Rust: AST support for variables

[hvitved]

This PR adds (initial) AST support for variables. For now, two classes Variable and VariableAccess are exposed, and a VariableAccess is linked to the Variable that it accesses.

Commit-by-commit review is suggested.

Care must be taken when taking shadowing into account: A variable can be shadowed both in a nested scope

let x = 0;
use(x);         // x = 0
{
    let x = "";
    use(x);     // x = ""
}
use(x);         // x = 0

as well as in the same scope

let x = 0;
use(x);     // x = 0
let x = "";
use(x);     // x = ""

In order to identify which variable is being accessed, we employ a purely location-based analysis, as we want to avoid an analysis that depends on the CFG.

In almost all cases, a Variable is identified by the IdentPat node that introduces it, however in case of or patterns like

match either {
    Either::Left(x) | Either::Right(x) => println!(x),
}

x can be introduced by either of the two branches, so we instead identify x with the whole Either::Left(x) | Either::Right(x) pattern.

[aibaars]

Looks good to me.

[aibaars]

Rust also has block comments starting with /* or /** .

[hvitved]

Yes, but I skipped that for now, as it is not needed for inline test expectations.

[aibaars]

What about ParenPat nodes? Things like a | (b | c) | d

[hvitved]

Those should be handled as well; getAPatAncestor uses the generic getImmediateParent predicate which also works for ParenPat.

[aibaars]

Ok, could you add a test case, just to be sure?

[hvitved]

Sure.

[aibaars]

This type looked a little strange to me at first sight. Perhaps you could add a comment explaining the idea of algorithm. If I understand correctly you rank all variable definitions and uses, and bind each use to the highest ranking definition that has a lower rank than the variable use.

[hvitved]

Fair; I have tried to document the class with the algorithm that is used.