Redundant discriminator checks in AccountLoader::load* methods

[acheroncrypto]

Problem

load* methods check the discriminator of the account each time they're called. This results in unnecessary performance penalty, especially when used in account constraints.

The new custom discriminator support also added a bit of additional overhead when comparing the discriminators.

It looks like these checks are all redundant because the only way to create an AccountLoader instance is to use AccountLoader::try_from or AccountLoader::try_from_unchecked.

try_from:

• Used when the account already exists

• Checks both the owner and the discriminator:

  anchor/lang/src/accounts/account_loader.rs

  Lines 121 to 135 in 48c732d

  	if acc_info.owner != &T::owner() {
  	return Err(Error::from(ErrorCode::AccountOwnedByWrongProgram)
  	.with_pubkeys((*acc_info.owner, T::owner())));
  	}
  	let data = &acc_info.try_borrow_data()?;
  	let disc = T::DISCRIMINATOR;
  	if data.len() < disc.len() {
  	return Err(ErrorCode::AccountDiscriminatorNotFound.into());
  	}
  	let given_disc = &data[..disc.len()];
  	if given_disc != disc {
  	return Err(ErrorCode::AccountDiscriminatorMismatch.into());
  	}

try_from_unchecked:

• Used when the account doesn't exist or the zero constraint is used constraints

• Checks only the owner:

  anchor/lang/src/accounts/account_loader.rs

  Lines 146 to 149 in 48c732d

  	if acc_info.owner != &T::owner() {
  	return Err(Error::from(ErrorCode::AccountOwnedByWrongProgram)
  	.with_pubkeys((*acc_info.owner, T::owner())));
  	}

Given this, the discriminator is already checked when it should be (using try_from), and if it shouldn't get checked (like when using init), it's not checked.

This also means load_init method is fully redundant.

Solution

If the above assumptions are correct, it should be safe to remove them:

• Remove the discriminator checks from the load and load_mut methods
• Remove the load_init method completely

But before doing so, we must be certain that those methods are indeed redundant, and this change would not affect the security of programs in any way.