Auto merge of #145993 - lcnr:allow-calling-opaques, r=BoxyUwU

`-Znext-solver` allow `ExprKind::Call` for not-yet defined opaques

Based on #146329. Revival of #140496. See the comment on `OpaqueTypesJank`. I've used the following document while working on this https://hackmd.io/Js61f8PRTcyaiyqS-fH9iQ.

Fixes rust-lang/trait-system-refactor-initiative#181. It does introduce one subtle footgun we may want to handle before stabilization, opened rust-lang/trait-system-refactor-initiative#230 for that. Also cc rust-lang/trait-system-refactor-initiative#231 for deref and index operations

r? `@BoxyUwU`

Author: bors

compiler/rustc_hir_analysis/src/autoderef.rs

@@ -68,7 +68,14 @@ impl<'a, 'tcx> Iterator for Autoderef<'a, 'tcx> {
             return None;
         }
 
-        if self.state.cur_ty.is_ty_var() {
+        // We want to support method and function calls for `impl Deref<Target = ..>`.
+        //
+        // To do so we don't eagerly bail if the current type is the hidden type of an
+        // opaque type and instead return `None` in `fn overloaded_deref_ty` if the
+        // opaque does not have a `Deref` item-bound.
+        if let &ty::Infer(ty::TyVar(vid)) = self.state.cur_ty.kind()
+            && !self.infcx.has_opaques_with_sub_unified_hidden_type(vid)
+        {
             return None;
         }
 
@@ -160,7 +167,11 @@ impl<'a, 'tcx> Autoderef<'a, 'tcx> {
             self.param_env,
             ty::Binder::dummy(trait_ref),
         );
-        if !self.infcx.next_trait_solver() && !self.infcx.predicate_may_hold(&obligation) {
+        // We detect whether the self type implements `Deref` before trying to
+        // structurally normalize. We use `predicate_may_hold_opaque_types_jank`
+        // to support not-yet-defined opaque types. It will succeed for `impl Deref`
+        // but fail for `impl OtherTrait`.
+        if !self.infcx.predicate_may_hold_opaque_types_jank(&obligation) {
             debug!("overloaded_deref_ty: cannot match obligation");
             return None;
         }

compiler/rustc_hir_typeck/src/callee.rs

@@ -25,6 +25,7 @@ use tracing::{debug, instrument};
 use super::method::MethodCallee;
 use super::method::probe::ProbeScope;
 use super::{Expectation, FnCtxt, TupleArgumentsFlag};
+use crate::method::TreatNotYetDefinedOpaques;
 use crate::{errors, fluent_generated};
 
 /// Checks that it is legal to call methods of the trait corresponding
@@ -78,7 +79,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             _ => self.check_expr(callee_expr),
         };
 
-        let expr_ty = self.structurally_resolve_type(call_expr.span, original_callee_ty);
+        let expr_ty = self.try_structurally_resolve_type(call_expr.span, original_callee_ty);
 
         let mut autoderef = self.autoderef(callee_expr.span, expr_ty);
         let mut result = None;
@@ -200,7 +201,8 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         arg_exprs: &'tcx [hir::Expr<'tcx>],
         autoderef: &Autoderef<'a, 'tcx>,
     ) -> Option<CallStep<'tcx>> {
-        let adjusted_ty = self.structurally_resolve_type(autoderef.span(), autoderef.final_ty());
+        let adjusted_ty =
+            self.try_structurally_resolve_type(autoderef.span(), autoderef.final_ty());
 
         // If the callee is a function pointer or a closure, then we're all set.
         match *adjusted_ty.kind() {
@@ -297,6 +299,15 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 return None;
             }
 
+            ty::Infer(ty::TyVar(vid)) => {
+                // If we end up with an inference variable which is not the hidden type of
+                // an opaque, emit an error.
+                if !self.has_opaques_with_sub_unified_hidden_type(vid) {
+                    self.type_must_be_known_at_this_point(autoderef.span(), adjusted_ty);
+                    return None;
+                }
+            }
+
             ty::Error(_) => {
                 return None;
             }
@@ -367,26 +378,33 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 Ty::new_tup_from_iter(self.tcx, arg_exprs.iter().map(|e| self.next_ty_var(e.span)))
             });
 
+            // We use `TreatNotYetDefinedOpaques::AsRigid` here so that if the `adjusted_ty`
+            // is `Box<impl FnOnce()>` we choose  `FnOnce` instead of `Fn`.
+            //
+            // We try all the different call traits in order and choose the first
+            // one which may apply. So if we treat opaques as inference variables
+            // `Box<impl FnOnce()>: Fn` is considered ambiguous and chosen.
             if let Some(ok) = self.lookup_method_for_operator(
                 self.misc(call_expr.span),
                 method_name,
                 trait_def_id,
                 adjusted_ty,
                 opt_input_type,
+                TreatNotYetDefinedOpaques::AsRigid,
             ) {
                 let method = self.register_infer_ok_obligations(ok);
                 let mut autoref = None;
                 if borrow {
                     // Check for &self vs &mut self in the method signature. Since this is either
                     // the Fn or FnMut trait, it should be one of those.
-                    let ty::Ref(_, _, mutbl) = method.sig.inputs()[0].kind() else {
+                    let ty::Ref(_, _, mutbl) = *method.sig.inputs()[0].kind() else {
                         bug!("Expected `FnMut`/`Fn` to take receiver by-ref/by-mut")
                     };
 
                     // For initial two-phase borrow
                     // deployment, conservatively omit
                     // overloaded function call ops.
-                    let mutbl = AutoBorrowMutability::new(*mutbl, AllowTwoPhase::No);
+                    let mutbl = AutoBorrowMutability::new(mutbl, AllowTwoPhase::No);
 
                     autoref = Some(Adjustment {
                         kind: Adjust::Borrow(AutoBorrow::Ref(mutbl)),

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -1469,24 +1469,25 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     pub(crate) fn structurally_resolve_type(&self, sp: Span, ty: Ty<'tcx>) -> Ty<'tcx> {
         let ty = self.try_structurally_resolve_type(sp, ty);
 
-        if !ty.is_ty_var() {
-            ty
-        } else {
-            let e = self.tainted_by_errors().unwrap_or_else(|| {
-                self.err_ctxt()
-                    .emit_inference_failure_err(
-                        self.body_id,
-                        sp,
-                        ty.into(),
-                        TypeAnnotationNeeded::E0282,
-                        true,
-                    )
-                    .emit()
-            });
-            let err = Ty::new_error(self.tcx, e);
-            self.demand_suptype(sp, err, ty);
-            err
-        }
+        if !ty.is_ty_var() { ty } else { self.type_must_be_known_at_this_point(sp, ty) }
+    }
+
+    #[cold]
+    pub(crate) fn type_must_be_known_at_this_point(&self, sp: Span, ty: Ty<'tcx>) -> Ty<'tcx> {
+        let guar = self.tainted_by_errors().unwrap_or_else(|| {
+            self.err_ctxt()
+                .emit_inference_failure_err(
+                    self.body_id,
+                    sp,
+                    ty.into(),
+                    TypeAnnotationNeeded::E0282,
+                    true,
+                )
+                .emit()
+        });
+        let err = Ty::new_error(self.tcx, guar);
+        self.demand_suptype(sp, err, ty);
+        err
     }
 
     pub(crate) fn structurally_resolve_const(

compiler/rustc_hir_typeck/src/method/mod.rs

@@ -317,7 +317,26 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         )?;
         Ok(pick)
     }
+}
+
+/// Used by [FnCtxt::lookup_method_for_operator] with `-Znext-solver`.
+///
+/// With `AsRigid` we error on `impl Opaque: NotInItemBounds` while
+/// `AsInfer` just treats it as ambiguous and succeeds. This is necessary
+/// as we want [FnCtxt::check_expr_call] to treat not-yet-defined opaque
+/// types as rigid to support `impl Deref<Target = impl FnOnce()>` and
+/// `Box<impl FnOnce()>`.
+///
+/// We only want to treat opaque types as rigid if we need to eagerly choose
+/// between multiple candidates. We otherwise treat them as ordinary inference
+/// variable to avoid rejecting otherwise correct code.
+#[derive(Debug)]
+pub(super) enum TreatNotYetDefinedOpaques {
+    AsInfer,
+    AsRigid,
+}
 
+impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// `lookup_method_in_trait` is used for overloaded operators.
     /// It does a very narrow slice of what the normal probe/confirm path does.
     /// In particular, it doesn't really do any probing: it simply constructs
@@ -331,6 +350,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         trait_def_id: DefId,
         self_ty: Ty<'tcx>,
         opt_rhs_ty: Option<Ty<'tcx>>,
+        treat_opaques: TreatNotYetDefinedOpaques,
     ) -> Option<InferOk<'tcx, MethodCallee<'tcx>>> {
         // Construct a trait-reference `self_ty : Trait<input_tys>`
         let args = GenericArgs::for_item(self.tcx, trait_def_id, |param, _| match param.kind {
@@ -360,7 +380,14 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         );
 
         // Now we want to know if this can be matched
-        if !self.predicate_may_hold(&obligation) {
+        let matches_trait = match treat_opaques {
+            TreatNotYetDefinedOpaques::AsInfer => self.predicate_may_hold(&obligation),
+            TreatNotYetDefinedOpaques::AsRigid => {
+                self.predicate_may_hold_opaque_types_jank(&obligation)
+            }
+        };
+
+        if !matches_trait {
             debug!("--> Cannot match obligation");
             // Cannot be matched, no such method resolution is possible.
             return None;

compiler/rustc_hir_typeck/src/op.rs

@@ -21,6 +21,7 @@ use {rustc_ast as ast, rustc_hir as hir};
 use super::FnCtxt;
 use super::method::MethodCallee;
 use crate::Expectation;
+use crate::method::TreatNotYetDefinedOpaques;
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
     /// Checks a `a <op>= b`
@@ -974,8 +975,18 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             },
         );
 
-        let method =
-            self.lookup_method_for_operator(cause.clone(), opname, trait_did, lhs_ty, opt_rhs_ty);
+        // We don't consider any other candidates if this lookup fails
+        // so we can freely treat opaque types as inference variables here
+        // to allow more code to compile.
+        let treat_opaques = TreatNotYetDefinedOpaques::AsInfer;
+        let method = self.lookup_method_for_operator(
+            cause.clone(),
+            opname,
+            trait_did,
+            lhs_ty,
+            opt_rhs_ty,
+            treat_opaques,
+        );
         match method {
             Some(ok) => {
                 let method = self.register_infer_ok_obligations(ok);

compiler/rustc_hir_typeck/src/opaque_types.rs

@@ -117,21 +117,25 @@ impl<'tcx> FnCtxt<'_, 'tcx> {
                     )
                 }
                 UsageKind::UnconstrainedHiddenType(hidden_type) => {
-                    let infer_var = hidden_type
-                        .ty
-                        .walk()
-                        .filter_map(ty::GenericArg::as_term)
-                        .find(|term| term.is_infer())
-                        .unwrap_or_else(|| hidden_type.ty.into());
-                    self.err_ctxt()
-                        .emit_inference_failure_err(
-                            self.body_id,
-                            hidden_type.span,
-                            infer_var,
-                            TypeAnnotationNeeded::E0282,
-                            false,
-                        )
-                        .emit()
+                    if let Some(guar) = self.tainted_by_errors() {
+                        guar
+                    } else {
+                        let infer_var = hidden_type
+                            .ty
+                            .walk()
+                            .filter_map(ty::GenericArg::as_term)
+                            .find(|term| term.is_infer())
+                            .unwrap_or_else(|| hidden_type.ty.into());
+                        self.err_ctxt()
+                            .emit_inference_failure_err(
+                                self.body_id,
+                                hidden_type.span,
+                                infer_var,
+                                TypeAnnotationNeeded::E0282,
+                                false,
+                            )
+                            .emit()
+                    }
                 }
                 UsageKind::HasDefiningUse => continue,
             };

compiler/rustc_hir_typeck/src/place_op.rs

@@ -12,7 +12,7 @@ use rustc_span::{Span, sym};
 use tracing::debug;
 use {rustc_ast as ast, rustc_hir as hir};
 
-use crate::method::MethodCallee;
+use crate::method::{MethodCallee, TreatNotYetDefinedOpaques};
 use crate::{FnCtxt, PlaceOp};
 
 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
@@ -210,7 +210,17 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return None;
         };
 
-        self.lookup_method_for_operator(self.misc(span), imm_op, imm_tr, base_ty, opt_rhs_ty)
+        // FIXME(trait-system-refactor-initiative#231): we may want to treat
+        // opaque types as rigid here to support `impl Deref<Target = impl Index<usize>>`.
+        let treat_opaques = TreatNotYetDefinedOpaques::AsInfer;
+        self.lookup_method_for_operator(
+            self.misc(span),
+            imm_op,
+            imm_tr,
+            base_ty,
+            opt_rhs_ty,
+            treat_opaques,
+        )
     }
 
     fn try_mutable_overloaded_place_op(
@@ -230,7 +240,19 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             return None;
         };
 
-        self.lookup_method_for_operator(self.misc(span), mut_op, mut_tr, base_ty, opt_rhs_ty)
+        // We have to replace the operator with the mutable variant for the
+        // program to compile, so we don't really have a choice here and want
+        // to just try using `DerefMut` even if its not in the item bounds
+        // of the opaque.
+        let treat_opaques = TreatNotYetDefinedOpaques::AsInfer;
+        self.lookup_method_for_operator(
+            self.misc(span),
+            mut_op,
+            mut_tr,
+            base_ty,
+            opt_rhs_ty,
+            treat_opaques,
+        )
     }
 
     /// Convert auto-derefs, indices, etc of an expression from `Deref` and `Index`

compiler/rustc_infer/src/infer/context.rs

@@ -302,6 +302,9 @@ impl<'tcx> rustc_type_ir::InferCtxtLike for InferCtxt<'tcx> {
             .map(|(k, h)| (k, h.ty))
             .collect()
     }
+    fn opaques_with_sub_unified_hidden_type(&self, ty: ty::TyVid) -> Vec<ty::AliasTy<'tcx>> {
+        self.opaques_with_sub_unified_hidden_type(ty)
+    }
 
     fn register_hidden_type_in_storage(
         &self,

compiler/rustc_infer/src/infer/mod.rs

@@ -1004,6 +1004,60 @@ impl<'tcx> InferCtxt<'tcx> {
         self.inner.borrow_mut().opaque_type_storage.iter_opaque_types().collect()
     }
 
+    pub fn has_opaques_with_sub_unified_hidden_type(&self, ty_vid: TyVid) -> bool {
+        if !self.next_trait_solver() {
+            return false;
+        }
+
+        let ty_sub_vid = self.sub_unification_table_root_var(ty_vid);
+        let inner = &mut *self.inner.borrow_mut();
+        let mut type_variables = inner.type_variable_storage.with_log(&mut inner.undo_log);
+        inner.opaque_type_storage.iter_opaque_types().any(|(_, hidden_ty)| {
+            if let ty::Infer(ty::TyVar(hidden_vid)) = *hidden_ty.ty.kind() {
+                let opaque_sub_vid = type_variables.sub_unification_table_root_var(hidden_vid);
+                if opaque_sub_vid == ty_sub_vid {
+                    return true;
+                }
+            }
+
+            false
+        })
+    }
+
+    /// Searches for an opaque type key whose hidden type is related to `ty_vid`.
+    ///
+    /// This only checks for a subtype relation, it does not require equality.
+    pub fn opaques_with_sub_unified_hidden_type(&self, ty_vid: TyVid) -> Vec<ty::AliasTy<'tcx>> {
+        // Avoid accidentally allowing more code to compile with the old solver.
+        if !self.next_trait_solver() {
+            return vec![];
+        }
+
+        let ty_sub_vid = self.sub_unification_table_root_var(ty_vid);
+        let inner = &mut *self.inner.borrow_mut();
+        // This is iffy, can't call `type_variables()` as we're already
+        // borrowing the `opaque_type_storage` here.
+        let mut type_variables = inner.type_variable_storage.with_log(&mut inner.undo_log);
+        inner
+            .opaque_type_storage
+            .iter_opaque_types()
+            .filter_map(|(key, hidden_ty)| {
+                if let ty::Infer(ty::TyVar(hidden_vid)) = *hidden_ty.ty.kind() {
+                    let opaque_sub_vid = type_variables.sub_unification_table_root_var(hidden_vid);
+                    if opaque_sub_vid == ty_sub_vid {
+                        return Some(ty::AliasTy::new_from_args(
+                            self.tcx,
+                            key.def_id.into(),
+                            key.args,
+                        ));
+                    }
+                }
+
+                None
+            })
+            .collect()
+    }
+
     #[inline(always)]
     pub fn can_define_opaque_ty(&self, id: impl Into<DefId>) -> bool {
         debug_assert!(!self.next_trait_solver());