Introduce MaxEncodedLenNoBound

max-encoded-len/derive/src/lib.rs

@@ -42,6 +42,16 @@ fn generate_crate_access_2018(def_crate: &str) -> Result<syn::Ident, Error> {
 /// Derive `MaxEncodedLen`.
 #[proc_macro_derive(MaxEncodedLen, attributes(max_encoded_len_crate))]
 pub fn derive_max_encoded_len(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+	parse_and_generate_mel(input, false)
+}
+
+/// Derive `MaxEncodedLen` but do not bound any generics.
+#[proc_macro_derive(MaxEncodedLenNoBound, attributes(max_encoded_len_crate))]
+pub fn derive_max_encoded_len_no_bound(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+	parse_and_generate_mel(input, true)
+}
+
+fn parse_and_generate_mel(input: proc_macro::TokenStream, no_bound: bool) -> proc_macro::TokenStream {
 	let input: DeriveInput = match syn::parse(input) {
 		Ok(input) => input,
 		Err(e) => return e.to_compile_error().into(),
@@ -53,7 +63,11 @@ pub fn derive_max_encoded_len(input: proc_macro::TokenStream) -> proc_macro::Tok
 	};
 
 	let name = &input.ident;
-	let generics = add_trait_bounds(input.generics, mel_trait.clone());
+	let generics = if no_bound {
+		input.generics
+	} else {
+		add_trait_bounds(input.generics, mel_trait.clone())
+	};
 	let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();
 
 	let data_expr = data_length_expr(&input.data);

max-encoded-len/src/lib.rs

@@ -48,6 +48,11 @@ use primitive_types::{H160, H256, H512};
 #[cfg(feature = "derive")]
 pub use max_encoded_len_derive::MaxEncodedLen;
 
+/// Same as `MaxEncodedLen`, but without bounding the generics on the derived type with
+/// `MaxEncodedLen`.
+#[cfg(feature = "derive")]
+pub use max_encoded_len_derive::MaxEncodedLenNoBound;
+
 /// Items implementing `MaxEncodedLen` have a statically known maximum encoded size.
 ///
 /// Some containers, such as `BoundedVec`, have enforced size limits and this trait

max-encoded-len/tests/max_encoded_len.rs

@@ -19,7 +19,8 @@
 
 #![cfg(feature = "derive")]
 
-use max_encoded_len::MaxEncodedLen;
+use max_encoded_len::{MaxEncodedLen, MaxEncodedLenNoBound};
+use std::marker::PhantomData;
 use codec::{Compact, Encode};
 
 // These structs won't even compile if the macro isn't working right.
@@ -76,6 +77,16 @@ fn two_generics_max_length() {
 	);
 }
 
+#[derive(Encode, MaxEncodedLenNoBound)]
+struct NoBoundGeneric<T> {
+	one: PhantomData<T>,
+}
+
+#[test]
+fn no_bound_generic() {
+	assert_eq!(NoBoundGeneric::<u8>::max_encoded_len(), 0);
+}
+
 #[derive(Encode, MaxEncodedLen)]
 struct UnitStruct;