VIP: Support @pure decorator

[fubuloubu]

Simple Summary

Support the use of @pure as a decorator for methods

Motivation

To be consistent with the ABI stateMutability field, we should add support for specifying a method as @pure.

Specification

A method that is decorated as @pure must not read any blockchain state such as storage variables, environment variables, querying contract code or account balances, or making a call to nonpure methods, or to other contracts. In the event this happens, this should raise an exception that uniquely details a violation of reading blockchain state. Such a function might be useful for complex math or other purposes.

A method decorated with @pure looks like:

@pure
@public
def getMaff() -> uint256:
    return 1 + 2

Backwards Compatibility

No backwards compatibility issues

Dependencies

No dependencies. Related to #2040

References

• Support the stateMutability field of the ABI JSON #1931
• https://solidity.readthedocs.io/en/latest/abi-spec.html#json

Copyright

Copyright and related rights waived via CC0

[fubuloubu]

Remove this line in docs/index.rst:
Anything marked constant is not allowed to change the state