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

Method resolution constrains hidden types instead of rejecting method candidates

Some of these are in probes and may affect inference.

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!()
}
```

r? `@compiler-errors`

Author: bors

compiler/rustc_hir_typeck/src/callee.rs

@@ -637,7 +637,7 @@ 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

@@ -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

@@ -27,8 +27,8 @@ fn ham() -> Foo {
 
 fn oof(_: Foo) -> impl std::fmt::Debug {
     let mut bar = ham();
-    let func = bar.next().unwrap();
-    return func(&"oof"); //~ ERROR opaque type's hidden type cannot be another opaque type
+    let func = bar.next().unwrap(); //~ ERROR: type annotations needed
+    return func(&"oof");
 }
 
 fn main() {

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

@@ -1,19 +1,16 @@
-error: opaque type's hidden type cannot be another opaque type from the same scope
-  --> $DIR/issue-70877.rs:31:12
+error[E0282]: type annotations needed
+  --> $DIR/issue-70877.rs:30:9
    |
+LL |     let func = bar.next().unwrap();
+   |         ^^^^
 LL |     return func(&"oof");
-   |            ^^^^^^^^^^^^ one of the two opaque types used here has to be outside its defining scope
+   |            ------------ type must be known at this point
    |
-note: opaque type whose hidden type is being assigned
-  --> $DIR/issue-70877.rs:28:19
+help: consider giving `func` an explicit type
    |
-LL | fn oof(_: Foo) -> impl std::fmt::Debug {
-   |                   ^^^^^^^^^^^^^^^^^^^^
-note: opaque type being used as hidden type
-  --> $DIR/issue-70877.rs:4:15
-   |
-LL | type FooRet = impl std::fmt::Debug;
-   |               ^^^^^^^^^^^^^^^^^^^^
+LL |     let func: /* Type */ = bar.next().unwrap();
+   |             ++++++++++++
 
 error: aborting due to 1 previous error
 
+For more information about this error, try `rustc --explain E0282`.

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

@@ -0,0 +1,21 @@
+//@ check-pass
+
+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();
+    }
+    todo!()
+}
+
+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/methods/opaque_param_in_ufc.stderr

@@ -0,0 +1,22 @@
+#![feature(type_alias_impl_trait)]
+
+type Foo = impl Sized;
+
+struct Bar<T>(T);
+
+impl Bar<Foo> {
+    fn bar(self) {}
+}
+
+impl Bar<u32> {
+    fn foo(self) {
+        self.bar()
+        //~^ ERROR: no method named `bar`
+    }
+}
+
+fn foo() -> Foo {
+    42_u32
+}
+
+fn main() {}