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Fix stylist indentation with a formfeed #7489

Merged

konstin merged 1 commit into main from stylist-formfeed

Sep 19, 2023

Commits on Sep 18, 2023

Fix stylist indentation with a formfeed

**Summary** In python, a formfeed is technically undefined behaviour (https://docs.python.org/3/reference/lexical_analysis.html#indentation):
> A formfeed character may be present at the start of the line; it will be ignored for
> the indentation calculations above. Formfeed characters occurring elsewhere in the
> leading whitespace have an undefined effect (for instance, they may reset the space
> count to zero).

In practice, they just reset the indentation:

https://github.com/python/cpython/blob/df8b3a46a7aa369f246a09ffd11ceedf1d34e921/Parser/tokenizer.c#L1819-L1821
https://github.com/astral-sh/ruff/blob/a41bb2733fe75a71f4cf6d4bb21e659fc4630b30/crates/ruff_python_parser/src/lexer.rs#L664-L667

The stylist didn't handle formfeeds previously and would produce invalid indents. The remedy is to cut everything before a form feed.

Checks box for #7455 (comment)

**Test Plan** Unit test for the stylist and a regression test for the rule

konstin committed Sep 18, 2023