perf: avoid retrieving possible_values unless they're used #5267
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epage
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Dec 27, 2023
I will say that the general assumption is Possible Values are fast to look up. I'm ok with this optimization being "best effort" but not willing to guarantee we won't look up possible values if they are hidden. So long as you understand that you'll be playing wack-a-mole with this as new problems crop up, I'm fine moving forward with it. |
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Whack-a-mole seems fine; I can certainly own the fixes to any problems in this space (at least by default). As long as the unit test remains in place :-) |
In some sophisticated situations, these may be expensive to calculate. One example might be a '--branch' option accepting any single Git branch that exists on the remote -- in such a case, the remote would need to be queried for all possible_values. The cost is ultimately unavoidable at runtime since this validation has to happen eventually, but there's no need to pay it when generating help text if `is_hide_possible_values_set`. To keep '-h' fast, avoid collecting `possible_values` during '-h' unless we're actually going to use the values in display. This optimization is repeated for the manpage renderer. This is trivially based on the short-circuiting logic at [1], which at least supports the idea that actually consuming the iterator is not generally-guaranteed behavior when `hide_possible_values` is set. Note on the 'expensive' mod: This keeps all the possible_values tests in one file but allows the entire set of tests to be controlled by the 'strings' feature (which is required to be able to use String rather than str for each possible value). [1]: clap_builder/src/builder/command.rs:long_help_exists_
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In some sophisticated situations, these may be expensive to calculate. One example might be a '--branch' option accepting any single Git branch that exists on the remote -- in such a case, the remote would need to be queried for all possible_values. The cost is ultimately unavoidable at runtime since this validation has to happen eventually, but there's no need to pay it when generating help text if
is_hide_possible_values_set.To keep '-h' fast, avoid collecting
possible_valuesduring '-h' unless we're actually going to use the values in display.This optimization is repeated for the manpage renderer.
This is trivially based on the short-circuiting logic at
clap_builder/src/builder/command.rs:long_help_exists_, which at least supports the idea that actually consuming the iterator is not generally-guaranteed behavior whenhide_possible_valuesis set.This seems like an obvious bug 'to me', but I recognize that there might be some angle to this that warrants discussion. If so, let me know and we can put this PR on ice while discussion happens in a tracking issue.