Issue 2506

[kngwyu]

For #2506, added dyn check when rewriting TraitObject.
I added 2 tests, but I'm not sure they're enough comprehensive.

[topecongiro]

Thank you for your contribution! I added some inline comments, please take a look.

[topecongiro]

Could you please add .offset_left(4) when dyn is used to make sure that we do not exceed max width with a long trait? For example, we need something like the following:

let shape = if tobj_syntax == ast::TraitObjectSyntax::Dyn {
    // 4 = "dyn "
    shape.offset_left(4)?
} else {
    shape
};

And it would be nice if there is a test for this, though I cannot come up with a good example.

[kngwyu]

I added, but it doesn't seem enough.
When trying to format this,

fn main() {
    fn f3() -> Box<Very__________________________Long__________________Name____________________Trait>
    {

    }
}

rustfmt cause an error, error: line exceeded maximum width (maximum: 100, found: 101).
However, in this case,

fn main() {
    fn f3() -> Box<dyn Very__________________________Long__________________Name____________________Trait>
    {

    }
}

rustfmt outputs

fn main() {
    fn f3(
) -> Box<dyn Very__________________________Long__________________Name____________________Trait>
    {

    }
}

Edited:
However, just

fn f3() -> Box<dyn Very__________________________Long__________________Name____________________Trait>
{
}

cause error: line exceeded maximum width (maximum: 100, found: 101)!

[kngwyu]

I checked rustfmt formats

fn main() {
    fn f3() -> Box<Very______________________________Long__________________Name____________________Trait>
    {
    }
}

into

fn main() {
    fn f3(
) -> Box<Very______________________________Long__________________Name____________________Trait>
    {
    }
}

So, this odd formatting is not because of dyn, but the length of trait name.
Though I'm not sure it's a bug, I think it's good to write tests after checking this is intended.

[topecongiro]

The opening bracket should not be indented, but I guess this is a seperate issue. So this is fine as is.

[kngwyu]

Thanks for your review.
I added offset_left(4) to shape, but I didn't write a test for it.
See https://github.com/rust-lang-nursery/rustfmt/pull/2507/files/f8f5d5c68cdc5168c32f19d7e09b81fe733c9bac#diff-dfd26227c4842bd3288e9b1f53f0089f
for the reason.

[topecongiro]

Thank you for the update and looking deeper to the underlying issue!

Looks like that rustfmt cannot handle trait with a long name. That is a separate issue from #2506, so I am going to merge this PR.