unused variable / dead assignment warnings should have lint modes

[nikomatsakis]

RIght now, they are always issued as warnings without any control from the user. To hook into the lint system, the liveness code has to be modified to thread along an expr/item id. Not a huge deal but sufficiently annoying I didn't do it.

[nikomatsakis]

I did drop in some fixme's into the code.

[brson]

If we do this can we remove feature that allows the _ prefix turn off the warning?

[nikomatsakis]

I was wondering whether that would be a good thing or not. Currently I think not for two reasons.

First, the lint annotations are very chatty. Something like

fn foo(_v: uint) { }

is a lot less chatty than

#[no(warn_unused_variables)]
fn foo(v: uint) { }

of course maybe we should just disable this warning for function parameters. Most compilers do. Similarly, let _ignored = something_with_a_dtor() is tolerable, but adding an annotation, not so much.

Second, I think right now they are too coarse-grained. We'd need them to be applicable at the level of individual statements.

I guess both of these problems are fixable though.

[graydon]

non-critical for 0.6, de-milestoning

[metajack]

I would like this for Servo, specifically for the auto-generated bindings. I already use allow(unused_imports) now for similar reasons. think _ as a prefix suppression should also be kept.

[alexcrichton]

I think this can be closed now.