Fix a variable capture during pattern desugaring #5562
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Pattern compilation was causing variable captures in some particular corner cases. Rather than try to figure out exactly what situation was causing the capture, and how to avoid it, I've just changed the way the recursive splitting functions works to thread the fresh ids through the entire process. This means that sub-expressions never make up the same variables as parent expressions, and should mean that no further processing is needed to avoid captures.
pchiusano
approved these changes
Jan 30, 2025
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Yeah. I think Vim deleted the trailing whitespace that the transcript runner put in the file. |
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Oh yeah. Or, what I mean is, it only sometimes passes CI. It runs successfully, but sometimes with different output than expected. |
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This fixes a variable capture issue that @stew found. I think it would only occur when you have code that is internally a case inside the scrutinee of a case.
I didn't try to find the exact spot that the variable capture occurs, and add calls to freshen variables. The capture was occurring on variables that are introduced by the pattern compiler. Before each stage of compilation would make up new variables like this starting from 0. Instead, I threaded the 'fresh number' through the whole process, so that variables made up are unique to the whole definition, and there's no need to worry about capture and re-freshening. This was pretty easy because it just involved tweaking how the
visitfunction is used to rewrite the expression.I added Stew's example as a transcript test case as well.