Auto merge of #106607 - compiler-errors:be-more-accurate-abt-method-suggestions, r=oli-obk

Consider return type when giving various method suggestions

1. Fix a bug in method probe where we weren't normalizing `xform_ret_ty` for non-`impl` method candidates. This shouldn't affect happy-path code, since we only use `xform_ret_ty` when probing methods for diagnostics (I think).
2. Pass the return type expectation down to `lookup_probe`/`probe_for_name` usages in diagnostics. Added a few UI tests to gate against bad suggestions.
3. Make a `FnCtxt::lookup_probe_for_diagnostic` which properly passes down `IsSuggestion(true)`. Should help suppress other weird notes in some corner cases.

Author: bors

compiler/rustc_hir_typeck/src/callee.rs

@@ -1,4 +1,4 @@
-use super::method::probe::{IsSuggestion, Mode, ProbeScope};
+use super::method::probe::ProbeScope;
 use super::method::MethodCallee;
 use super::{Expectation, FnCtxt, TupleArgumentsFlag};
 
@@ -496,15 +496,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // any strange errors. If it's successful, then we'll do a true
             // method lookup.
             let Ok(pick) = self
-            .probe_for_name(
-                Mode::MethodCall,
+            .lookup_probe_for_diagnostic(
                 segment.ident,
-                IsSuggestion(true),
                 callee_ty,
-                call_expr.hir_id,
+                call_expr,
                 // We didn't record the in scope traits during late resolution
                 // so we need to probe AllTraits unfortunately
                 ProbeScope::AllTraits,
+                expected.only_has_type(self),
             ) else {
                 return None;
             };

compiler/rustc_hir_typeck/src/demand.rs

@@ -303,11 +303,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 // Get the evaluated type *after* calling the method call, so that the influence
                 // of the arguments can be reflected in the receiver type. The receiver
                 // expression has the type *before* theis analysis is done.
-                let ty = match self.lookup_probe(
+                let ty = match self.lookup_probe_for_diagnostic(
                     segment.ident,
                     rcvr_ty,
                     expr,
                     probe::ProbeScope::TraitsInScope,
+                    None,
                 ) {
                     Ok(pick) => pick.self_ty,
                     Err(_) => rcvr_ty,
@@ -557,19 +558,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let Some(self_ty) = self.typeck_results.borrow().expr_ty_adjusted_opt(base) else { return; };
 
         let Ok(pick) = self
-            .probe_for_name(
-                probe::Mode::MethodCall,
+            .lookup_probe_for_diagnostic(
                 path.ident,
-                probe::IsSuggestion(true),
                 self_ty,
-                deref.hir_id,
+                deref,
                 probe::ProbeScope::TraitsInScope,
+                None,
             ) else {
                 return;
             };
         let in_scope_methods = self.probe_for_name_many(
             probe::Mode::MethodCall,
             path.ident,
+            Some(expected),
             probe::IsSuggestion(true),
             self_ty,
             deref.hir_id,
@@ -581,6 +582,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let all_methods = self.probe_for_name_many(
             probe::Mode::MethodCall,
             path.ident,
+            Some(expected),
             probe::IsSuggestion(true),
             self_ty,
             deref.hir_id,
@@ -1832,7 +1834,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub fn check_for_range_as_method_call(
         &self,
         err: &mut Diagnostic,
-        expr: &hir::Expr<'_>,
+        expr: &hir::Expr<'tcx>,
         checked_ty: Ty<'tcx>,
         expected_ty: Ty<'tcx>,
     ) {
@@ -1850,10 +1852,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return;
         }
         let mut expr = end.expr;
+        let mut expectation = Some(expected_ty);
         while let hir::ExprKind::MethodCall(_, rcvr, ..) = expr.kind {
             // Getting to the root receiver and asserting it is a fn call let's us ignore cases in
             // `src/test/ui/methods/issues/issue-90315.stderr`.
             expr = rcvr;
+            // If we have more than one layer of calls, then the expected ty
+            // cannot guide the method probe.
+            expectation = None;
         }
         let hir::ExprKind::Call(method_name, _) = expr.kind else { return; };
         let ty::Adt(adt, _) = checked_ty.kind() else { return; };
@@ -1869,13 +1875,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let hir::ExprKind::Path(hir::QPath::Resolved(None, p)) = method_name.kind else { return; };
         let [hir::PathSegment { ident, .. }] = p.segments else { return; };
         let self_ty = self.typeck_results.borrow().expr_ty(start.expr);
-        let Ok(_pick) = self.probe_for_name(
-            probe::Mode::MethodCall,
+        let Ok(_pick) = self.lookup_probe_for_diagnostic(
             *ident,
-            probe::IsSuggestion(true),
             self_ty,
-            expr.hir_id,
+            expr,
             probe::ProbeScope::AllTraits,
+            expectation,
         ) else { return; };
         let mut sugg = ".";
         let mut span = start.expr.span.between(end.expr.span);

compiler/rustc_hir_typeck/src/expr.rs

@@ -351,7 +351,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             ExprKind::Struct(qpath, fields, ref base_expr) => {
                 self.check_expr_struct(expr, expected, qpath, fields, base_expr)
             }
-            ExprKind::Field(base, field) => self.check_field(expr, &base, field),
+            ExprKind::Field(base, field) => self.check_field(expr, &base, field, expected),
             ExprKind::Index(base, idx) => self.check_expr_index(base, idx, expr),
             ExprKind::Yield(value, ref src) => self.check_expr_yield(value, expr, src),
             hir::ExprKind::Err => tcx.ty_error(),
@@ -1244,6 +1244,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         SelfSource::MethodCall(rcvr),
                         error,
                         Some((rcvr, args)),
+                        expected,
                     ) {
                         err.emit();
                     }
@@ -2186,6 +2187,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         expr: &'tcx hir::Expr<'tcx>,
         base: &'tcx hir::Expr<'tcx>,
         field: Ident,
+        expected: Expectation<'tcx>,
     ) -> Ty<'tcx> {
         debug!("check_field(expr: {:?}, base: {:?}, field: {:?})", expr, base, field);
         let base_ty = self.check_expr(base);
@@ -2244,12 +2246,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             // (#90483) apply adjustments to avoid ExprUseVisitor from
             // creating erroneous projection.
             self.apply_adjustments(base, adjustments);
-            self.ban_private_field_access(expr, base_ty, field, did);
+            self.ban_private_field_access(expr, base_ty, field, did, expected.only_has_type(self));
             return self.tcx().ty_error();
         }
 
         if field.name == kw::Empty {
-        } else if self.method_exists(field, base_ty, expr.hir_id, true) {
+        } else if self.method_exists(
+            field,
+            base_ty,
+            expr.hir_id,
+            true,
+            expected.only_has_type(self),
+        ) {
             self.ban_take_value_of_method(expr, base_ty, field);
         } else if !base_ty.is_primitive_ty() {
             self.ban_nonexisting_field(field, base, expr, base_ty);
@@ -2423,10 +2431,11 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
 
     fn ban_private_field_access(
         &self,
-        expr: &hir::Expr<'_>,
+        expr: &hir::Expr<'tcx>,
         expr_t: Ty<'tcx>,
         field: Ident,
         base_did: DefId,
+        return_ty: Option<Ty<'tcx>>,
     ) {
         let struct_path = self.tcx().def_path_str(base_did);
         let kind_name = self.tcx().def_kind(base_did).descr(base_did);
@@ -2438,7 +2447,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         );
         err.span_label(field.span, "private field");
         // Also check if an accessible method exists, which is often what is meant.
-        if self.method_exists(field, expr_t, expr.hir_id, false) && !self.expr_in_place(expr.hir_id)
+        if self.method_exists(field, expr_t, expr.hir_id, false, return_ty)
+            && !self.expr_in_place(expr.hir_id)
         {
             self.suggest_method_call(
                 &mut err,
@@ -2452,7 +2462,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         err.emit();
     }
 
-    fn ban_take_value_of_method(&self, expr: &hir::Expr<'_>, expr_t: Ty<'tcx>, field: Ident) {
+    fn ban_take_value_of_method(&self, expr: &hir::Expr<'tcx>, expr_t: Ty<'tcx>, field: Ident) {
         let mut err = type_error_struct!(
             self.tcx().sess,
             field.span,

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -820,6 +820,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         SelfSource::QPath(qself),
                         error,
                         None,
+                        Expectation::NoExpectation,
                     ) {
                         e.emit();
                     }

compiler/rustc_hir_typeck/src/fn_ctxt/suggestions.rs

@@ -1343,6 +1343,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 if let Ok(pick) = self.probe_for_name(
                     Mode::Path,
                     Ident::new(capitalized_name, segment.ident.span),
+                    Some(expected_ty),
                     IsSuggestion(true),
                     self_ty,
                     expr.hir_id,

compiler/rustc_hir_typeck/src/method/mod.rs

@@ -97,10 +97,12 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         self_ty: Ty<'tcx>,
         call_expr_id: hir::HirId,
         allow_private: bool,
+        return_type: Option<Ty<'tcx>>,
     ) -> bool {
         match self.probe_for_name(
             probe::Mode::MethodCall,
             method_name,
+            return_type,
             IsSuggestion(false),
             self_ty,
             call_expr_id,
@@ -118,7 +120,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             Err(Ambiguity(..)) => true,
             Err(PrivateMatch(..)) => allow_private,
             Err(IllegalSizedBound { .. }) => true,
-            Err(BadReturnType) => bug!("no return type expectations but got BadReturnType"),
+            Err(BadReturnType) => false,
         }
     }
 
@@ -130,17 +132,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         msg: &str,
         method_name: Ident,
         self_ty: Ty<'tcx>,
-        call_expr: &hir::Expr<'_>,
+        call_expr: &hir::Expr<'tcx>,
         span: Option<Span>,
     ) {
         let params = self
-            .probe_for_name(
-                probe::Mode::MethodCall,
+            .lookup_probe_for_diagnostic(
                 method_name,
-                IsSuggestion(true),
                 self_ty,
-                call_expr.hir_id,
+                call_expr,
                 ProbeScope::TraitsInScope,
+                None,
             )
             .map(|pick| {
                 let sig = self.tcx.fn_sig(pick.item.def_id);
@@ -221,25 +222,30 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             }
 
             // We probe again, taking all traits into account (not only those in scope).
-            let candidates =
-                match self.lookup_probe(segment.ident, self_ty, call_expr, ProbeScope::AllTraits) {
-                    // If we find a different result the caller probably forgot to import a trait.
-                    Ok(ref new_pick) if pick.differs_from(new_pick) => {
-                        vec![new_pick.item.container_id(self.tcx)]
-                    }
-                    Err(Ambiguity(ref sources)) => sources
-                        .iter()
-                        .filter_map(|source| {
-                            match *source {
-                                // Note: this cannot come from an inherent impl,
-                                // because the first probing succeeded.
-                                CandidateSource::Impl(def) => self.tcx.trait_id_of_impl(def),
-                                CandidateSource::Trait(_) => None,
-                            }
-                        })
-                        .collect(),
-                    _ => Vec::new(),
-                };
+            let candidates = match self.lookup_probe_for_diagnostic(
+                segment.ident,
+                self_ty,
+                call_expr,
+                ProbeScope::AllTraits,
+                None,
+            ) {
+                // If we find a different result the caller probably forgot to import a trait.
+                Ok(ref new_pick) if pick.differs_from(new_pick) => {
+                    vec![new_pick.item.container_id(self.tcx)]
+                }
+                Err(Ambiguity(ref sources)) => sources
+                    .iter()
+                    .filter_map(|source| {
+                        match *source {
+                            // Note: this cannot come from an inherent impl,
+                            // because the first probing succeeded.
+                            CandidateSource::Impl(def) => self.tcx.trait_id_of_impl(def),
+                            CandidateSource::Trait(_) => None,
+                        }
+                    })
+                    .collect(),
+                _ => Vec::new(),
+            };
 
             return Err(IllegalSizedBound { candidates, needs_mut, bound_span: span, self_expr });
         }
@@ -252,12 +258,13 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         &self,
         method_name: Ident,
         self_ty: Ty<'tcx>,
-        call_expr: &'tcx hir::Expr<'tcx>,
+        call_expr: &hir::Expr<'_>,
         scope: ProbeScope,
     ) -> probe::PickResult<'tcx> {
         let pick = self.probe_for_name(
             probe::Mode::MethodCall,
             method_name,
+            None,
             IsSuggestion(false),
             self_ty,
             call_expr.hir_id,
@@ -267,6 +274,26 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         Ok(pick)
     }
 
+    pub fn lookup_probe_for_diagnostic(
+        &self,
+        method_name: Ident,
+        self_ty: Ty<'tcx>,
+        call_expr: &hir::Expr<'_>,
+        scope: ProbeScope,
+        return_type: Option<Ty<'tcx>>,
+    ) -> probe::PickResult<'tcx> {
+        let pick = self.probe_for_name(
+            probe::Mode::MethodCall,
+            method_name,
+            return_type,
+            IsSuggestion(true),
+            self_ty,
+            call_expr.hir_id,
+            scope,
+        )?;
+        Ok(pick)
+    }
+
     pub(super) fn obligation_for_method(
         &self,
         cause: ObligationCause<'tcx>,
@@ -484,6 +511,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         let pick = self.probe_for_name(
             probe::Mode::Path,
             method_name,
+            None,
             IsSuggestion(false),
             self_ty,
             expr_id,