wip: PackedLayout doc. fixs use-ink#637.

crates/storage/derive/Cargo.toml

@@ -24,6 +24,7 @@ proc-macro2 = "1.0"
 synstructure = "0.12.4"
 
 [dev-dependencies]
+scale = { package = "parity-scale-codec", version = "2.0", default-features = false, features = ["derive", "full"] }
 ink_env = { version = "3.0.0-rc2", path = "../../env" }
 ink_primitives = { version = "3.0.0-rc2", path = "../../primitives" }
 ink_metadata = { version = "3.0.0-rc2", path = "../../metadata" }

crates/storage/derive/src/lib.rs

@@ -97,6 +97,36 @@ synstructure::decl_derive!(
 synstructure::decl_derive!(
     [PackedLayout] =>
     /// Derives `ink_storage`'s `PackedLayout` trait for the given `struct` or `enum`.
+    ///
+    /// # Examples
+    /// ```
+    /// use scale::{Encode, Decode};
+    /// use ink_primitives::Key;
+    /// use ink_storage::traits::{
+    ///     SpreadLayout,
+    ///     PackedLayout,
+    ///     push_packed_root,
+    ///     pull_packed_root
+    /// };
+    ///
+    /// # ink_env::test::run_test::<ink_env::DefaultEnvironment, _>(|_| {
+    /// #[derive(Encode, Decode, SpreadLayout, PackedLayout)]
+    /// struct NamedFields {
+    ///     a: u32,
+    ///     b: [u32; 32],
+    /// }
+    ///
+    /// let mut value = NamedFields {
+    ///     a: 123,
+    ///     b: [22; 32],
+    /// };
+    ///
+    /// push_packed_root(&value, &mut Key::from([0x42; 32]));
+    /// let value2: NamedFields = pull_packed_root(&mut Key::from([0x42; 32]));
+    /// assert_eq!(value.a, value2.a);
+    /// # Ok(())
+    /// # });
+    /// ```
     packed_layout_derive
 );
 synstructure::decl_derive!(