Compare empty blocks for equality based on tokens

[Jarcho]

fixes: #1390

This only considers empty blocks for now, though we should also catch something like this:

match 0 {
    0 => {
        do_something();
        trace!(0);
        0
    }
    1 => {
        do_something();
        trace!(1);
        1
    }
    x => x,
}

As far as I can tell there aren't any negative effects on other lints. These blocks only happen to be the same for a given compilation, not all compilations.

changelog: Fix match_on_same_arms and others. Only consider empty blocks equal if the tokens contained are the same.

[rust-highfive]

r? @Manishearth

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[Manishearth]

Hmmm. This seems like it would be a large perf impact from reparsing, no? cc @flip1995

Can we potentially have a tokenize boolean argument that does equality checking based on tokens, and first do a regular equality check and only then do a token based one? This also means this function stays useful for other use cases.

[Jarcho]

It's only running the lexer, not the full parser. It will also only run once a given block is determined to be empty. The performance impact should be basically zero.

[Manishearth]

Oh, only for empty blocks, missed that sorry

[flip1995]

Why tokenize at all and not just compare the snippets directly? (You probably will have to trim away the braces and all white spaces and line breaks before comparing though)

This would also take care of comments, that are not identical inside the blocks.

[Jarcho]

The tokenizing is to deal with the whitespace. e.g. x y isn't the same as xy, but ( x ) is the same as (x).

Should probably not skip comments though.

[Manishearth]

@bors r+

I think it's fine to not skip comments

[bors]

📌 Commit 39c5e86 has been approved by Manishearth

[bors]

⌛ Testing commit 39c5e86 with merge 7be3a32...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Manishearth
Pushing 7be3a32 to master...