VIP: safe erc20 transfer builtin

[charles-cooper]

Simple Summary

add a builtin to handle non-compliant ERC20 tokens

Motivation

I see this in the wild all the time:

res: Bytes[32] = raw_call(_abi_encode(sender, receiver, amount), method_id=method_id("transferFrom(address,address,uint256)"))
if len(res) > 0:
    assert convert(res, bool)

We could do it more efficiently as a builtin.

Specification

add a function, (name: safeTransferFromERC20?), which implements a safe ERC20 transfer as above.

Dependencies

May be good to implement this as a library function once #2431 is implemented.

Copyright

Copyright and related rights waived via CC0

[charles-cooper]

whoops, i searched for the "VIP" label and clicked the first one. obviously needs to go up for discussion first!

[charles-cooper]

By the way, I've been thinking about this and i think we might want to add a "default" returnvalue to calls. this is a more general mechanism for dealing with contracts which return less data than expected, so we do not have to add a builtin for every ERC20 (or, could be other standards) function implementation which is buggy. something like

ExternalContract(msg.sender).some_function(<args>, default=False)

The compiler can issue code to fill the return buffer with the default value if returndatasize is empty.

[charles-cooper]

note: default is just a placeholder, real name of the kwarg TBD if we decide to move forward with this idea.

[charles-cooper]

closed in favor of #2812