Properly check the defining scope of existential types

Fixes #52632

Existential types (soon to be 'impl trait' aliases) can either be
delcared at a top-level crate/module scope, or within another item such
as an fn. Previously, we were handling the second case incorrectly when
recursively searching for defining usages - we would check children of
the item, but not the item itself. This lead to us missing closures
that consituted a defining use of the existential type, as their opaque
type instantiations are stored in the TypeckTables of their parent
function.

This commit ensures that we explicitly visit the defining item itself,
not just its children.

src/librustc/infer/opaque_types/mod.rs

@@ -1189,11 +1189,7 @@ pub fn may_define_existential_type(
     opaque_hir_id: hir::HirId,
 ) -> bool {
     let mut hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
-    trace!(
-        "may_define_existential_type(def={:?}, opaque_node={:?})",
-        tcx.hir().get(hir_id),
-        tcx.hir().get(opaque_hir_id)
-    );
+
 
     // Named existential types can be defined by any siblings or children of siblings.
     let scope = tcx.hir().get_defining_scope(opaque_hir_id).expect("could not get defining scope");
@@ -1202,5 +1198,12 @@ pub fn may_define_existential_type(
         hir_id = tcx.hir().get_parent_item(hir_id);
     }
     // Syntactically, we are allowed to define the concrete type if:
-    hir_id == scope
+    let res = hir_id == scope;
+    trace!(
+        "may_define_existential_type(def={:?}, opaque_node={:?}) = {}",
+        tcx.hir().get(hir_id),
+        tcx.hir().get(opaque_hir_id),
+        res
+    );
+    res
 }

src/librustc_typeck/collect.rs

@@ -1664,6 +1664,7 @@ fn find_existential_constraints(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
             intravisit::NestedVisitorMap::All(&self.tcx.hir())
         }
         fn visit_item(&mut self, it: &'tcx Item) {
+            debug!("find_existential_constraints: visiting {:?}", it);
             let def_id = self.tcx.hir().local_def_id(it.hir_id);
             // The existential type itself or its children are not within its reveal scope.
             if def_id != self.def_id {
@@ -1672,6 +1673,7 @@ fn find_existential_constraints(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
             }
         }
         fn visit_impl_item(&mut self, it: &'tcx ImplItem) {
+            debug!("find_existential_constraints: visiting {:?}", it);
             let def_id = self.tcx.hir().local_def_id(it.hir_id);
             // The existential type itself or its children are not within its reveal scope.
             if def_id != self.def_id {
@@ -1680,6 +1682,7 @@ fn find_existential_constraints(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
             }
         }
         fn visit_trait_item(&mut self, it: &'tcx TraitItem) {
+            debug!("find_existential_constraints: visiting {:?}", it);
             let def_id = self.tcx.hir().local_def_id(it.hir_id);
             self.check(def_id);
             intravisit::walk_trait_item(self, it);
@@ -1703,9 +1706,21 @@ fn find_existential_constraints(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
     } else {
         debug!("find_existential_constraints: scope={:?}", tcx.hir().get(scope));
         match tcx.hir().get(scope) {
-            Node::Item(ref it) => intravisit::walk_item(&mut locator, it),
-            Node::ImplItem(ref it) => intravisit::walk_impl_item(&mut locator, it),
-            Node::TraitItem(ref it) => intravisit::walk_trait_item(&mut locator, it),
+            // We explicitly call 'visit_*' methods, instead of using intravisit::walk_* methods
+            // This allows our visitor to process the defining item itself, causing
+            // it to pick up any 'sibling' defining uses.
+            //
+            // For example, this code:
+            // fn foo() {
+            //     existential type Blah: Debug;
+            //     let my_closure = || -> Blah { true };
+            // }
+            //
+            // requires us to explicitly process 'foo()' in order
+            // to notice the defining usage of 'Blah'
+            Node::Item(ref it) => locator.visit_item(it),
+            Node::ImplItem(ref it) => locator.visit_impl_item(it),
+            Node::TraitItem(ref it) => locator.visit_trait_item(it),
             other => bug!(
                 "{:?} is not a valid scope for an existential type item",
                 other

src/test/ui/existential-type/issue-52843.rs

@@ -0,0 +1,11 @@
+#![feature(existential_type)]
+
+use std::fmt::Debug;
+
+fn main() {
+    existential type Existential: Debug;
+    fn _unused() -> Existential { String::new() }
+    //~^ ERROR: concrete type differs from previous defining existential type use
+    let null = || -> Existential { 0 };
+    println!("{:?}", null());
+}

src/test/ui/existential-type/issue-52843.stderr

@@ -0,0 +1,20 @@
+error: concrete type differs from previous defining existential type use
+  --> $DIR/issue-52843.rs:7:5
+   |
+LL |     fn _unused() -> Existential { String::new() }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `i32`, got `std::string::String`
+   |
+note: previous use here
+  --> $DIR/issue-52843.rs:5:1
+   |
+LL | / fn main() {
+LL | |     existential type Existential: Debug;
+LL | |     fn _unused() -> Existential { String::new() }
+LL | |
+LL | |     let null = || -> Existential { 0 };
+LL | |     println!("{:?}", null());
+LL | | }
+   | |_^
+
+error: aborting due to previous error
+