Deprecate the var keyword

[axic]

I do not think there is any proper legitimate use left for the var keyword therefore I suggest we remove it.

In for loops already a type is suggested, instead of tuple declarations tuple assignments can be used and there is no need for the var for functions as function types can be used now.

Is there any other use case for var?

In #2558 I proposed to replace it with auto, but this issue would override that and just drop it.

[axic]

@chriseth any arguments for keeping var?

[chriseth]

No, I am not a big fan of auto in C++ either.

[axic]

The task here is to extend libsolidity/analysis/SyntaxChecker with a visitor for VariableDeclaration where the typeName is empty (means "var").

The compiler should emit a warning for such instances, unless the experimental mode V050 is turned on, in which case it should be an error.

[briandsutton]

Hi, I'm trying to understand something here. I understand, the use of 'var' keyword was deprecated in solidity version 0.4.20 (https://github.com/ethereum/solidity/releases/tag/v0.4.20), however the documentation for tuple assignments for that version shows the use of the 'var' keyword (https://solidity.readthedocs.io/en/v0.4.20/control-structures.html#assignment).

Is this use still correct? In case it's not, how should it be used?

[axic]

@briandsutton thanks for noticing, it was left by mistake. Tuple assignments will be changed as proposed in #3314.

[jtakalai]

So... what's the resolution on how tuple assignments are done in the future? Instead of

var (,a,,,,b,,,,,c,,,,,) = crazyfunction()

we'd have to first introduce the types like so:

uint a;
string b;
bytes32 c;
(,a,,,,b,,,,,c,,,,,) = crazyfunction()

I don't see how that is an improvement. Even the following would be neat enough:

(,uint a,,,,string b,,,,,bytes32 c,,,,,) = crazyfunction()

[OTTTO]

potentially var could only be used for tuple destructuring.