Delegation: support coercion for target expression

Author: Bryanskiy

compiler/rustc_ast_lowering/src/delegation.rs

@@ -38,7 +38,7 @@
 
 use crate::{ImplTraitPosition, ResolverAstLoweringExt};
 
-use super::{ImplTraitContext, LoweringContext, ParamMode};
+use super::{ImplTraitContext, LoweringContext, ParamMode, ParenthesizedGenericArgs};
 
 use ast::visit::Visitor;
 use hir::def::{DefKind, PartialRes, Res};
@@ -66,17 +66,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let Ok(sig_id) = sig_id else {
             return false;
         };
-        if let Some(local_sig_id) = sig_id.as_local() {
-            // The value may be missing due to recursive delegation.
-            // Error will be emmited later during HIR ty lowering.
-            self.resolver.delegation_fn_sigs.get(&local_sig_id).map_or(false, |sig| sig.has_self)
-        } else {
-            match self.tcx.def_kind(sig_id) {
-                DefKind::Fn => false,
-                DefKind::AssocFn => self.tcx.associated_item(sig_id).fn_has_self_parameter,
-                _ => span_bug!(span, "unexpected DefKind for delegation item"),
-            }
-        }
+        self.has_self(sig_id, span)
     }
 
     pub(crate) fn lower_delegation(
@@ -107,12 +97,29 @@ impl<'hir> LoweringContext<'_, 'hir> {
         span: Span,
     ) -> Result<DefId, ErrorGuaranteed> {
         let sig_id = if self.is_in_trait_impl { item_id } else { path_id };
-        let sig_id =
-            self.resolver.get_partial_res(sig_id).and_then(|r| r.expect_full_res().opt_def_id());
-        sig_id.ok_or_else(|| {
-            self.tcx
-                .dcx()
-                .span_delayed_bug(span, "LoweringContext: couldn't resolve delegation item")
+        self.get_resolution_id(sig_id, span)
+    }
+
+    fn has_self(&self, def_id: DefId, span: Span) -> bool {
+        if let Some(local_sig_id) = def_id.as_local() {
+            self.resolver.delegation_fn_sigs.get(&local_sig_id).map_or(false, |sig| sig.has_self)
+        } else {
+            match self.tcx.def_kind(def_id) {
+                DefKind::Fn => false,
+                DefKind::AssocFn => self.tcx.associated_item(def_id).fn_has_self_parameter,
+                _ => span_bug!(span, "unexpected DefKind for delegation item"),
+            }
+        }
+    }
+
+    fn get_resolution_id(&self, node_id: NodeId, span: Span) -> Result<DefId, ErrorGuaranteed> {
+        let def_id =
+            self.resolver.get_partial_res(node_id).and_then(|r| r.expect_full_res().opt_def_id());
+        def_id.ok_or_else(|| {
+            self.tcx.dcx().span_delayed_bug(
+                span,
+                format!("LoweringContext: couldn't resolve node {:?} in delegation item", node_id),
+            )
         })
     }
 
@@ -122,7 +129,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             predicates: &[],
             has_where_clause_predicates: false,
             where_clause_span: span,
-            span: span,
+            span,
         })
     }
 
@@ -222,12 +229,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }));
 
         let path = self.arena.alloc(hir::Path { span, res: Res::Local(param_id), segments });
-
-        hir::Expr {
-            hir_id: self.next_id(),
-            kind: hir::ExprKind::Path(hir::QPath::Resolved(None, path)),
-            span,
-        }
+        self.mk_expr(hir::ExprKind::Path(hir::QPath::Resolved(None, path)), span)
     }
 
     fn lower_delegation_body(
@@ -236,25 +238,18 @@ impl<'hir> LoweringContext<'_, 'hir> {
         param_count: usize,
         span: Span,
     ) -> BodyId {
-        let path = self.lower_qpath(
-            delegation.id,
-            &delegation.qself,
-            &delegation.path,
-            ParamMode::Optional,
-            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
-            None,
-        );
         let block = delegation.body.as_deref();
 
         self.lower_body(|this| {
-            let mut parameters: Vec<hir::Param<'_>> = Vec::new();
-            let mut args: Vec<hir::Expr<'hir>> = Vec::new();
+            let mut parameters: Vec<hir::Param<'_>> = Vec::with_capacity(param_count);
+            let mut args: Vec<hir::Expr<'_>> = Vec::with_capacity(param_count);
+            let mut target_expr = None;
 
             for idx in 0..param_count {
                 let (param, pat_node_id) = this.generate_param(span);
                 parameters.push(param);
 
-                let arg = if let Some(block) = block
+                if let Some(block) = block
                     && idx == 0
                 {
                     let mut self_resolver = SelfResolver {
@@ -263,56 +258,103 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         self_param_id: pat_node_id,
                     };
                     self_resolver.visit_block(block);
-                    let block = this.lower_block(block, false);
-                    hir::Expr {
-                        hir_id: this.next_id(),
-                        kind: hir::ExprKind::Block(block, None),
-                        span: block.span,
-                    }
+                    target_expr = Some(this.lower_block_noalloc(block, false));
                 } else {
                     let pat_hir_id = this.lower_node_id(pat_node_id);
-                    this.generate_arg(pat_hir_id, span)
+                    let arg = this.generate_arg(pat_hir_id, span);
+                    args.push(arg);
                 };
-                args.push(arg);
             }
 
-            let args = self.arena.alloc_from_iter(args);
-            let final_expr = this.generate_call(path, args);
+            let final_expr = this.finalize_body_lowering(delegation, target_expr, args, span);
             (this.arena.alloc_from_iter(parameters), final_expr)
         })
     }
 
-    fn generate_call(
+    // Generates expression for the resulting body. If possible, `MethodCall` is used
+    // instead of fully qualified call for the self type coercion. See `consider_candidates`
+    // for more information.
+    fn finalize_body_lowering(
         &mut self,
-        path: hir::QPath<'hir>,
-        args: &'hir [hir::Expr<'hir>],
+        delegation: &Delegation,
+        target_expr: Option<hir::Block<'hir>>,
+        mut args: Vec<hir::Expr<'hir>>,
+        span: Span,
     ) -> hir::Expr<'hir> {
-        let callee = self.arena.alloc(hir::Expr {
-            hir_id: self.next_id(),
-            kind: hir::ExprKind::Path(path),
-            span: path.span(),
-        });
+        let (stmts, call, block_id) = if self
+            .get_resolution_id(delegation.id, span)
+            .and_then(|def_id| Ok(self.has_self(def_id, span)))
+            .unwrap_or_default()
+            && delegation.qself.is_none()
+        {
+            let ast_segment = delegation.path.segments.last().unwrap();
+            let segment = self.lower_path_segment(
+                delegation.path.span,
+                ast_segment,
+                ParamMode::Optional,
+                ParenthesizedGenericArgs::Err,
+                ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                None,
+                None,
+            );
+            let segment = self.arena.alloc(segment);
+
+            let args = &*self.arena.alloc_from_iter(args);
+            let (stmts, receiver, args, block_id) = if let Some(target_expr) = target_expr {
+                // Use unit type as a receiver if no expression is provided.
+                let receiver = target_expr.expr.unwrap_or_else(|| {
+                    self.arena.alloc(self.mk_expr(hir::ExprKind::Tup(&[]), span))
+                });
+                (target_expr.stmts, receiver, args, target_expr.hir_id)
+            } else {
+                (&*self.arena.alloc_from_iter([]), &args[0], &args[1..], self.next_id())
+            };
 
-        let expr = self.arena.alloc(hir::Expr {
-            hir_id: self.next_id(),
-            kind: hir::ExprKind::Call(callee, args),
-            span: path.span(),
-        });
+            let method_call_id = self.next_id();
+            if let Some(traits) = self.resolver.delegation_trait_map.remove(&delegation.id) {
+                self.trait_map.insert(method_call_id.local_id, traits.into_boxed_slice());
+            }
+
+            let method_call = self.arena.alloc(hir::Expr {
+                hir_id: method_call_id,
+                kind: hir::ExprKind::MethodCall(segment, receiver, args, span),
+                span,
+            });
+
+            (stmts, method_call, block_id)
+        } else {
+            let path = self.lower_qpath(
+                delegation.id,
+                &delegation.qself,
+                &delegation.path,
+                ParamMode::Optional,
+                ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                None,
+            );
+
+            if let Some(target_expr) = target_expr {
+                let target_expr = self.arena.alloc(target_expr);
+                let fst_arg =
+                    self.mk_expr(hir::ExprKind::Block(target_expr, None), target_expr.span);
+                args.insert(0, fst_arg);
+            }
 
+            let args = self.arena.alloc_from_iter(args);
+            let callee_path = self.arena.alloc(self.mk_expr(hir::ExprKind::Path(path), span));
+
+            let call = self.arena.alloc(self.mk_expr(hir::ExprKind::Call(callee_path, args), span));
+
+            (&*self.arena.alloc_from_iter([]), call, self.next_id())
+        };
         let block = self.arena.alloc(hir::Block {
-            stmts: &[],
-            expr: Some(expr),
-            hir_id: self.next_id(),
+            stmts,
+            expr: Some(call),
+            hir_id: block_id,
             rules: hir::BlockCheckMode::DefaultBlock,
-            span: path.span(),
+            span,
             targeted_by_break: false,
         });
-
-        hir::Expr {
-            hir_id: self.next_id(),
-            kind: hir::ExprKind::Block(block, None),
-            span: path.span(),
-        }
+        self.mk_expr(hir::ExprKind::Block(block, None), span)
     }
 
     fn generate_delegation_error(
@@ -333,11 +375,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let header = self.generate_header_error();
         let sig = hir::FnSig { decl, header, span };
 
-        let body_id = self.lower_body(|this| {
-            let expr =
-                hir::Expr { hir_id: this.next_id(), kind: hir::ExprKind::Err(err), span: span };
-            (&[], expr)
-        });
+        let body_id = self.lower_body(|this| (&[], this.mk_expr(hir::ExprKind::Err(err), span)));
         DelegationResults { generics, body_id, sig }
     }
 
@@ -349,6 +387,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
             abi: abi::Abi::Rust,
         }
     }
+
+    #[inline]
+    fn mk_expr(&mut self, kind: hir::ExprKind<'hir>, span: Span) -> hir::Expr<'hir> {
+        hir::Expr { hir_id: self.next_id(), kind, span }
+    }
 }
 
 struct SelfResolver<'a> {

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -1218,8 +1218,54 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         )>,
         mut unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
     ) -> Option<PickResult<'tcx>> {
+        // Delegation item can be expanded into method calls or fully qualified calls
+        // depending on the callee's signature. Method calls are used to allow
+        // autoref/autoderef for target expression. For example in:
+        //
+        // trait Trait : Sized {
+        //     fn by_value(self) -> i32 { 1 }
+        //     fn by_mut_ref(&mut self) -> i32 { 2 }
+        //     fn by_ref(&self) -> i32 { 3 }
+        // }
+        //
+        // struct NewType(SomeType);
+        // impl Trait for NewType {
+        //     reuse Trait::* { self.0 }
+        // }
+        //
+        // `self.0` will automatically coerce. The difference with existing method lookup
+        // is that methods in delegation items are pre-resolved by callee path (`Trait::*`).
+        // Therefore, we are only forced to consider the pre-resolved method here.
+        let delegation_res = if self.fcx.tcx.hir().opt_delegation_sig_id(self.body_id).is_some() {
+            let body = self.tcx.hir().body_owned_by(self.body_id);
+            let block = match body.value.kind {
+                hir::ExprKind::Block(block, _) => block,
+                _ => unreachable!(),
+            };
+            let expr = block.expr.unwrap();
+            let res_id = match expr.kind {
+                hir::ExprKind::MethodCall(segment, ..) => segment.res.def_id(),
+                hir::ExprKind::Call(
+                    hir::Expr { kind: hir::ExprKind::Path(hir::QPath::Resolved(_, path)), .. },
+                    _,
+                ) => path.res.def_id(),
+                _ => unreachable!(),
+            };
+            Some((expr.hir_id, res_id))
+        } else {
+            None
+        };
         let mut applicable_candidates: Vec<_> = candidates
             .iter()
+            .filter(|candidate| {
+                if let Some((expr_id, res_id)) = delegation_res
+                    && self.scope_expr_id == expr_id
+                    && candidate.item.def_id != res_id
+                {
+                    return false;
+                }
+                true
+            })
             .map(|probe| {
                 (probe, self.consider_probe(self_ty, probe, possibly_unsatisfied_predicates))
             })

compiler/rustc_middle/src/hir/map/mod.rs

@@ -728,6 +728,21 @@ impl<'hir> Map<'hir> {
         }
     }
 
+    pub fn opt_delegation_sig_id(self, def_id: LocalDefId) -> Option<DefId> {
+        if let Some(ret) = self.get_fn_output(def_id)
+            && let FnRetTy::Return(ty) = ret
+            && let TyKind::InferDelegation(sig_id, _) = ty.kind
+        {
+            return Some(sig_id);
+        }
+        None
+    }
+
+    #[inline]
+    pub fn delegation_sig_id(self, def_id: LocalDefId) -> DefId {
+        self.opt_delegation_sig_id(def_id).unwrap()
+    }
+
     #[inline]
     fn opt_ident(self, id: HirId) -> Option<Ident> {
         match self.tcx.hir_node(id) {

compiler/rustc_middle/src/ty/mod.rs

@@ -224,8 +224,10 @@ pub struct ResolverAstLowering {
     /// Lints that were emitted by the resolver and early lints.
     pub lint_buffer: Steal<LintBuffer>,
 
-    /// Information about functions signatures for delegation items expansion
+    /// Information about functions signatures for delegation items expansion.
     pub delegation_fn_sigs: LocalDefIdMap<DelegationFnSig>,
+    /// Separate `trait_map` for delegation items only.
+    pub delegation_trait_map: NodeMap<Vec<hir::TraitCandidate>>,
 }
 
 #[derive(Debug)]

compiler/rustc_resolve/src/late.rs

@@ -3315,15 +3315,19 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
         if let Some(qself) = &delegation.qself {
             self.visit_ty(&qself.ty);
         }
+        let last_segment = delegation.path.segments.last().unwrap();
+        let traits = self.traits_in_scope(last_segment.ident, ValueNS);
+        // Saving traits for a `MethodCall` that has not yet been generated.
+        self.r.delegation_trait_map.insert(delegation.id, traits);
+
         self.visit_path(&delegation.path, delegation.id);
         if let Some(body) = &delegation.body {
             self.with_rib(ValueNS, RibKind::FnOrCoroutine, |this| {
                 // `PatBoundCtx` is not necessary in this context
                 let mut bindings = smallvec![(PatBoundCtx::Product, Default::default())];
 
-                let span = delegation.path.segments.last().unwrap().ident.span;
                 this.fresh_binding(
-                    Ident::new(kw::SelfLower, span),
+                    Ident::new(kw::SelfLower, last_segment.ident.span),
                     delegation.id,
                     PatternSource::FnParam,
                     &mut bindings,