According to this:
https://blog.bitbucket.org/2012/10/30/building-inline-comments-for-pull-requests-and-commits/
They exist but I couldn't find how to add inline comment within the source code.
That's because they don't exist.
You can only add inline comments to commits, not source. So (if you're a project member) https://bitbucket.org/username/project/commits/12345678 should let add comments by clicking the button that appears on the left hand side when you mouse over a line, but https://bitbucket.org/username/project/src/12345678/file.name has no such feature.
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