Method resolution constrains hidden types instead of rejecting method…

… candidates

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/fn_ctxt/_impl.rs

@@ -1448,7 +1448,7 @@ 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

@@ -1488,7 +1488,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         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,
+                DefineOpaqueTypes::Yes,
                 probe.xform_self_ty,
                 self_ty,
             ) {

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.current.stderr

@@ -1,30 +1,27 @@
-error[E0599]: no method named `bar` found for struct `Bar<impl Sized>` in the current scope
-  --> $DIR/method-resolution.rs:23:11
+error[E0599]: no method named `foo` found for struct `Bar` in the current scope
+  --> $DIR/method-resolution.rs:22:11
    |
 LL | struct Bar<T>(T);
-   | ------------- method `bar` not found for this struct
+   | ------------- method `foo` not found for this struct
 ...
-LL |         x.bar();
+LL |         x.foo();
    |           ^^^ method not found in `Bar<impl Sized>`
-   |
-   = note: the method was found for
-           - `Bar<u32>`
 
 error[E0391]: cycle detected when computing type of opaque `foo::{opaque#0}`
-  --> $DIR/method-resolution.rs:19:24
+  --> $DIR/method-resolution.rs:18:24
    |
 LL | fn foo(x: bool) -> Bar<impl Sized> {
    |                        ^^^^^^^^^^
    |
 note: ...which requires type-checking `foo`...
-  --> $DIR/method-resolution.rs:23:9
+  --> $DIR/method-resolution.rs:22:9
    |
-LL |         x.bar();
+LL |         x.foo();
    |         ^
    = note: ...which requires evaluating trait selection obligation `Bar<foo::{opaque#0}>: core::marker::Unpin`...
    = note: ...which again requires computing type of opaque `foo::{opaque#0}`, completing the cycle
 note: cycle used when computing type of `foo::{opaque#0}`
-  --> $DIR/method-resolution.rs:19:24
+  --> $DIR/method-resolution.rs:18:24
    |
 LL | fn foo(x: bool) -> Bar<impl Sized> {
    |                        ^^^^^^^^^^

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

@@ -0,0 +1,12 @@
+error[E0599]: no method named `foo` found for struct `Bar` in the current scope
+  --> $DIR/method-resolution.rs:22:11
+   |
+LL | struct Bar<T>(T);
+   | ------------- method `foo` not found for this struct
+...
+LL |         x.foo();
+   |           ^^^ method not found in `Bar<_>`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0599`.

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

@@ -4,7 +4,6 @@
 
 //@ revisions: current next
 //@[next] compile-flags: -Znext-solver
-//@[next] check-pass
 
 trait Trait {}
 
@@ -17,11 +16,11 @@ impl Bar<u32> {
 }
 
 fn foo(x: bool) -> Bar<impl Sized> {
-    //[current]~^ ERROR: cycle detected
+    //[current]~^ ERROR: cycle
     if x {
         let x = foo(false);
-        x.bar();
-        //[current]~^ ERROR: no method named `bar` found
+        x.foo();
+        //~^ ERROR: no method named `foo` found
     }
     todo!()
 }

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

@@ -1,15 +1,11 @@
-error[E0599]: no method named `bar` found for struct `Bar<impl Sized>` in the current scope
+error[E0599]: no method named `foo` found for struct `Bar` in the current scope
   --> $DIR/method-resolution3.rs:22:11
    |
 LL | struct Bar<T>(T);
-   | ------------- method `bar` not found for this struct
+   | ------------- method `foo` not found for this struct
 ...
-LL |         x.bar();
+LL |         x.foo();
    |           ^^^ method not found in `Bar<impl Sized>`
-   |
-   = note: the method was found for
-           - `Bar<i32>`
-           - `Bar<u32>`
 
 error[E0391]: cycle detected when computing type of opaque `foo::{opaque#0}`
   --> $DIR/method-resolution3.rs:18:24
@@ -20,7 +16,7 @@ LL | fn foo(x: bool) -> Bar<impl Sized> {
 note: ...which requires type-checking `foo`...
   --> $DIR/method-resolution3.rs:22:9
    |
-LL |         x.bar();
+LL |         x.foo();
    |         ^
    = note: ...which requires evaluating trait selection obligation `Bar<foo::{opaque#0}>: core::marker::Unpin`...
    = note: ...which again requires computing type of opaque `foo::{opaque#0}`, completing the cycle

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

@@ -1,20 +1,12 @@
-error[E0034]: multiple applicable items in scope
+error[E0599]: no method named `foo` found for struct `Bar` in the current scope
   --> $DIR/method-resolution3.rs:22:11
    |
-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) {}
-   |     ^^^^^^^^^^^^
+LL | struct Bar<T>(T);
+   | ------------- method `foo` not found for this struct
+...
+LL |         x.foo();
+   |           ^^^ method not found in `Bar<_>`
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0034`.
+For more information about this error, try `rustc --explain E0599`.

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

@@ -19,9 +19,8 @@ fn foo(x: bool) -> Bar<impl Sized> {
     //[current]~^ ERROR: cycle
     if x {
         let x = foo(false);
-        x.bar();
-        //[current]~^ ERROR: no method named `bar`
-        //[next]~^^ ERROR: multiple applicable items in scope
+        x.foo();
+        //~^ ERROR: no method named `foo` found
     }
     todo!()
 }

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

@@ -0,0 +1,16 @@
+error[E0308]: mismatched types
+  --> $DIR/method-resolution4.rs:15:5
+   |
+LL |     std::iter::empty()
+   |     ^^^^^^^^^^^^^^^^^^ expected `&mut _`, found `Empty<_>`
+   |
+   = note: expected mutable reference `&mut _`
+                         found struct `std::iter::Empty<_>`
+help: consider mutably borrowing here
+   |
+LL |     &mut std::iter::empty()
+   |     ++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0308`.

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

@@ -1,11 +1,11 @@
 error[E0282]: type annotations needed
-  --> $DIR/method-resolution4.rs:13:9
+  --> $DIR/method-resolution4.rs:12:9
    |
 LL |         foo(false).next().unwrap();
    |         ^^^^^^^^^^ cannot infer type
 
 error[E0308]: mismatched types
-  --> $DIR/method-resolution4.rs:16:5
+  --> $DIR/method-resolution4.rs:15:5
    |
 LL | fn foo(b: bool) -> impl Iterator<Item = ()> {
    |                    ------------------------ the expected opaque type

tests/ui/impl-trait/method-resolution4.rs

@@ -4,7 +4,6 @@
 //! variable, but get a type mismatch when comparing `&mut _` with
 //! `std::iter::Empty`.
 
-//@[current] check-pass
 //@ revisions: current next
 //@[next] compile-flags: -Znext-solver
 
@@ -14,7 +13,7 @@ fn foo(b: bool) -> impl Iterator<Item = ()> {
         //[next]~^ type annotations needed
     }
     std::iter::empty()
-    //[next]~^ mismatched types
+    //~^ mismatched types
 }
 
 fn main() {}

tests/ui/methods/opaque_param_in_ufc.rs

@@ -1,4 +1,7 @@
 #![feature(type_alias_impl_trait)]
+
+//@ check-pass
+
 struct Foo<T>(T);
 
 impl Foo<u32> {
@@ -15,14 +18,11 @@ fn bar() -> Bar {
 impl Foo<Bar> {
     fn foo() -> Bar {
         Self::method();
-        //~^ ERROR: no function or associated item named `method` found for struct `Foo<Bar>`
         Foo::<Bar>::method();
-        //~^ ERROR: no function or associated item named `method` found for struct `Foo<Bar>`
         let x = Foo(bar());
         Foo::method2(x);
         let x = Self(bar());
         Self::method2(x);
-        //~^ ERROR: no function or associated item named `method2` found for struct `Foo<Bar>`
         todo!()
     }
 }

tests/ui/type-alias-impl-trait/method_resolution2.rs

@@ -3,19 +3,17 @@
 
 //@ revisions: current next
 //@[next] compile-flags: -Znext-solver
-//@[next] check-pass
+//@ check-pass
 
 #![feature(type_alias_impl_trait)]
 
 type Foo = impl Sized;
-//[current]~^ ERROR: cycle
 
 struct Bar<T>(T);
 
 impl Bar<Foo> {
     fn bar(self) {
         self.foo()
-        //[current]~^ ERROR: no method named `foo`
     }
 }
 

tests/ui/type-alias-impl-trait/method_resolution_trait_method_from_opaque.current.stderr

@@ -0,0 +1,15 @@
+error[E0284]: type annotations needed
+  --> $DIR/method_resolution_trait_method_from_opaque.rs:25:18
+   |
+LL |         self.bar.next().unwrap();
+   |                  ^^^^
+   |
+   = note: cannot satisfy `<_ as Iterator>::Item == _`
+help: try using a fully qualified path to specify the expected types
+   |
+LL |         <_ as Iterator>::next(self.bar).unwrap();
+   |         ++++++++++++++++++++++        ~
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0284`.

tests/ui/type-alias-impl-trait/method_resolution_trait_method_from_opaque.next.stderr

@@ -1,5 +1,5 @@
 error[E0282]: type annotations needed
-  --> $DIR/method_resolution_trait_method_from_opaque.rs:26:9
+  --> $DIR/method_resolution_trait_method_from_opaque.rs:25:9
    |
 LL |         self.bar.next().unwrap();
    |         ^^^^^^^^ cannot infer type