Auto merge of rust-lang#123962 - oli-obk:define_opaque_types5, r=<try>

change method resolution to constrain hidden types instead of rejecting method candidates

Some of these are in probes and may affect inference. This is therefore a breaking change.

This allows new code to compile on stable:

```rust
trait Trait {}

impl Trait for u32 {}

struct Bar<T>(T);

impl Bar<u32> {
    fn foo(self) {}
}

fn foo(x: bool) -> Bar<impl Sized> {
    if x {
        let x = foo(false);
        x.foo();
        //^ this used to not find the `foo` method, because while we did equate `x`'s type with possible candidates, we didn't allow opaque type inference while doing so
    }
    todo!()
}
```

But it is also a breaking change, since `&self` and `&mut self` method calls on recursive RPIT function calls now constrain the hidden type to `&_` and `&mut _` respectively. This is not what users really expect or want from this, but there's way around this.

r? `@compiler-errors`

fixes  rust-lang#121404

cc rust-lang#116652

compiler/rustc_data_structures/src/obligation_forest/mod.rs

@@ -359,7 +359,10 @@ impl<O: ForestObligation> ObligationForest<O> {
             Entry::Vacant(v) => {
                 let obligation_tree_id = match parent {
                     Some(parent_index) => self.nodes[parent_index].obligation_tree_id,
-                    None => self.obligation_tree_id_generator.next().unwrap(),
+                    // FIXME(type_alias_impl_trait): with `#[defines]` attributes required to define hidden
+                    // types we can convert this back to a `next` method call, as this function shouldn't be
+                    // defining a hidden type anyway.
+                    None => Iterator::next(&mut self.obligation_tree_id_generator).unwrap(),
                 };
 
                 let already_failed = parent.is_some()

compiler/rustc_hir_typeck/src/callee.rs

@@ -627,7 +627,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                 return;
             };
 
-            let pick = self.confirm_method(
+            let pick = self.confirm_method_for_diagnostic(
                 call_expr.span,
                 callee_expr,
                 call_expr,

compiler/rustc_hir_typeck/src/fn_ctxt/_impl.rs

@@ -1435,7 +1435,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             let impl_ty = self.normalize(span, tcx.type_of(impl_def_id).instantiate(tcx, args));
             let self_ty = self.normalize(span, self_ty);
             match self.at(&self.misc(span), self.param_env).eq(
-                DefineOpaqueTypes::No,
+                DefineOpaqueTypes::Yes,
                 impl_ty,
                 self_ty,
             ) {

compiler/rustc_hir_typeck/src/method/confirm.rs

@@ -499,7 +499,7 @@ impl<'a, 'tcx> ConfirmContext<'a, 'tcx> {
                 args,
             })),
         );
-        match self.at(&cause, self.param_env).sup(DefineOpaqueTypes::No, method_self_ty, self_ty) {
+        match self.at(&cause, self.param_env).sup(DefineOpaqueTypes::Yes, method_self_ty, self_ty) {
             Ok(InferOk { obligations, value: () }) => {
                 self.register_predicates(obligations);
             }

compiler/rustc_hir_typeck/src/method/probe.rs

@@ -651,8 +651,8 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         }
     }
 
+    #[instrument(level = "debug", skip(self))]
     fn assemble_probe(&mut self, self_ty: &Canonical<'tcx, QueryResponse<'tcx, Ty<'tcx>>>) {
-        debug!("assemble_probe: self_ty={:?}", self_ty);
         let raw_self_ty = self_ty.value.value;
         match *raw_self_ty.kind() {
             ty::Dynamic(data, ..) if let Some(p) = data.principal() => {
@@ -730,13 +730,12 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         }
     }
 
+    #[instrument(level = "debug", skip(self))]
     fn assemble_inherent_impl_probe(&mut self, impl_def_id: DefId) {
         if !self.impl_dups.insert(impl_def_id) {
             return; // already visited
         }
 
-        debug!("assemble_inherent_impl_probe {:?}", impl_def_id);
-
         for item in self.impl_or_trait_item(impl_def_id) {
             if !self.has_applicable_self(&item) {
                 // No receiver declared. Not a candidate.
@@ -786,9 +785,8 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         }
     }
 
+    #[instrument(level = "debug", skip(self))]
     fn assemble_inherent_candidates_from_object(&mut self, self_ty: Ty<'tcx>) {
-        debug!("assemble_inherent_candidates_from_object(self_ty={:?})", self_ty);
-
         let principal = match self_ty.kind() {
             ty::Dynamic(ref data, ..) => Some(data),
             _ => None,
@@ -835,9 +833,9 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         });
     }
 
+    #[instrument(level = "debug", skip(self))]
     fn assemble_inherent_candidates_from_param(&mut self, param_ty: ty::ParamTy) {
         // FIXME: do we want to commit to this behavior for param bounds?
-        debug!("assemble_inherent_candidates_from_param(param_ty={:?})", param_ty);
 
         let bounds = self.param_env.caller_bounds().iter().filter_map(|predicate| {
             let bound_predicate = predicate.kind();
@@ -904,6 +902,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         }
     }
 
+    #[instrument(level = "debug", skip(self))]
     fn assemble_extension_candidates_for_traits_in_scope(&mut self) {
         let mut duplicates = FxHashSet::default();
         let opt_applicable_traits = self.tcx.in_scope_traits(self.scope_expr_id);
@@ -920,6 +919,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         }
     }
 
+    #[instrument(level = "debug", skip(self))]
     fn assemble_extension_candidates_for_all_traits(&mut self) {
         let mut duplicates = FxHashSet::default();
         for trait_info in suggest::all_traits(self.tcx) {
@@ -956,12 +956,12 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         }
     }
 
+    #[instrument(level = "debug", skip(self))]
     fn assemble_extension_candidates_for_trait(
         &mut self,
         import_ids: &SmallVec<[LocalDefId; 1]>,
         trait_def_id: DefId,
     ) {
-        debug!("assemble_extension_candidates_for_trait(trait_def_id={:?})", trait_def_id);
         let trait_args = self.fresh_args_for_item(self.span, trait_def_id);
         let trait_ref = ty::TraitRef::new(self.tcx, trait_def_id, trait_args);
 
@@ -1066,6 +1066,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     ///////////////////////////////////////////////////////////////////////////
     // THE ACTUAL SEARCH
 
+    #[instrument(level = "debug", skip(self))]
     fn pick(mut self) -> PickResult<'tcx> {
         assert!(self.method_name.is_some());
 
@@ -1474,6 +1475,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         }
     }
 
+    #[instrument(level = "trace", skip(self, possibly_unsatisfied_predicates), ret)]
     fn consider_probe(
         &self,
         self_ty: Ty<'tcx>,
@@ -1484,20 +1486,29 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             Option<ObligationCause<'tcx>>,
         )>,
     ) -> ProbeResult {
-        debug!("consider_probe: self_ty={:?} probe={:?}", self_ty, probe);
-
         self.probe(|_| {
             // First check that the self type can be related.
-            let sub_obligations = match self.at(&ObligationCause::dummy(), self.param_env).sup(
-                DefineOpaqueTypes::No,
-                probe.xform_self_ty,
-                self_ty,
-            ) {
-                Ok(InferOk { obligations, value: () }) => obligations,
-                Err(err) => {
-                    debug!("--> cannot relate self-types {:?}", err);
+            let sub_obligations = match self_ty.kind() {
+                // HACK: opaque types will match anything for which their bounds hold.
+                // Thus we need to prevent them from trying to match the `&_` autoref
+                // candidates that get created for `&self` trait methods.
+                ty::Alias(ty::Opaque, alias_ty)
+                    if self.infcx.can_define_opaque_ty(alias_ty.def_id)
+                        && !probe.xform_self_ty.is_ty_var() =>
+                {
                     return ProbeResult::NoMatch;
                 }
+                _ => match self.at(&ObligationCause::dummy(), self.param_env).sup(
+                    DefineOpaqueTypes::Yes,
+                    probe.xform_self_ty,
+                    self_ty,
+                ) {
+                    Ok(InferOk { obligations, value: () }) => obligations,
+                    Err(err) => {
+                        debug!("--> cannot relate self-types {:?}", err);
+                        return ProbeResult::NoMatch;
+                    }
+                },
             };
 
             let mut result = ProbeResult::Match;
@@ -1755,6 +1766,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     /// Similarly to `probe_for_return_type`, this method attempts to find the best matching
     /// candidate method where the method name may have been misspelled. Similarly to other
     /// edit distance based suggestions, we provide at most one such suggestion.
+    #[instrument(level = "debug", skip(self))]
     pub(crate) fn probe_for_similar_candidate(
         &mut self,
     ) -> Result<Option<ty::AssocItem>, MethodError<'tcx>> {

compiler/rustc_middle/src/traits/query.rs

@@ -162,7 +162,7 @@ pub struct CandidateStep<'tcx> {
 
 #[derive(Copy, Clone, Debug, HashStable)]
 pub struct MethodAutoderefStepsResult<'tcx> {
-    /// The valid autoderef steps that could be find.
+    /// The valid autoderef steps that could be found.
     pub steps: &'tcx [CandidateStep<'tcx>],
     /// If Some(T), a type autoderef reported an error on.
     pub opt_bad_ty: Option<&'tcx MethodAutoderefBadTy<'tcx>>,

tests/ui/impl-trait/issues/issue-70877.rs

@@ -27,7 +27,9 @@ fn ham() -> Foo {
 
 fn oof(_: Foo) -> impl std::fmt::Debug {
     let mut bar = ham();
-    let func = bar.next().unwrap();
+    // Need to UFC invoke `Iterator::next`,
+    // as otherwise the hidden type gets constrained to `&mut _`
+    let func = Iterator::next(&mut bar).unwrap();
     return func(&"oof"); //~ ERROR opaque type's hidden type cannot be another opaque type
 }
 

tests/ui/impl-trait/issues/issue-70877.stderr

@@ -1,5 +1,5 @@
 error: opaque type's hidden type cannot be another opaque type from the same scope
-  --> $DIR/issue-70877.rs:31:12
+  --> $DIR/issue-70877.rs:33:12
    |
 LL |     return func(&"oof");
    |            ^^^^^^^^^^^^ one of the two opaque types used here has to be outside its defining scope

tests/ui/impl-trait/method-resolution.rs

@@ -0,0 +1,26 @@
+//! Since there is only one possible `bar` method, we invoke it and subsequently
+//! constrain `foo`'s RPIT to `u32`.
+
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@ check-pass
+
+trait Trait {}
+
+impl Trait for u32 {}
+
+struct Bar<T>(T);
+
+impl Bar<u32> {
+    fn bar(self) {}
+}
+
+fn foo(x: bool) -> Bar<impl Sized> {
+    if x {
+        let x = foo(false);
+        x.bar();
+    }
+    todo!()
+}
+
+fn main() {}

tests/ui/impl-trait/method-resolution2.next.stderr

@@ -0,0 +1,20 @@
+error[E0034]: multiple applicable items in scope
+  --> $DIR/method-resolution2.rs:25:11
+   |
+LL |         x.bar();
+   |           ^^^ multiple `bar` found
+   |
+note: candidate #1 is defined in an impl for the type `Bar<T>`
+  --> $DIR/method-resolution2.rs:19:5
+   |
+LL |     fn bar(self) {}
+   |     ^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Bar<u32>`
+  --> $DIR/method-resolution2.rs:15:5
+   |
+LL |     fn bar(self) {}
+   |     ^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0034`.

tests/ui/impl-trait/method-resolution2.rs

@@ -0,0 +1,31 @@
+//! Check that the method call does not constrain the RPIT to `i32`, even though
+//! `i32` is the only type that satisfies the RPIT's trait bounds.
+
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+//@[current] check-pass
+
+trait Trait {}
+
+impl Trait for i32 {}
+
+struct Bar<T>(T);
+
+impl Bar<u32> {
+    fn bar(self) {}
+}
+
+impl<T: Trait> Bar<T> {
+    fn bar(self) {}
+}
+
+fn foo(x: bool) -> Bar<impl Trait> {
+    if x {
+        let x = foo(false);
+        x.bar();
+        //[next]~^ ERROR: multiple applicable items in scope
+    }
+    Bar(42_i32)
+}
+
+fn main() {}

tests/ui/impl-trait/method-resolution3.current.stderr

@@ -0,0 +1,20 @@
+error[E0034]: multiple applicable items in scope
+  --> $DIR/method-resolution3.rs:21:11
+   |
+LL |         x.bar();
+   |           ^^^ multiple `bar` found
+   |
+note: candidate #1 is defined in an impl for the type `Bar<i32>`
+  --> $DIR/method-resolution3.rs:15:5
+   |
+LL |     fn bar(self) {}
+   |     ^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Bar<u32>`
+  --> $DIR/method-resolution3.rs:11:5
+   |
+LL |     fn bar(self) {}
+   |     ^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0034`.

tests/ui/impl-trait/method-resolution3.next.stderr

@@ -0,0 +1,20 @@
+error[E0034]: multiple applicable items in scope
+  --> $DIR/method-resolution3.rs:21:11
+   |
+LL |         x.bar();
+   |           ^^^ multiple `bar` found
+   |
+note: candidate #1 is defined in an impl for the type `Bar<i32>`
+  --> $DIR/method-resolution3.rs:15:5
+   |
+LL |     fn bar(self) {}
+   |     ^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Bar<u32>`
+  --> $DIR/method-resolution3.rs:11:5
+   |
+LL |     fn bar(self) {}
+   |     ^^^^^^^^^^^^
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0034`.

tests/ui/impl-trait/method-resolution3.rs

@@ -0,0 +1,27 @@
+//! Check that we consider `Bar<impl Sized>` to successfully unify
+//! with both `Bar<u32>` and `Bar<i32>` (in isolation), so we bail
+//! out with ambiguity.
+
+//@ revisions: current next
+//@[next] compile-flags: -Znext-solver
+
+struct Bar<T>(T);
+
+impl Bar<u32> {
+    fn bar(self) {}
+}
+
+impl Bar<i32> {
+    fn bar(self) {}
+}
+
+fn foo(x: bool) -> Bar<impl Sized> {
+    if x {
+        let x = foo(false);
+        x.bar();
+        //~^ ERROR: multiple applicable items in scope
+    }
+    todo!()
+}
+
+fn main() {}

tests/ui/impl-trait/method-resolution4.next.stderr

@@ -0,0 +1,22 @@
+error[E0282]: type annotations needed
+  --> $DIR/method-resolution4.rs:13:9
+   |
+LL |         foo(false).next().unwrap();
+   |         ^^^^^^^^^^ cannot infer type
+
+error[E0308]: mismatched types
+  --> $DIR/method-resolution4.rs:16:5
+   |
+LL | fn foo(b: bool) -> impl Iterator<Item = ()> {
+   |                    ------------------------ the expected opaque type
+...
+LL |     std::iter::empty()
+   |     ^^^^^^^^^^^^^^^^^^ types differ
+   |
+   = note: expected opaque type `impl Iterator<Item = ()>`
+                   found struct `std::iter::Empty<_>`
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0282, E0308.
+For more information about an error, try `rustc --explain E0282`.