Avoid some clone when creating keys to use in StorageMap methods #1876

diff --git a/srml/example/src/lib.rs b/srml/example/src/lib.rs
index 4f4e0b96..98b93d81 100644
--- a/srml/example/src/lib.rs
+++ b/srml/example/src/lib.rs
@@ -20,7 +20,7 @@
 // Ensure we're `no_std` when compiling for Wasm.
 #![cfg_attr(not(feature = "std"), no_std)]
 
-use srml_support::{StorageValue, dispatch::Result, decl_module, decl_storage, decl_event};
+use srml_support::{StorageValue, dispatch::Result, decl_module, decl_storage, decl_event, storage::generator::StorageMap, storage::RuntimeStorage};
 use system::ensure_signed;
 
 /// Our module's configuration trait. All our types and consts go in here. If the
@@ -63,8 +63,10 @@ decl_storage! {
 		// `fn getter_name(key: KeyType) -> ValueType` for map items.
 		Dummy get(dummy) config(): Option<T::Balance>;
 
+		TestDouble: map (T::AccountId, T::AccountId) => u32;
+
 		// A map that has enumerable entries.
-		Bar get(bar) config(): linked_map T::AccountId => T::Balance;
+		// Bar get(bar) config(): linked_map T::AccountId => T::Balance;
 
 
 		// this one uses the default, we'll demonstrate the usage of 'mutate' API.
@@ -162,6 +164,8 @@ decl_module! {
 			// This is a public call, so we ensure that the origin is some signed account.
 			let _sender = ensure_signed(origin)?;
 
+			<TestDouble<T>>::insert(&(&_sender, &_sender), &3, &RuntimeStorage);
+
 			// Read the value of dummy from storage.
 			// let dummy = Self::dummy();
 			// Will also work using the `::get` on the storage item type itself:
diff --git a/srml/support/procedural/src/storage/impls.rs b/srml/support/procedural/src/storage/impls.rs
index e008c81f..5920f0ed 100644
--- a/srml/support/procedural/src/storage/impls.rs
+++ b/srml/support/procedural/src/storage/impls.rs
@@ -145,7 +145,7 @@ impl<'a> Impls<'a> {
 				}
 
 				/// Get the storage key used to fetch a value corresponding to a specific key.
-				fn key_for(x: &#kty) -> #scrate::rstd::vec::Vec<u8> {
+				fn key_for<KSame: #scrate::storage::generator::SameEncodeAs<#kty>>(x: &KSame) -> Vec<u8> {
 					let mut key = #prefix.as_bytes().to_vec();
 					#scrate::codec::Encode::encode_to(x, &mut key);
 					key
diff --git a/srml/support/src/storage/generator.rs b/srml/support/src/storage/generator.rs
index e9c38e87..a49098a3 100644
--- a/srml/support/src/storage/generator.rs
+++ b/srml/support/src/storage/generator.rs
@@ -181,6 +181,11 @@ pub trait StorageList<T: codec::Codec> {
 	fn clear<S: Storage>(storage: &S);
 }
 
+pub trait SameEncodeAs<T>: codec::Encode {}
+
+impl<'a, 'b, A: codec::Encode, B: codec::Encode> SameEncodeAs<(A, B)> for (&'a A, &'b B) {}
+impl<T: codec::Encode> SameEncodeAs<T> for T {}
+
 /// A strongly-typed map in storage.
 pub trait StorageMap<K: codec::Codec, V: codec::Codec> {
 	/// The type that get/take returns.
@@ -190,7 +195,7 @@ pub trait StorageMap<K: codec::Codec, V: codec::Codec> {
 	fn prefix() -> &'static [u8];
 
 	/// Get the storage key used to fetch a value corresponding to a specific key.
-	fn key_for(x: &K) -> Vec<u8>;
+	fn key_for<KSame: SameEncodeAs<K>>(x: &KSame) -> Vec<u8>;
 
 	/// true if the value is defined in storage.
 	fn exists<S: Storage>(key: &K, storage: &S) -> bool {
@@ -204,7 +209,7 @@ pub trait StorageMap<K: codec::Codec, V: codec::Codec> {
 	fn take<S: Storage>(key: &K, storage: &S) -> Self::Query;
 
 	/// Store a value to be associated with the given key from the map.
-	fn insert<S: Storage>(key: &K, val: &V, storage: &S) {
+	fn insert<S: Storage, KSame: SameEncodeAs<K>>(key: &KSame, val: &V, storage: &S) {
 		storage.put(&Self::key_for(key)[..], val);
 	}