Lint: => \ Deprecated at Cylc 6.

[wxtim]

New rules proposal.

The backslash in => \ hasn't been required but there was no upgrade or obsolete message because it's inside the graph. Lint should probably point this out.

Additionally, consider a second almost identical rule for & \ and | \ both of which are unecessary at Cylc 8. I'm in favour because;

• Users should be upgrading from Cylc 7 and back and forward compatible workflows should be becoming less common.
• Users can always use pyproject.toml to disable the rule.

[oliver-sanders]

(changed milestone to match PR)