docs: explain cast trait and vault double-spend checks

programs/drift/src/instructions/user.rs

@@ -1032,6 +1032,8 @@ pub fn handle_withdraw<'c: 'info, 'info>(
 
     // reload the spot market vault balance so it's up-to-date
     ctx.accounts.spot_market_vault.reload()?;
+    // Verify that the vault's on-chain token balance matches the protocol's internal ledger.
+    // If the vault has fewer tokens than expected, it indicates insolvency or a double-spend attempt, and aborts.
     math::spot_withdraw::validate_spot_market_vault_amount(
         &spot_market,
         ctx.accounts.spot_market_vault.amount,

programs/drift/src/math/casting.rs

@@ -4,6 +4,15 @@ use crate::msg;
 use std::convert::TryInto;
 use std::panic::Location;
 
+/// A helper trait for safe type casting in the Drift protocol.
+///
+/// Rust does not perform implicit type casting, and standard `as` casting
+/// (e.g., `x as u64`) can cause silent truncation or overflows if the value
+/// is too large or negative.
+///
+/// This trait utilizes `try_into()` to safely convert types. If the conversion
+/// fails, it logs the exact file and line number of the failure using `#[track_caller]`
+/// and returns a `CastingFailure` error instead of panicking.
 pub trait Cast: Sized {
     #[track_caller]
     #[inline(always)]