Rollup merge of rust-lang#126699 - Bryanskiy:delegation-coercion, r=petrochenkov

Delegation: support coercion for target expression

(solves rust-lang#118212 (comment))

The implementation consist of 3 parts. Firstly, method call is generated instead of fully qualified call in AST->HIR lowering if there were no generic arguments or `Qpath` were provided. These restrictions are imposed due to the loss of information after desugaring. For example in

```rust
trait Trait {
  fn foo(&self) {}
}

reuse <u8 as Trait>::foo;
```

We would like to generate such a code:

```rust
fn foo<u8: Trait>(x: &u8) {
  x.foo(x)
}
```

however, the signature is inherited during HIR analysis where `u8` was discarded.

Secondly, traits found in the callee path are also added to `trait_map`. This is done to avoid the side effects of converting body into method calls:

```rust
mod inner {
    pub trait Trait {
      fn foo(&self) {}
    }
}

reuse inner::Trait::foo; // Ok, `Trait` is in scope for implicit method call.
```

Finally, applicable candidates are filtered with pre-resolved method during method lookup.

P.S In the future, we would like to avoid restrictions on the callee path by `Self` autoref/autoderef in fully qualified calls, but at the moment it didn't work out.

r? `@petrochenkov`

Author: matthiaskrgr

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};
@@ -259,8 +259,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         self_param_id: pat_node_id,
                     };
                     self_resolver.visit_block(block);
-                    let block = this.lower_block(block, false);
-                    this.mk_expr(hir::ExprKind::Block(block, None), block.span)
+                    this.lower_target_expr(&block)
                 } else {
                     let pat_hir_id = this.lower_node_id(pat_node_id);
                     this.generate_arg(pat_hir_id, span)
@@ -273,26 +272,74 @@ impl<'hir> LoweringContext<'_, 'hir> {
         })
     }
 
-    // Generates fully qualified call for the resulting body.
+    // FIXME(fn_delegation): Alternatives for target expression lowering:
+    // https://github.com/rust-lang/rfcs/pull/3530#issuecomment-2197170600.
+    fn lower_target_expr(&mut self, block: &Block) -> hir::Expr<'hir> {
+        if block.stmts.len() == 1
+            && let StmtKind::Expr(expr) = &block.stmts[0].kind
+        {
+            return self.lower_expr_mut(expr);
+        }
+
+        let block = self.lower_block(block, false);
+        self.mk_expr(hir::ExprKind::Block(block, None), block.span)
+    }
+
+    // Generates expression for the resulting body. If possible, `MethodCall` is used
+    // instead of fully qualified call for the self type coercion.
     fn finalize_body_lowering(
         &mut self,
         delegation: &Delegation,
         args: Vec<hir::Expr<'hir>>,
         span: Span,
     ) -> hir::Expr<'hir> {
-        let path = self.lower_qpath(
-            delegation.id,
-            &delegation.qself,
-            &delegation.path,
-            ParamMode::Optional,
-            ImplTraitContext::Disallowed(ImplTraitPosition::Path),
-            None,
-        );
-
         let args = self.arena.alloc_from_iter(args);
-        let path_expr = self.arena.alloc(self.mk_expr(hir::ExprKind::Path(path), span));
-        let call = self.arena.alloc(self.mk_expr(hir::ExprKind::Call(path_expr, args), span));
 
+        let has_generic_args =
+            delegation.path.segments.iter().rev().skip(1).any(|segment| segment.args.is_some());
+
+        let call = 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()
+            && !has_generic_args
+        {
+            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 method_call_id = self.next_id();
+            if let Some(traits) = self.resolver.trait_map.remove(&delegation.id) {
+                self.trait_map.insert(method_call_id.local_id, traits.into_boxed_slice());
+            }
+
+            self.arena.alloc(hir::Expr {
+                hir_id: method_call_id,
+                kind: hir::ExprKind::MethodCall(segment, &args[0], &args[1..], span),
+                span,
+            })
+        } else {
+            let path = self.lower_qpath(
+                delegation.id,
+                &delegation.qself,
+                &delegation.path,
+                ParamMode::Optional,
+                ImplTraitContext::Disallowed(ImplTraitPosition::Path),
+                None,
+            );
+
+            let callee_path = self.arena.alloc(self.mk_expr(hir::ExprKind::Path(path), span));
+            self.arena.alloc(self.mk_expr(hir::ExprKind::Call(callee_path, args), span))
+        };
         let block = self.arena.alloc(hir::Block {
             stmts: &[],
             expr: Some(call),

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -583,6 +583,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     IsSuggestion(true),
                     callee_ty.peel_refs(),
                     callee_expr.unwrap().hir_id,
+                    None,
                     TraitsInScope,
                     |mut ctxt| ctxt.probe_for_similar_candidate(),
                 )

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -1610,6 +1610,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         IsSuggestion(true),
                         self_ty,
                         expr.hir_id,
+                        None,
                         ProbeScope::TraitsInScope,
                     )
                 {

compiler/rustc_hir_typeck/src/method/mod.rs

@@ -104,6 +104,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             IsSuggestion(true),
             self_ty,
             call_expr_id,
+            None,
             ProbeScope::TraitsInScope,
         ) {
             Ok(pick) => {
@@ -182,8 +183,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self_expr: &'tcx hir::Expr<'tcx>,
         args: &'tcx [hir::Expr<'tcx>],
     ) -> Result<MethodCallee<'tcx>, MethodError<'tcx>> {
-        let pick =
-            self.lookup_probe(segment.ident, self_ty, call_expr, ProbeScope::TraitsInScope)?;
+        let pick = self.lookup_probe(
+            segment.ident,
+            self_ty,
+            call_expr,
+            segment.res.opt_def_id(),
+            ProbeScope::TraitsInScope,
+        )?;
 
         self.lint_edition_dependent_dot_call(
             self_ty, segment, span, call_expr, self_expr, &pick, args,
@@ -208,6 +214,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     segment.ident,
                     trait_type,
                     call_expr,
+                    None,
                     ProbeScope::TraitsInScope,
                 ) {
                     Ok(ref new_pick) if pick.differs_from(new_pick) => {
@@ -276,6 +283,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         method_name: Ident,
         self_ty: Ty<'tcx>,
         call_expr: &hir::Expr<'_>,
+        expected_def_id: Option<DefId>,
         scope: ProbeScope,
     ) -> probe::PickResult<'tcx> {
         let pick = self.probe_for_name(
@@ -285,6 +293,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             IsSuggestion(false),
             self_ty,
             call_expr.hir_id,
+            expected_def_id,
             scope,
         )?;
         pick.maybe_emit_unstable_name_collision_hint(self.tcx, method_name.span, call_expr.hir_id);
@@ -306,6 +315,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             IsSuggestion(true),
             self_ty,
             call_expr.hir_id,
+            None,
             scope,
         )?;
         Ok(pick)
@@ -529,6 +539,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             IsSuggestion(false),
             self_ty,
             expr_id,
+            None,
             ProbeScope::TraitsInScope,
         );
         let pick = match (pick, struct_variant) {

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -97,6 +97,27 @@ pub(crate) struct ProbeContext<'a, 'tcx> {
 
     scope_expr_id: HirId,
 
+    /// 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:
+    ///
+    /// ```ignore (illustrative)
+    /// 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::*`).
+    expected_def_id: Option<DefId>,
+
     /// Is this probe being done for a diagnostic? This will skip some error reporting
     /// machinery, since we don't particularly care about, for example, similarly named
     /// candidates if we're *reporting* similarly named candidates.
@@ -249,6 +270,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 IsSuggestion(true),
                 self_ty,
                 scope_expr_id,
+                None,
                 ProbeScope::AllTraits,
                 |probe_cx| Ok(probe_cx.candidate_method_names(candidate_filter)),
             )
@@ -264,6 +286,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                     IsSuggestion(true),
                     self_ty,
                     scope_expr_id,
+                    None,
                     ProbeScope::AllTraits,
                     |probe_cx| probe_cx.pick(),
                 )
@@ -282,6 +305,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         is_suggestion: IsSuggestion,
         self_ty: Ty<'tcx>,
         scope_expr_id: HirId,
+        expected_def_id: Option<DefId>,
         scope: ProbeScope,
     ) -> PickResult<'tcx> {
         self.probe_op(
@@ -292,6 +316,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             is_suggestion,
             self_ty,
             scope_expr_id,
+            expected_def_id,
             scope,
             |probe_cx| probe_cx.pick(),
         )
@@ -316,6 +341,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             is_suggestion,
             self_ty,
             scope_expr_id,
+            None,
             scope,
             |probe_cx| {
                 Ok(probe_cx
@@ -336,6 +362,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         is_suggestion: IsSuggestion,
         self_ty: Ty<'tcx>,
         scope_expr_id: HirId,
+        expected_def_id: Option<DefId>,
         scope: ProbeScope,
         op: OP,
     ) -> Result<R, MethodError<'tcx>>
@@ -477,6 +504,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 &orig_values,
                 steps.steps,
                 scope_expr_id,
+                expected_def_id,
                 is_suggestion,
             );
 
@@ -572,6 +600,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         orig_steps_var_values: &'a OriginalQueryValues<'tcx>,
         steps: &'tcx [CandidateStep<'tcx>],
         scope_expr_id: HirId,
+        expected_def_id: Option<DefId>,
         is_suggestion: IsSuggestion,
     ) -> ProbeContext<'a, 'tcx> {
         ProbeContext {
@@ -591,6 +620,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             static_candidates: RefCell::new(Vec::new()),
             unsatisfied_predicates: RefCell::new(Vec::new()),
             scope_expr_id,
+            expected_def_id,
             is_suggestion,
         }
     }
@@ -1221,6 +1251,9 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     ) -> Option<PickResult<'tcx>> {
         let mut applicable_candidates: Vec<_> = candidates
             .iter()
+            .filter(|candidate| {
+                !matches!(self.expected_def_id, Some(def_id) if def_id != candidate.item.def_id)
+            })
             .map(|probe| {
                 (probe, self.consider_probe(self_ty, probe, possibly_unsatisfied_predicates))
             })
@@ -1679,6 +1712,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 self.orig_steps_var_values,
                 self.steps,
                 self.scope_expr_id,
+                None,
                 IsSuggestion(true),
             );
             pcx.allow_similar_names = true;

compiler/rustc_hir_typeck/src/method/suggest.rs

@@ -2034,6 +2034,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                             IsSuggestion(true),
                             rcvr_ty,
                             expr_id,
+                            None,
                             ProbeScope::TraitsInScope,
                         )
                         .is_ok()
@@ -3095,6 +3096,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 IsSuggestion(true),
                 deref_ty,
                 ty.hir_id,
+                None,
                 ProbeScope::TraitsInScope,
             ) {
                 if deref_ty.is_suggestable(self.tcx, true)

compiler/rustc_resolve/src/late.rs

@@ -3317,14 +3317,32 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             self.visit_ty(&qself.ty);
         }
         self.visit_path(&delegation.path, delegation.id);
+        let last_ident = delegation.path.segments.last().unwrap().ident;
+
+        // Saving traits for a `MethodCall` that has not yet been generated.
+        // Traits found in the path are also considered visible:
+        //
+        // impl Trait for Type {
+        //    reuse inner::TraitFoo::*; // OK, even `TraitFoo` is not in scope.
+        // }
+        let mut traits = self.traits_in_scope(last_ident, ValueNS);
+        for segment in &delegation.path.segments {
+            if let Some(partial_res) = self.r.partial_res_map.get(&segment.id)
+                && let Some(def_id) = partial_res.full_res().and_then(|res| res.opt_def_id())
+                && self.r.tcx.def_kind(def_id) == DefKind::Trait
+            {
+                traits.push(TraitCandidate { def_id, import_ids: smallvec![] });
+            }
+        }
+        self.r.trait_map.insert(delegation.id, traits);
+
         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_ident.span),
                     delegation.id,
                     PatternSource::FnParam,
                     &mut bindings,

tests/ui/delegation/bad-resolve.rs

@@ -34,6 +34,9 @@ impl Trait for S {
 
     reuse foo { &self.0 }
     //~^ ERROR cannot find function `foo` in this scope
+    reuse Trait::foo2 { self.0 }
+    //~^ ERROR cannot find function `foo2` in trait `Trait`
+    //~| ERROR method `foo2` is not a member of trait `Trait`
 }
 
 mod prefix {}