IndexOutOfBounds in SourceFile.scala `contents()`

[Xavientois]

Given a SourcePosition pos where pos.exists and pos.source.exists, I would expect that it is safe to call startColumnPadding on pos. This is not always the case, as can be seen when building the community lib Lucre here.

This exception issue occurs at startColumnPadding in SourceFile.scala, when content() is empty, but length is non-zero.

It seems content().length != length happens when length is computed from Tasty using setLineIndicesFromLineSizes instead of using calculateLineIndicesFromContents.

Originally posted by @olhotak in #14683 (comment)

[Xavientois]

This issue was also identified here: #11210 (comment)

[olhotak]

#14683 fixed startColumnPadding by replacing length with content().length, so we can probably close this.

Before #10363, length and content().length were synonyms and various functions in SourceFile called length to get content().length. #10363 made them different. Over time, various functions that called length were changed to call content().length instead. Now only one reference to length remains, in offsetToLine.

For future users, it would be good to document the intended meaning of length and how it now differs from content().length, to give a hint as to when to use which one. I would add such a comment myself but I don't fully understand the subtleties of the intended meaning of the new length.