[Merged by Bors] - doc(Cache/Hashing): add docstring for HashMemo

[joneugster]

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[github-actions]

PR summary e88a97b477

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

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No changes to technical debt.

You can run this locally as

./scripts/technical-debt-metrics.sh pr_summary

• The relative value is the weighted sum of the differences with weight given by the inverse of the current value of the statistic.
• The absolute value is the relative value divided by the total sum of the inverses of the current values (i.e. the weighted average of the differences).

[eric-wieser]

Is it none only for source files not in mathlib?

[joneugster]

updated the docstring. Files downstream will fall into the latter category, but not necessarily exclusively.

E.g. if you'll write import Mathlib.Data in another mathlib file and try to get cache, this file will have none as Mathlib/Data.lean doesn't exist. (meaning no cache will be retrieved for the file with bad import)

[eric-wieser]

bors d+

Thanks!

[mathlib-bors]

✌️ joneugster can now approve this pull request. To approve and merge a pull request, simply reply with bors r+. More detailed instructions are available here.

[joneugster]

bors r+

[mathlib-bors]

Pull request successfully merged into master.

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