Parse assoc type bounds in generic params and provide custom diagnostic

src/test/ui/parser/assoc-type-in-type-arg.rs

@@ -0,0 +1,11 @@
+trait Tr {
+    type TrSubtype;
+}
+
+struct Bar<'a, Item: Tr, <Item as Tr>::TrSubtype: 'a> {
+    //~^ ERROR associated type bounds do not belong here
+    item: Item,
+    item_sub: &'a <Item as Tr>::TrSubtype,
+}
+
+fn main() {}

src/test/ui/parser/assoc-type-in-type-arg.stderr

@@ -0,0 +1,8 @@
+error: associated type bounds do not belong here
+  --> $DIR/assoc-type-in-type-arg.rs:5:26
+   |
+LL | struct Bar<'a, Item: Tr, <Item as Tr>::TrSubtype: 'a> {
+   |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^ belongs in `where` clause
+
+error: aborting due to previous error
+