Fix "var_universe invoked on bound variable" crash

This should fix rust-lang/rust-analyzer#5495.

`unify_ty_ty` calls `normalize_ty_shallow` on both types and then assumes that
they can't be bound variables. With integer type variables, this assumption was
broken, because you could have a general type variable resolving to an integer
type variable resolving to i32. To fix this, `normalize_ty_shallow` probes
twice, to make sure to fully resolve the variable.

Author: flodiebold

chalk-solve/src/infer.rs

@@ -119,6 +119,15 @@ impl<I: Interner> InferenceTable<I> {
     }
 
     pub fn normalize_ty_shallow(&mut self, interner: &I, leaf: &Ty<I>) -> Option<Ty<I>> {
+        // An integer/float type variable will never normalize to another
+        // variable; but a general type variable might normalize to an
+        // integer/float variable. So we potentially need to normalize twice to
+        // get at the actual value.
+        self.normalize_ty_shallow_inner(interner, leaf)
+            .map(|ty| self.normalize_ty_shallow_inner(interner, &ty).unwrap_or(ty))
+    }
+
+    fn normalize_ty_shallow_inner(&mut self, interner: &I, leaf: &Ty<I>) -> Option<Ty<I>> {
         self.probe_var(leaf.inference_var(interner)?)
             .map(|p| p.assert_ty_ref(interner).clone())
     }

tests/test/numerics.rs

@@ -271,3 +271,26 @@ fn shl_ice() {
         }
     }
 }
+
+/// Regression test for rust-analyzer#5495 ("var_universe invoked on bound
+/// variable" crash).
+#[test]
+fn unify_general_then_specific_ty() {
+    test! {
+        program {
+            #[non_enumerable]
+            trait Foo {}
+            struct Bar<T> {}
+
+            impl<T> Foo for Bar<(T, T, i32, i32)> {}
+        }
+
+        goal {
+            exists<T, int N> {
+                Bar<(N, T, T, T)>: Foo
+            }
+        } yields {
+            "Unique"
+        }
+    }
+}