Introduce the DEFAULT_SUB_ID constant (#5246)

## Description

The following pull request does two things:
1. Introduces the `DEFAULT_SUB_ID` constant. This is equivalent to the
`ZERO_B256` constant. This constant makes the Sway code easier to read
and comprehend improving the developer's UX. The actual value of the
default is something the developer no longer needs to think about.

Before:
```sway
let my_asset = AssetId::new(my_contract, ZERO_B256);
```
Now:
```sway
let my_asset = AssetId::new(my_contract, DEFAULT_SUB_ID);
```

2. Removes the `ContractId` argument from the `AssetId::default()`
function. In 90% of cases `default()` is used within the contract that
issued the token, making this an unnecessary argument. This function now
fetches the contract id of the contract it's called in, thus eliminating
the need to import another function and add additional arguments. In the
case a developer needs the default of another contract, the
`DEFAULT_SUB_ID` can be used instead making the distinction more legible
and apparent.

Before:
```sway
use std::call_frames::contract_id;

fn foo(other_contract: ContractId) {
     let other_asset = AssetId::default(other_contract);
     let my_asset = AssetId::default(contract_id());
}
```
After:
```sway
fn foo(other_contract: ContractId) {
     let other_asset = AssetId::new(other_contract, DEFAULT_SUB_ID);
     let my_asset = AssetId::default();
}
```

## Checklist

- [ ] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

---------

Co-authored-by: SwayStar123 <46050679+SwayStar123@users.noreply.github.com>

examples/liquidity_pool/src/main.sw

@@ -5,7 +5,7 @@ use std::{
         contract_id,
         msg_asset_id,
     },
-    constants::ZERO_B256,
+    constants::DEFAULT_SUB_ID,
     context::msg_amount,
     hash::*,
     token::{
@@ -32,11 +32,11 @@ impl LiquidityPool for Contract {
         let amount_to_mint = msg_amount() * 2;
 
         // Mint some LP token based upon the amount of the base token.
-        mint_to_address(recipient, ZERO_B256, amount_to_mint);
+        mint_to_address(recipient, DEFAULT_SUB_ID, amount_to_mint);
     }
 
     fn withdraw(recipient: Address) {
-        let asset_id = AssetId::default(contract_id());
+        let asset_id = AssetId::default();
         assert(msg_asset_id() == asset_id);
         assert(msg_amount() > 0);
 

examples/native_token/src/main.sw

@@ -1,6 +1,6 @@
 contract;
 
-use std::{constants::ZERO_B256, context::*, token::*};
+use std::{constants::DEFAULT_SUB_ID, context::*, token::*};
 
 abi NativeAssetToken {
     fn mint_coins(mint_amount: u64);
@@ -16,12 +16,12 @@ abi NativeAssetToken {
 impl NativeAssetToken for Contract {
     /// Mint an amount of this contracts native asset to the contracts balance.
     fn mint_coins(mint_amount: u64) {
-        mint(ZERO_B256, mint_amount);
+        mint(DEFAULT_SUB_ID, mint_amount);
     }
 
     /// Burn an amount of this contracts native asset.
     fn burn_coins(burn_amount: u64) {
-        burn(ZERO_B256, burn_amount);
+        burn(DEFAULT_SUB_ID, burn_amount);
     }
 
     /// Transfer coins to a target contract.
@@ -46,11 +46,11 @@ impl NativeAssetToken for Contract {
 
     /// Mint and send this contracts native token to a destination contract.
     fn mint_and_send_to_contract(amount: u64, destination: ContractId) {
-        mint_to_contract(destination, ZERO_B256, amount);
+        mint_to_contract(destination, DEFAULT_SUB_ID, amount);
     }
 
     /// Mind and send this contracts native token to a destination address.
     fn mint_and_send_to_address(amount: u64, recipient: Address) {
-        mint_to_address(recipient, ZERO_B256, amount);
+        mint_to_address(recipient, DEFAULT_SUB_ID, amount);
     }
 }

sway-lib-std/src/constants.sw

@@ -32,3 +32,17 @@ pub const BASE_ASSET_ID: AssetId = AssetId::from(ZERO_B256);
 /// }
 /// ```
 pub const ZERO_B256 = 0x0000000000000000000000000000000000000000000000000000000000000000;
+
+/// The default Sub Id for assets.
+///
+/// # Examples
+/// 
+/// ```sway
+/// use std::{call_frames::contract_id, constants::DEFAULT_SUB_ID};
+///
+/// fn foo() {
+///     let asset = AssetId::default();
+///     assert(AssetId::new(contract_id(), DEFAULT_SUB_ID) == msg_asset_id());
+/// }
+/// ```
+pub const DEFAULT_SUB_ID = ZERO_B256;

sway-lib-std/src/contract_id.sw

@@ -178,31 +178,24 @@ impl AssetId {
         Self { value: result_buffer }
     }
 
-    /// Creates a new AssetId from a ContractId and the zero SubId.
-    ///
-    /// # Arguments
-    ///
-    /// * `contract_id`: [ContractId] - The ContractId of the contract that created the asset.
+    /// Creates a new AssetId with the default SubId for the current contract.
     ///
     /// # Returns
     ///
-    /// * [AssetId] - The AssetId of the asset. Computed by hashing the ContractId and the zero SubId.
+    /// * [AssetId] - The AssetId of the asset. Computed by hashing the ContractId and the default SubId.
     ///
     /// # Examples
     ///
     /// ```sway
-    /// use std::{callframes::contract_id, constants::ZERO_B256};
+    /// use std::{callframes::contract_id, constants::DEFAULT_SUB_ID};
     ///
     /// fn foo() {
-    ///     let contract_id = contract_id();
-    ///     let sub_id = ZERO_B256;
-    ///
-    ///     let asset_id = AssetId::default(contract_id);
-    ///
-    ///     assert(asset_id == AssetId::new(contract_id, sub_id));
+    ///     let asset_id = AssetId::default();
+    ///     assert(asset_id == AssetId::new(contract_id(), DEFAULT_SUB_ID));
     /// }
     /// ```
-    pub fn default(contract_id: ContractId) -> Self {
+    pub fn default() -> Self {
+        let contract_id = asm() { fp: b256 };
         let result_buffer = 0x0000000000000000000000000000000000000000000000000000000000000000;
         asm(asset_id: result_buffer, ptr: (contract_id, 0x0000000000000000000000000000000000000000000000000000000000000000), bytes: 64) {
             s256 asset_id ptr bytes;

test/src/e2e_vm_tests/test_programs/should_pass/require_contract_deployment/token_ops_test/src/main.sw

@@ -3,6 +3,7 @@ script;
 use std::context::balance_of;
 use std::token::*;
 use std::contract_id::*;
+use std::constants::DEFAULT_SUB_ID;
 use test_fuel_coin_abi::*;
 
 struct Opts {
@@ -16,7 +17,7 @@ fn main() -> bool {
 
     // the deployed fuel_coin Contract_Id:
     let fuelcoin_id = ContractId::from(0x5d10689c7eecb405937a3f35fab7baf05a3f6189f9a2993ee70e21ccc1212460);
-    let fuelcoin_asset_id = AssetId::default(fuelcoin_id);
+    let fuelcoin_asset_id = AssetId::new(fuelcoin_id, DEFAULT_SUB_ID);
 
     // contract ID for sway/test/src/e2e_vm_tests/test_programs/should_pass/test_contracts/balance_test_contract/
     let balance_test_id = ContractId::from(0x4a00baa517980432b9274a0e2f03c88735bdb483730816679c6eb37b4046d060);

test/src/sdk-harness/test_artifacts/context_caller_contract/src/main.sw

@@ -18,7 +18,7 @@ impl ContextCaller for Contract {
     fn call_get_this_balance_with_coins(send_amount: u64, target: ContractId) -> u64 {
         let id = target.value;
         let context_contract = abi(ContextTesting, id);
-        let asset_id = AssetId::default(contract_id());
+        let asset_id = AssetId::default();
 
         context_contract.get_this_balance {
             gas: 500_000,
@@ -30,7 +30,7 @@ impl ContextCaller for Contract {
     fn call_get_balance_of_contract_with_coins(send_amount: u64, target: ContractId) -> u64 {
         let id = target.value;
         let context_contract = abi(ContextTesting, id);
-        let asset_id = AssetId::default(contract_id());
+        let asset_id = AssetId::default();
 
         context_contract.get_balance_of_contract {
             gas: 500_000,
@@ -42,7 +42,7 @@ impl ContextCaller for Contract {
     fn call_get_amount_with_coins(send_amount: u64, target: ContractId) -> u64 {
         let id = target.value;
         let context_contract = abi(ContextTesting, id);
-        let asset_id = AssetId::default(contract_id());
+        let asset_id = AssetId::default();
 
         context_contract.get_amount {
             gas: 500_000,
@@ -54,7 +54,7 @@ impl ContextCaller for Contract {
     fn call_get_asset_id_with_coins(send_amount: u64, target: ContractId) -> b256 {
         let id = target.value;
         let context_contract = abi(ContextTesting, id);
-        let asset_id = AssetId::default(contract_id());
+        let asset_id = AssetId::default();
 
         context_contract.get_asset_id {
             gas: 500_000,
@@ -66,7 +66,7 @@ impl ContextCaller for Contract {
     fn call_get_gas_with_coins(send_amount: u64, target: ContractId) -> u64 {
         let id = target.value;
         let context_contract = abi(ContextTesting, id);
-        let asset_id = AssetId::default(contract_id());
+        let asset_id = AssetId::default();
 
         context_contract.get_gas {
             gas: 500_000,
@@ -78,7 +78,7 @@ impl ContextCaller for Contract {
     fn call_get_global_gas_with_coins(send_amount: u64, target: ContractId) -> u64 {
         let id = target.value;
         let context_contract = abi(ContextTesting, id);
-        let asset_id = AssetId::default(contract_id());
+        let asset_id = AssetId::default();
 
         context_contract.get_global_gas {
             gas: 500_000,
@@ -90,7 +90,7 @@ impl ContextCaller for Contract {
     fn call_receive_coins(send_amount: u64, target: ContractId) {
         let id = target.value;
         let context_contract = abi(ContextTesting, id);
-        let asset_id = AssetId::default(contract_id());
+        let asset_id = AssetId::default();
 
         context_contract.receive_coins {
             gas: 500_000,