Tests for --file-lines

[Manishearth]

I think the existing formatting tests can be made to work for testing --file-lines.

Basically, what you can do is run diff between source/target, and identify the source lines that get formatted. Then use --file-lines to:

• only format those lines, check if it produces something identical to target
• only format lines that aren't those, check if it produces something identical to source

This may cover many of the cases @randomPoison has been finding.

There may be cleverer ways of doing this that cover more cases

[randomPoison]

I think this works as an idempotency test for --file-lines, but I don't think it's sufficient for verifying that we can partially format language constructs. It would verify that selecting things with --file-lines does result in those things being formatted, but if something needs to support partial formatting and doesn't this test wouldn't catch that.

Take for example a function parameter list. We might want to be able to partially format it, e.g. format one of the args without modifying the others (tbd if that's actually something we want to support, but I'm just using that as an example for now). We might have something like:

fn whatever(
a: char,
b: bool,
c: i32,
) {}

If we run that through the full formatter that'll all get turned into:

fn whatever(a: char, b: bool, c: i32) {}

When we diff that, it'll show up as all lines (1-5) being changed. If we then use those lines for --file-lines we're selecting the whole thing, which doesn't work as a test for partial formatting. To test partial formatting we need to be able to select e.g. just line 3 and verify that we can adjust that line without touching the others, which we can't get from diffing after doing a full reformat.

I think ultimately we need some amount of explicit tests for partially formatting language constructs that need to support that. We need to have test cases that are mis-formatted in multiple places, and only target part of the thing with --file-lines, so that we can verify that we can format some of the item without touching the unselected parts.

[Manishearth]

That seems fine. We should at least start with the "free" idempotency tests and then write explicit file lines tests.