added auto un-boxing

README.md

@@ -166,6 +166,57 @@ struct SomeOptions<T> {
 
 this will generate a standard builder method as if `T` wasn't generic.
 
+#### `Box`
+
+The macro detects if a field is a `Box` and generates a builder method that
+accepts the inner type (without `Box`) and adds the box in the body.
+
+In case it's a `Box<dyn Trait>` the builder method will have an argument of type
+`impl Trait`.
+
+If you want to prevent this auto un-boxing you can use the `#[builder(keep_box)]` attribute.
+
+```rust
+trait Test {}
+
+#[derive(DefaultBuilder)]
+struct SomeOptions {
+    the_field: Box<dyn Test + 'static>,
+    other_field: Box<String>,
+
+    #[builder(keep_box)]
+    keep: Box<String>,
+}
+```
+
+This will generate the following code:
+
+```rust
+impl SomeOptions {
+    pub fn the_field(self, value: impl Test + 'static) -> Self {
+        Self {
+            the_field: Box::new(value),
+            ..self
+        }   
+    }
+
+    pub fn other_field(self, value: String) -> Self {
+        Self {
+            other_field: Box::new(value),
+            ..self
+        }
+    }
+
+    pub fn keep(self, value: Box<String>) -> Self {
+        Self {
+            keep: value,
+            ..self
+        }   
+    }
+}
+```
+
+
 ### Related Work
 
 For more general purposes please check out the much more powerful