VIP: automatic detection of payable functions

[AlbertoCentonze]

Simple Summary

This VIP proposes to completely remove the @payable and @nonpayable decorators from the language syntax.

Motivation

• We live in a world where native currency transfers are less and less used compared to ERC20s.
• People ended up saving gas by using the @payable decorator just to remove the runtime check that a @nonpayable modifier implies.
• The compiler is smart enough to know when a function is payable.
• This change reduces the cognitive overhead of using native currency transfers for developers.

Specification

• @payable and @nonpayable should be disallowed in the syntax.
• When a function accesses msg.value the abi of the function should be payable, otherwise it's always non-payable.
• The runtime check to make sure that a function is non-payable is turned into a no-op.

Backwards Compatibility

This is a breaking change that requires changes in existing codebases. This VIP can be implemented without waiting for a major release by turning the @nonpayable modifier into a no-op, which is already the case for @payable and showing a deprecation warning.

Copyright

Copyright and related rights waived via CC0

[pcaversaccio]

I personally don't think it's a general blocker, but I want to highlight that in the EIP-721 standard we have the following specifications:

function safeTransferFrom(address _from, address _to, uint256 _tokenId, bytes data) external payable;
function safeTransferFrom(address _from, address _to, uint256 _tokenId) external payable;
function transferFrom(address _from, address _to, uint256 _tokenId) external payable;
function approve(address _approved, uint256 _tokenId) external payable;

In probably 99% of the implementations, these functions do not access msg.value but would require to be payable at the ABI level. The current proposal would lead to an inconsistent ABI for EIP-721 token implementations. We could add maybe a kwarg to the function parameters, is_payable = True to enforce it:

@external
def safeTransferFrom(owner: address, to: address, token_id: uint256, data: Bytes[1_024] = b"", is_payable = True):