feat(erc20): implement Metadata extension

contracts/Cargo.toml

@@ -22,6 +22,7 @@ wavm-shims = { path = "../lib/wavm-shims" }
 [features]
 default = []
 erc20 = []
+erc20_metadata = ["erc20"]
 erc721 = []
 
 [lib]

contracts/src/erc20/extensions/metadata.rs

@@ -0,0 +1,125 @@
+//! Optional metadata of the ERC-20 standard.
+use alloc::string::String;
+use alloy_primitives::U8;
+use stylus_proc::{external, sol_storage};
+
+/// Number of decimals used by default on implementors of [`Metadata`].
+pub const DEFAULT_DECIMALS: u8 = 18;
+
+sol_storage! {
+    /// Optional metadata of the ERC-20 standard.
+    pub struct Metadata {
+        /// Token name.
+        string _name;
+        /// Token symbol.
+        string _symbol;
+        /// Token symbol.
+        uint8 _decimals;
+    }
+}
+
+#[external]
+impl Metadata {
+    /// Initializes a [`Metadata`] instance with the passed `name` and
+    /// `symbol`. It also sets `decimals` to [`DEFAULT_DECIMALS`].
+    ///
+    /// Note that there are no setters for these fields. This makes them
+    /// immutable: they can only be set once at construction.
+    ///
+    /// # Arguments
+    ///
+    /// * `&mut self` - Write access to the contract's state.
+    /// * `name` - The name of the token.
+    /// * `symbol` - The symbol of the token.
+    pub fn constructor(&mut self, name: String, symbol: String) {
+        self._name.set_str(name);
+        self._symbol.set_str(symbol);
+        self._decimals.set(U8::from(DEFAULT_DECIMALS));
+    }
+
+    /// Returns the name of the token.
+    ///
+    /// # Arguments
+    ///
+    /// * `&self` - Read access to the contract's state.
+    pub fn name(&self) -> String {
+        self._name.get_string()
+    }
+
+    /// Returns the symbol of the token, usually a shorter version of the name.
+    ///
+    /// # Arguments
+    ///
+    /// * `&self` - Read access to the contract's state.
+    pub fn symbol(&self) -> String {
+        self._symbol.get_string()
+    }
+
+    /// Returns the number of decimals used to get a user-friendly
+    /// representation of values of this token.
+    ///
+    /// For example, if `decimals` equals `2`, a balance of `505` tokens should
+    /// be displayed to a user as `5.05` (`505 / 10 ** 2`).
+    ///
+    /// Tokens usually opt for a value of `18`, imitating the relationship
+    /// between Ether and Wei. This is the default value returned by this
+    /// function ([`DEFAULT_DECIMALS`]), unless it's overridden.
+    ///
+    /// NOTE: This information is only used for *display* purposes: in
+    /// no way it affects any of the arithmetic of the contract, including
+    /// [`ERC20::balance_of`] and [`ERC20::transfer`].
+    pub fn decimals(&self) -> u8 {
+        // TODO: Use `U8` an avoid the conversion once https://github.com/OffchainLabs/stylus-sdk-rs/issues/117
+        // gets resolved.
+        self._decimals.get().byte(0)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use alloy_primitives::U256;
+    use stylus_sdk::storage::{StorageString, StorageType, StorageU8};
+
+    #[allow(unused_imports)]
+    use crate::test_utils;
+
+    use super::Metadata;
+    use super::DEFAULT_DECIMALS;
+
+    impl Default for Metadata {
+        fn default() -> Self {
+            let root = U256::ZERO;
+            Metadata {
+                _name: unsafe { StorageString::new(root, 0) },
+                _symbol: unsafe {
+                    StorageString::new(root + U256::from(32), 0)
+                },
+                _decimals: unsafe { StorageU8::new(root + U256::from(64), 0) },
+            }
+        }
+    }
+
+    #[test]
+    fn constructs() {
+        test_utils::with_storage::<Metadata>(|meta| {
+            let name = meta.name();
+            let symbol = meta.symbol();
+            let decimals = meta.decimals();
+            assert_eq!(name, "");
+            assert_eq!(symbol, "");
+            assert_eq!(decimals, 0);
+
+            const NAME: &str = "Meta";
+            const SYMBOL: &str = "Symbol";
+
+            meta.constructor(NAME.to_owned(), SYMBOL.to_owned());
+
+            let name = meta.name();
+            let symbol = meta.symbol();
+            let decimals = meta.decimals();
+            assert_eq!(name, NAME);
+            assert_eq!(symbol, SYMBOL);
+            assert_eq!(decimals, DEFAULT_DECIMALS);
+        })
+    }
+}

contracts/src/erc20/extensions/mod.rs

@@ -0,0 +1,2 @@
+#[cfg(any(test, erc20_metadata))]
+pub mod metadata;

contracts/src/erc20/mod.rs

@@ -12,6 +12,8 @@ use stylus_sdk::{
     stylus_proc::{external, sol_storage},
 };
 
+pub mod extensions;
+
 sol! {
     /// Emitted when `value` tokens are moved from one account (`from`) to
     /// another (`to`).
@@ -356,17 +358,15 @@ mod tests {
     impl Default for ERC20 {
         fn default() -> Self {
             let root = U256::ZERO;
-            let token = ERC20 {
+            ERC20 {
                 _balances: unsafe { StorageMap::new(root, 0) },
                 _allowances: unsafe {
                     StorageMap::new(root + U256::from(32), 0)
                 },
                 _total_supply: unsafe {
                     StorageU256::new(root + U256::from(64), 0)
                 },
-            };
-
-            token
+            }
         }
     }