Add UI test for field_reassign_with_default

Author: Henri Lunnikivi

tests/ui/field_reassign_with_default.rs

@@ -0,0 +1,38 @@
+#![warn(clippy::field_reassign_with_default)]
+
+#[derive(Default)]
+struct A {
+    i: i32,
+    j: i64,
+}
+
+struct B {
+    i: i32,
+    j: i64,
+}
+
+#[derive(Default)]
+struct C {
+    i: i32,
+}
+
+fn main() {
+    // wrong
+    let mut a: A = Default::default();
+    a.i = 42;
+
+    // right
+    let mut a: A = Default::default();
+
+    // right
+    let a = A {
+        i: 42,
+        ..Default::default()
+    };
+
+    // right
+    let b = B { i: 42, j: 24 };
+
+    // right
+    let c: C = Default::default();
+}