Translation for linked map (#4052)

* make linked_map generic functions better

* implement translation of linked map

* proc-macro for linked map updated

* test linked map migration

* account for instances

* address grumbles

* cut map short if migration fails

substrate/srml/support/procedural/src/lib.rs

@@ -86,6 +86,10 @@ use proc_macro::TokenStream;
 ///   * `head_key`: `"head of " ++ $module_prefix ++ " " ++ $storage_name`
 ///   * `Hasher`: $hash
 ///
+///   All key formatting logic can be accessed in a type-agnostic format via the
+///   [`KeyFormat`](../srml_support/storage/generator/trait.KeyFormat.html) trait, which
+///   is implemented for the storage linked map type as well.
+///
 /// * Double map: `Foo: double_map hasher($hash1) u32, $hash2(u32) => u32`: Implements the
 ///   [`StorageDoubleMap`](../srml_support/storage/trait.StorageDoubleMap.html) trait using the
 ///   [`StorageDoubleMap generator`](../srml_support/storage/generator/trait.StorageDoubleMap.html).

substrate/srml/support/procedural/src/storage/storage_struct.rs

@@ -162,15 +162,12 @@ pub fn decl_and_impl(scrate: &TokenStream, def: &DeclStorageDefExt) -> TokenStre
 					{
 						type Query = #query_type;
 						type Hasher = #scrate::#hasher;
+						type KeyFormat = Self;
 
 						fn prefix() -> &'static [u8] {
 							#final_prefix
 						}
 
-						fn head_key() -> &'static [u8] {
-							#head_key
-						}
-
 						fn from_optional_value_to_query(v: Option<#value_type>) -> Self::Query {
 							#from_optional_value_to_query
 						}
@@ -179,6 +176,14 @@ pub fn decl_and_impl(scrate: &TokenStream, def: &DeclStorageDefExt) -> TokenStre
 							#from_query_to_optional_value
 						}
 					}
+
+					impl<#impl_trait> #scrate::storage::generator::LinkedMapKeyFormat for #storage_struct {
+						type Hasher = #scrate::#hasher;
+
+						fn head_key() -> &'static [u8] {
+							#head_key
+						}
+					}
 				)
 			},
 			StorageLineTypeDef::DoubleMap(map) => {