bug: Sniper printer drops whitespace after comment if next element is type member with modified modifier list

[slarse]

I've encountered a problem where the sniper printer will not insert newlines correctly on modified or new elements. I've encountered this when inserting import statements, as well as when modifying fields. This is a big problem for the fields, because inline comments can cause a modified field to become commented out. For example, if we have this class:

public class PrinterNewlines {
    // if a newline is not inserted on this line, the field below will be commented out
    public static int NON_FINAL_FIELD = 42;
}

And insert a modifier on the field (say final), this is the output:

public class PrinterNewlines {
    // if a newline is not inserted on this line, the field below will be commented outpublic static final int NON_FINAL_FIELD = 42;
}

I'm working on figuring out why this happens.

[monperrus]

Thanks for the bug report, looking forward to the fix!

[slarse]

Thanks for the bug report, looking forward to the fix!

So am I, this issue is messing with Sorald's patches :)

I've located the problem with the particular case I show in the opening post here, and it's a corner case of corner cases, in particular when there's a comment preceding a modified CollectionSourceFragment (e.g. modifiers). The problem of added import statements not getting newlines is related, but it's not exactly the same.

Overall, the logic for when or when not to print newlines is a bit complicated, and gets really involved when elements are modified, added or deleted. I'm working on a fix, but it's slow going as I still don't fully understand the sniper printer.

[monperrus]

Cool. The expertise you're building while understanding the sniper architecture will be very valuable for both Sorald and Spoon.

[slarse]

I've determined that the import statement newlines is a separate issue, see #3698

[slarse]

Alright, after a ton of debugging, I've found that the problem here is as follows:

When there is a comment just preceding a type member (method, field, inner type), and the type member's modifiers are altered, things break down. The typical thing that happens is that the whitespace (newline + indentation) between the comment and the type member simply isn't printed. I've however also managed to get the comment to simply disappear.

There are also problems with modifiers on local variables. For example, adding the final keyword to a local variable that was preceded by a comment gave me this funky output:

final
// comment here
int a = 2

The problems seem to be very much related to modifier lists, CollectionSourceFragment, and comments/whitespace (which are treated similarly by the sniper printer). I've found many fixes that have worked, but resulted in redundant whitespace elsewhere. I'll keep working on this, and will open a PR tomorrow with my test cases.

[slarse]

The comment disappears entirely whenever an initially non-empty modifier list is emptied.

[slarse]

The comment disappears entirely whenever an initially non-empty modifier list is emptied.

This is an entirely separate issue.