ref: add IErc20Capped trait

Refers #336

contracts/src/token/erc20/extensions/burnable.rs

@@ -9,7 +9,7 @@ use crate::token::erc20::{Erc20, Error};
 /// their own tokens and those that they have an allowance for,
 /// in a way that can be recognized off-chain (via event analysis).
 pub trait IErc20Burnable {
-    /// The error type associated to this ERC-20 Burnable trait implementation.
+    /// The error type associated to the trait implementation.
     type Error: Into<alloc::vec::Vec<u8>>;
 
     /// Destroys a `value` amount of tokens from the caller. lowering the total

contracts/src/token/erc20/extensions/capped.rs

@@ -1,4 +1,4 @@
-//! Capped Contract.
+//! Optional Capped Contract.
 //!
 //! Extension of ERC-20 standard that adds a cap to the supply of tokens.
 //!
@@ -43,10 +43,21 @@ sol_storage! {
     }
 }
 
+/// Extension of [`Erc-20`] that adds a cap to the supply of tokens.
+pub trait IErc20Capped {
+    /// The error type associated to the trait implementation.
+    type Error: Into<alloc::vec::Vec<u8>>;
+
+    /// Returns the cap on the token's total supply.
+    fn cap(&self) -> U256;
+}
+
 #[public]
-impl Capped {
+impl IErc20Capped for Capped {
+    type Error = Error;
+
     /// Returns the cap on the token's total supply.
-    pub fn cap(&self) -> U256 {
+    fn cap(&self) -> U256 {
         self._cap.get()
     }
 }
@@ -55,7 +66,7 @@ impl Capped {
 mod tests {
     use alloy_primitives::uint;
 
-    use super::Capped;
+    use super::{Capped, IErc20Capped};
 
     #[motsu::test]
     fn cap_works(contract: Capped) {

contracts/src/token/erc20/extensions/mod.rs

@@ -5,6 +5,6 @@ pub mod metadata;
 pub mod permit;
 
 pub use burnable::IErc20Burnable;
-pub use capped::Capped;
+pub use capped::{Capped, IErc20Capped};
 pub use metadata::{Erc20Metadata, IErc20Metadata};
 pub use permit::Erc20Permit;

examples/erc20/src/lib.rs

@@ -6,7 +6,9 @@ use alloc::vec::Vec;
 use alloy_primitives::{Address, FixedBytes, U256};
 use openzeppelin_stylus::{
     token::erc20::{
-        extensions::{capped, Capped, Erc20Metadata, IErc20Burnable},
+        extensions::{
+            capped, Capped, Erc20Metadata, IErc20Burnable, IErc20Capped,
+        },
         Erc20, IErc20,
     },
     utils::{introspection::erc165::IErc165, Pausable},