        // For trait impl's `sig_id` is always equal to the corresponding trait method.
+        (FnKind::AssocTraitImpl, _)
+        | (_, FnKind::AssocTraitImpl)
+        // Delegation to inherent methods is not yet supported.
+        | (_, FnKind::AssocInherentImpl) => unreachable!(),
     }
-
-    let param_def_id_to_index =
-        own_params.iter().map(|param| (param.def_id, param.index)).collect();
-
-    Some(ty::Generics {
-        parent: None,
-        parent_count: 0,
-        own_params,
-        param_def_id_to_index,
-        has_self: false,
-        has_late_bound_regions: callee_generics.has_late_bound_regions,
-        host_effect_index: callee_generics.host_effect_index,
-    })
 }
 
 pub(crate) fn inherit_predicates_for_delegation_item<'tcx>(
     tcx: TyCtxt<'tcx>,
     def_id: LocalDefId,
     sig_id: DefId,
-) -> Option<ty::GenericPredicates<'tcx>> {
-    // FIXME(fn_delegation): Support generics on associated delegation items.
-    // Error will be reported in `check_constraints`.
-    if fn_kind(tcx, def_id.into()) != FnKind::Free {
-        return None;
-    }
-
-    let callee_predicates = tcx.predicates_of(sig_id);
+) -> ty::GenericPredicates<'tcx> {
     let args = create_generic_args(tcx, def_id, sig_id);
+    let builder = PredicatesBuilder::new(tcx, args, sig_id);
 
-    let mut preds = vec![];
-    if let Some(parent_id) = callee_predicates.parent {
-        preds.extend(tcx.predicates_of(parent_id).instantiate_own(tcx, args));
+    let caller_kind = fn_kind(tcx, def_id.into());
+    let callee_kind = fn_kind(tcx, sig_id);
+    match (caller_kind, callee_kind) {
+        (FnKind::Free, FnKind::Free)
+        | (FnKind::Free, FnKind::AssocTrait) => {
+            builder.with_inheritance_kind(InheritanceKind::WithParent(true)).build()
+        }
+
+        (FnKind::AssocTraitImpl, FnKind::AssocTrait) => {
+            builder
+            .with_parent(tcx.parent(def_id.into()))
+            .with_inheritance_kind(InheritanceKind::Own)
+            .build()
+        }
+
+        (FnKind::AssocInherentImpl, FnKind::AssocTrait)
+        | (FnKind::AssocTrait, FnKind::AssocTrait)
+        | (FnKind::AssocInherentImpl, FnKind::Free)
+        | (FnKind::AssocTrait, FnKind::Free) => {
+            builder
+                .with_parent(tcx.parent(def_id.into()))
+                .build()
+        }
+
+        // For trait impl's `sig_id` is always equal to the corresponding trait method.
+        (FnKind::AssocTraitImpl, _)
+        | (_, FnKind::AssocTraitImpl)
+        // Delegation to inherent methods is not yet supported.
+        | (_, FnKind::AssocInherentImpl) => unreachable!(),
     }
-    preds.extend(callee_predicates.instantiate_own(tcx, args));
-
-    Some(ty::GenericPredicates {
-        parent: None,
-        predicates: tcx.arena.alloc_from_iter(preds),
-        effects_min_tys: ty::List::empty(),
-    })
 }
 
 fn check_constraints<'tcx>(
@@ -224,19 +482,6 @@ fn check_constraints<'tcx>(
         emit("recursive delegation is not supported yet");
     }
 
-    if fn_kind(tcx, def_id.into()) != FnKind::Free {
-        let sig_generics = tcx.generics_of(sig_id);
-        let parent = tcx.parent(def_id.into());
-        let parent_generics = tcx.generics_of(parent);
-
-        let parent_has_self = parent_generics.has_self as usize;
-        let sig_has_self = sig_generics.has_self as usize;
-
-        if sig_generics.count() > sig_has_self || parent_generics.count() > parent_has_self {
-            emit("early bound generics are not supported for associated delegation items");
-        }
-    }
-
     ret
 }