Error handling for macros

[detrumi]

Part of #720

[matklad]

overall, lgtm, but let's try to separate parsing & expansion errors. And perhaps it would be a good idea to write some unit-tests for parsing and exapnsion of erroneous macros.

[detrumi]

I've added some simple tests for error cases. The test organization could be better, but I didn't want to dump all tests in lib.rs.

There's also some weirdness with Var.kind, which is kinda best-effort while other parts aren't.

[matklad]

Excellent, bors r+

[matklad]

bors r+

[bors]

Build succeeded

• continuous-integration/travis-ci/push

[matklad]

A next step would be to fix

            while let Ok(t) = expand_subtree(&repeat.subtree, bindings, nesting) {
                let idx = nesting.pop().unwrap();
                nesting.push(idx + 1);
                token_trees.push(t.into())
            }

logic. This is for handling repeates: if $($name:ident)* matched foo bar baz in the input, than $(struct $name)* should be expanded three times in the result. The current logic is basically "repeat until any error occurs", while it should be more like "repeat the n times", where n is figured out somehow... That is, there are two problems with the current impl:

• it may mask legitimate errors
• I don't really understand why it works in the first place :)

It would be cool to dig into the rustc implementation and figure out to handle this properly. The useful bits of rustc are:

• NamedMatch which is analagous to our Bindings struct
• transcribe whihc handles the actual expansion.