Codegen for custom values in metadata (#1117)

* work in progress

* add custom types access

* nit

* custom values client

* adjust light client

* adjust doc comments

* adjust book for custom values in code gen

* format and check docs

* work in progress

* add custom types access

* nit

* custom values client

* adjust light client

* codegen and validation

* adjust docs

* use ignore in docs in book

* change iter implementation

* use validation hash and other codegen changes

* add ui test for custom values codegen

* allow 'latest' metadata to be returned from the fallback code (#1127)

* nits

* fix validation check

* fix comments

* nits

---------

Co-authored-by: James Wilson <james@jsdw.me>

subxt/src/custom_values/custom_value_address.rs

@@ -1,15 +1,23 @@
+use std::marker::PhantomData;
+
 use crate::dynamic::DecodedValueThunk;
 use crate::metadata::DecodeWithMetadata;
 
 /// This represents the address of a custom value in in the metadata.
 /// Anything, that implements the [CustomValueAddress] trait can be used, to fetch
 /// custom values from the metadata.
+/// The trait is implemented by [str] for dynamic loopup and [StaticAddress] for static queries.
 pub trait CustomValueAddress {
     /// The type of the custom value.
     type Target: DecodeWithMetadata;
 
     /// the name (key) by which the custom value can be accessed in the metadata.
     fn name(&self) -> &str;
+
+    /// An optional hash which, if present, can be checked against node metadata.
+    fn validation_hash(&self) -> Option<[u8; 32]> {
+        None
+    }
 }
 
 impl CustomValueAddress for str {
@@ -19,3 +27,43 @@ impl CustomValueAddress for str {
         self
     }
 }
+
+/// A static address to a custom value.
+pub struct StaticAddress<R> {
+    name: &'static str,
+    hash: Option<[u8; 32]>,
+    phantom: PhantomData<R>,
+}
+
+impl<R> StaticAddress<R> {
+    #[doc(hidden)]
+    /// Creates a new StaticAddress.
+    pub fn new_static(name: &'static str, hash: [u8; 32]) -> Self {
+        StaticAddress {
+            name,
+            hash: Some(hash),
+            phantom: PhantomData,
+        }
+    }
+
+    /// Do not validate this custom value prior to accessing it.
+    pub fn unvalidated(self) -> Self {
+        Self {
+            name: self.name,
+            hash: None,
+            phantom: self.phantom,
+        }
+    }
+}
+
+impl<R: DecodeWithMetadata> CustomValueAddress for StaticAddress<R> {
+    type Target = R;
+
+    fn name(&self) -> &str {
+        self.name
+    }
+
+    fn validation_hash(&self) -> Option<[u8; 32]> {
+        self.hash
+    }
+}