Implement weakening of variables introduced in branch patterns

Variables introduced in branch patterns should never be generalized in
the new weakening model. This implements that. The strategy is:

- when we have a let-binding that should be weakened, do not introduce
  its bound variables in a new (higher) rank
- instead, introduce them at the current rank, and also solve the
  let-binding at the current rank
- if any of those variables should then be generalized relative to the
  current rank, they will be so when the current rank is popped and
  generalized

crates/compiler/solve/tests/solve_expr.rs

@@ -8674,4 +8674,27 @@ mod solve_expr {
         @"main : (a -[[]]-> b) -[[main(0)]]-> (a -[[y(2) (a -[[]]-> b)]]-> b)"
         );
     }
+
+    #[test]
+    fn when_branch_variables_not_generalized() {
+        infer_queries!(
+            indoc!(
+                r#"
+                app "test" provides [main] to "./platform"
+
+                main = \{} -> when Red is
+                #^^^^{-1}
+                    x ->
+                        y : [Red]_
+                        y = x
+
+                        z : [Red, Green]_
+                        z = x
+
+                        {y, z}
+                "#
+            ),
+        @"main : {}* -[[main(0)]]-> { y : [Green, Red]a, z : [Green, Red]a }"
+        );
+    }
 }