Add unnecessary_import_braces lint.

[omasanori]

This PR adds a lint path checking unnecessary brackets around single imported item. (ex. use std::num::{abs};)

I was unsure whether the lint should be allow-by-default or warn-by-default, so I split changes into two commits. If the lint itself is OK but it shouldn't be warn-by-default, then I'll purge the second one. What do you think?

[sinistersnare]

could it check for unneccessary brackets around closures?

fn do_a_thing() -> || -> int {
    || { 2 } // This warns
}

[omasanori]

@sinistersnare No, I've never be aware of such unnecessary brackets.

[sinistersnare]

that actually does not compile, but something like that. hopefully someone can give a working example, i just wanted to throw in my 2 cents.

[omasanori]

I'll try to check such expressions if it is also desired. Any working example should be nice.

I wrote this lint because I always search commas when I see use foo::{very_long_single_identifier}; and I found that unnecessary brackets in use statement might be noise.

[sinistersnare]

I agree that this might be useful, but it can have more uses that originally intended too!

[omasanori]

Indeed. I'll dig into libsyntax to find more and more unnecessary brackets...

[ftxqxd]

The name of this lint is slightly confusing—unnecessary_parens already exists (for things like if (condition) { ... }), and I (and most people I’ve heard talk about them) normally use the term ‘braces’ to describe { and }. Maybe it could be renamed unnecessary_braces or even unnecessary_import_braces?

I also think that this shouldn’t detect unnecessary braces around closures. I’ve seen a lot of code that uses braces when it doesn’t strictly need to, but it really does make it clearer and easier to add more statements, especially if it spans multiple lines. If such a lint were to be added, it should be part of the unnecessary_parens lint (which might have to be renamed).

But overall, this is a good idea. I see unnecessary {}s in imports all the time, and it’s quite annoying.

[jfager]

'Brackets' are []. As @P1start said, this is for 'braces' or 'curly brackets': http://en.m.wikipedia.org/wiki/Bracket

I vote for allow-by-default. With globs being discouraged I've reflexively started using braces anticipating that I'll need to add imports as I go.

[omasanori]

I'm sorry for inaccurate wording. The lint is renamed to unnecessary_import_braces.

[omasanori]

@sinistersnare After a minute of thought, I have an opinion that redundant braces around expressions and ones in use statements are different kinds of style issue.
I'm unsure whether putting extra braces around expressions is good style or bad style, but at least I think it should be treated in any other lint. Your opinion?

[omasanori]

I decided to purge the second commit and make the lint allow-by-default for now. r?