SIMD-0312: CreatePrefundedAccount

[rustopian]

CreateAccountPrefunded

Emerging from discussions around work on a newer ATA program.

Someday, multi-instruction wrappers such as CreateAccount will not be as necessary for efficiency as they are today.

Until then, CreateAccountPrefunded offers quick and sizable compute savings for the common flow "fund account rent first, then create the account later".

[joncinque]

Looks good to me overall! Mostly some small points

[joncinque]

This section could use a little more detailed information for other teams implementing validators, like Firedancer, including the following:

• exact instruction data format, ie the discriminant and expected value
• exact accounts in order, and their role (readonly, writable, signer, writable-signer)
• special cases. For example, if the transfer amount is 0, is a payer account even required? If not, then we don't perform any checks on the payer account.

[joncinque]

nit: the term would be "prefunded account" then. And isn't a prefunded account one that may receive lamports?

[rustopian]

Updated to Prefunded Account in 145e947. However, no, if it hasn't received lamports, it is no different, and is treated no differently, than a non-prefunded account. Or how do you mean?

[joncinque]

I mean that in the instruction, the prefunded account may receive lamports. The possibilities are:

• the account has 0 lamports, it's not prefunded, so it does receive lamports
• the account is partially funded, but not enough for rent-exemption, so it's prefunded and it receives lamports
• the account has enough for rent-exemption, so it's prefunded but it receives no lamports

So what I mean: the optionality of receiving lamports is important

[rustopian]

Ah, I see what you mean. I'm defining it as an account that has received lamports prior to the create transaction/instruction, but the original definition sounds like it could refer the lamports transfer which occurs in the CreatePrefundedAccount call itself.

By "which receives lamports," I meant to refer to the funding in advance of creation (perhaps months before, for example).

I've clarified to:

* Prefunded Account: an account which has received lamports to pay for its rent
in whole or in part in any prior instruction or prior transaction, before the
instruction in which its space is `allocate`d and its owner is `assign`ed.

[joncinque]

Looks good to me! Just the last question that we've been kicking around a bit for the payer

[joncinque]

What do you think about making this account optional? This would make it different from CreateAccount, but that should be ok

[rustopian]

Updated in 25fddea

[joncinque]

This looks good to me!

[rustopian]

@t-nelson or @jstarry, requesting a review at your convenience.