Auto merge of rust-lang#125929 - Bryanskiy:delegation-generics-3, r=p…

…etrochenkov

Delegation: support generics for delegation from free functions

(The PR was split from rust-lang#123958, explainer - https://github.com/Bryanskiy/posts/blob/master/delegation%20in%20generic%20contexts.md)

This PR implements generics inheritance from free functions to free functions and trait methods.

#### free functions to free functions:

```rust
fn to_reuse<T: Clone>(_: T) {}

reuse to_reuse as bar;
// desugaring:
fn bar<T: Clone>(x: T) {
  to_reuse(x)
}
```

Generics, predicates and signature are simply copied. Generic arguments in paths are ignored during generics inheritance:

```rust
fn to_reuse<T: Clone>(_: T) {}

reuse to_reuse::<u8> as bar;
// desugaring:
fn bar<T: Clone>(x: T) {
  to_reuse::<u8>(x) // ERROR: mismatched types
}
```

Due to implementation limitations callee path is lowered without modifications. Therefore, it is a compilation error at the moment.

#### free functions to trait methods:

```rust
trait Trait<'a, A> {
    fn foo<'b, B>(&self, x: A, y: B) {...}
}

reuse Trait::foo;
// desugaring:
fn foo<'a, 'b, This: Trait<'a, A>, A, B>(this: &This, x: A, y: B) {
  Trait::foo(this, x, y)
}
```

The inheritance is similar to the previous case but with some corrections:

- `Self` parameter converted into `T: Trait`
- generic parameters need to be reordered so that lifetimes go first

Arguments are similarly ignored.

---

In the future, we plan to  support generic inheritance for delegating from all contexts to all contexts (from free/trait/impl to free/trait /impl). These cases were considered first as the simplest from the implementation perspective.

compiler/rustc_ast_lowering/src/delegation.rs

@@ -33,7 +33,7 @@
 //! HIR ty lowering.
 //!
 //! Similarly generics, predicates and header are set to the "default" values.
-//! In case of discrepancy with callee function the `NotSupportedDelegation` error will
+//! In case of discrepancy with callee function the `UnsupportedDelegation` error will
 //! also be emitted during HIR ty lowering.
 
 use std::iter;

compiler/rustc_hir_analysis/messages.ftl

@@ -341,8 +341,7 @@ hir_analysis_must_implement_not_function_span_note = required by this annotation
 
 hir_analysis_must_implement_one_of_attribute = the `#[rustc_must_implement_one_of]` attribute must be used with at least 2 args
 
-hir_analysis_not_supported_delegation =
-    {$descr} is not supported yet
+hir_analysis_not_supported_delegation = {$descr}
     .label = callee defined here
 
 hir_analysis_only_current_traits_adt = `{$name}` is not defined in the current crate

compiler/rustc_hir_analysis/src/collect/generics_of.rs

@@ -10,6 +10,7 @@ use rustc_session::lint;
 use rustc_span::symbol::{kw, Symbol};
 use rustc_span::Span;
 
+use crate::delegation::inherit_generics_for_delegation_item;
 use crate::middle::resolve_bound_vars as rbv;
 
 #[instrument(level = "debug", skip(tcx), ret)]
@@ -53,6 +54,13 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
         };
     }
 
+    // For a delegation item inherit generics from callee.
+    if let Some(sig_id) = tcx.hir().opt_delegation_sig_id(def_id)
+        && let Some(generics) = inherit_generics_for_delegation_item(tcx, def_id, sig_id)
+    {
+        return generics;
+    }
+
     let hir_id = tcx.local_def_id_to_hir_id(def_id);
 
     let node = tcx.hir_node(hir_id);

compiler/rustc_hir_analysis/src/collect/predicates_of.rs

@@ -12,6 +12,7 @@ use rustc_span::{Span, DUMMY_SP};
 use crate::bounds::Bounds;
 use crate::collect::ItemCtxt;
 use crate::constrained_generic_params as cgp;
+use crate::delegation::inherit_predicates_for_delegation_item;
 use crate::hir_ty_lowering::{HirTyLowerer, OnlySelfBounds, PredicateFilter, RegionInferReason};
 
 /// Returns a list of all type predicates (explicit and implicit) for the definition with
@@ -114,6 +115,13 @@ fn gather_explicit_predicates_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Gen
         None => {}
     }
 
+    // For a delegation item inherit predicates from callee.
+    if let Some(sig_id) = tcx.hir().opt_delegation_sig_id(def_id)
+        && let Some(predicates) = inherit_predicates_for_delegation_item(tcx, def_id, sig_id)
+    {
+        return predicates;
+    }
+
     let hir_id = tcx.local_def_id_to_hir_id(def_id);
     let node = tcx.hir_node(hir_id);
 

compiler/rustc_hir_analysis/src/delegation.rs

@@ -0,0 +1,259 @@
+use rustc_data_structures::fx::FxHashMap;
+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_type_ir::visit::TypeVisitableExt;
+
+type RemapTable = FxHashMap<u32, u32>;
+
+struct ParamIndexRemapper<'tcx> {
+    tcx: TyCtxt<'tcx>,
+    remap_table: RemapTable,
+}
+
+impl<'tcx> TypeFolder<TyCtxt<'tcx>> for ParamIndexRemapper<'tcx> {
+    fn cx(&self) -> TyCtxt<'tcx> {
+        self.tcx
+    }
+
+    fn fold_ty(&mut self, ty: Ty<'tcx>) -> Ty<'tcx> {
+        if !ty.has_param() {
+            return ty;
+        }
+
+        if let ty::Param(param) = ty.kind()
+            && let Some(index) = self.remap_table.get(&param.index)
+        {
+            return Ty::new_param(self.tcx, *index, param.name);
+        }
+        ty.super_fold_with(self)
+    }
+
+    fn fold_region(&mut self, r: ty::Region<'tcx>) -> ty::Region<'tcx> {
+        if let ty::ReEarlyParam(param) = r.kind()
+            && let Some(index) = self.remap_table.get(&param.index).copied()
+        {
+            return ty::Region::new_early_param(
+                self.tcx,
+                ty::EarlyParamRegion { index, name: param.name },
+            );
+        }
+        r
+    }
+
+    fn fold_const(&mut self, ct: ty::Const<'tcx>) -> ty::Const<'tcx> {
+        if let ty::ConstKind::Param(param) = ct.kind()
+            && let Some(idx) = self.remap_table.get(&param.index)
+        {
+            let param = ty::ParamConst::new(*idx, param.name);
+            return ty::Const::new_param(self.tcx, param);
+        }
+        ct.super_fold_with(self)
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq)]
+enum FnKind {
+    Free,
+    AssocInherentImpl,
+    AssocTrait,
+    AssocTraitImpl,
+}
+
+fn fn_kind<'tcx>(tcx: TyCtxt<'tcx>, def_id: DefId) -> FnKind {
+    debug_assert!(matches!(tcx.def_kind(def_id), DefKind::Fn | DefKind::AssocFn));
+
+    let parent = tcx.parent(def_id);
+    match tcx.def_kind(parent) {
+        DefKind::Trait => FnKind::AssocTrait,
+        DefKind::Impl { of_trait: true } => FnKind::AssocTraitImpl,
+        DefKind::Impl { of_trait: false } => FnKind::AssocInherentImpl,
+        _ => FnKind::Free,
+    }
+}
+
+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 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)
+        }
+        // FIXME(fn_delegation): Only `Self` param supported here.
+        (FnKind::AssocTraitImpl, FnKind::AssocTrait)
+        | (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))
+        }
+        _ => ty::GenericArgs::identity_for_item(tcx, sig_id),
+    }
+}
+
+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;
+    }
+
+    let mut own_params = vec![];
+
+    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;
+        }
+    }
+
+    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);
+    let args = create_generic_args(tcx, def_id, 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));
+    }
+    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>(
+    tcx: TyCtxt<'tcx>,
+    def_id: LocalDefId,
+    sig_id: DefId,
+) -> Result<(), ErrorGuaranteed> {
+    let mut ret = Ok(());
+
+    let mut emit = |descr| {
+        ret = Err(tcx.dcx().emit_err(crate::errors::UnsupportedDelegation {
+            span: tcx.def_span(def_id),
+            descr,
+            callee_span: tcx.def_span(sig_id),
+        }));
+    };
+
+    if tcx.has_host_param(sig_id) {
+        emit("delegation to a function with effect parameter is not supported yet");
+    }
+
+    if let Some(local_sig_id) = sig_id.as_local()
+        && tcx.hir().opt_delegation_sig_id(local_sig_id).is_some()
+    {
+        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
+}
+
+pub(crate) fn inherit_sig_for_delegation_item<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    def_id: LocalDefId,
+) -> &'tcx [Ty<'tcx>] {
+    let sig_id = tcx.hir().delegation_sig_id(def_id);
+    let caller_sig = tcx.fn_sig(sig_id);
+    if let Err(err) = check_constraints(tcx, def_id, sig_id) {
+        let sig_len = caller_sig.instantiate_identity().skip_binder().inputs().len() + 1;
+        let err_type = Ty::new_error(tcx, err);
+        return tcx.arena.alloc_from_iter((0..sig_len).map(|_| err_type));
+    }
+    let args = create_generic_args(tcx, def_id, sig_id);
+
+    // Bound vars are also inherited from `sig_id`.
+    // They will be rebound later in `lower_fn_ty`.
+    let sig = caller_sig.instantiate(tcx, args).skip_binder();
+    let sig_iter = sig.inputs().iter().cloned().chain(std::iter::once(sig.output()));
+    tcx.arena.alloc_from_iter(sig_iter)
+}

compiler/rustc_hir_analysis/src/errors.rs

@@ -1575,7 +1575,7 @@ pub struct RefOfMutStatic<'a> {
 
 #[derive(Diagnostic)]
 #[diag(hir_analysis_not_supported_delegation)]
-pub struct NotSupportedDelegation<'a> {
+pub struct UnsupportedDelegation<'a> {
     #[primary_span]
     pub span: Span,
     pub descr: &'a str,