Fix async fn lowering ICE with APIT.

This commit fixes an ICE where simple bindings (which aren't replaced
with replacement arguments during async fn lowering) were not being
visited in the def collector and thus caused an ICE during HIR lowering
for types that use their `DefId` at that point - such as `impl Trait`.

src/librustc/hir/map/def_collector.rs

@@ -92,10 +92,12 @@ impl<'a> DefCollector<'a> {
             visit::walk_generics(this, generics);
 
             // Walk the generated arguments for the `async fn`.
-            for a in arguments {
+            for (i, a) in arguments.iter().enumerate() {
                 use visit::Visitor;
                 if let Some(arg) = &a.arg {
                     this.visit_ty(&arg.ty);
+                } else {
+                    this.visit_ty(&decl.inputs[i].ty);
                 }
             }
 

src/test/ui/async-await/issue-60518.rs

@@ -0,0 +1,12 @@
+// compile-pass
+// edition:2018
+
+#![feature(async_await)]
+
+// This is a regression test to ensure that simple bindings (where replacement arguments aren't
+// created during async fn lowering) that have their DefId used during HIR lowering (such as impl
+// trait) are visited during def collection and thus have a DefId.
+
+async fn foo(ws: impl Iterator<Item = ()>) {}
+
+fn main() {}