Implement lazy loading mechanism for expensive metadata providers

[Chenguang-Zhu]

Summary

• Implement a lazy metadata loading mechanism
• Add a LazyValue class, which postpones the loading of some expensive metadata, e.g., qualified names, to the time of calling get_metadata
• Modify the implementation of QualifiedNameProvider to use this lazy loading mechanism
• Add support for lazy values in metadata matchers
• Modify existing unit tests to use the lazy loading

Test Plan

With this lazy loading, the running time of the function QualifiedNameVisitor.on_visit is supposed to be reduced by 20% on average.

[zsol]

Nice! Looks like there are some test failures, most of them in libcst.metadata.tests.test_name_provider.QualifiedNameProviderTest. Can you please fix these and add some new cases that cover the new functionality directly?

[zsol]

Let's fix test failures and add some new unit tests that exercise the new functionality directly. There are some type checker failures too, there are instructions in the readme about how you can set this up on your local machine without relying on CI.

[zsol]

This will need to be a bit more strictly typed. LazyValue will need to be generic over some T value, and this callable will be typed as Callable[[], T]

[Chenguang-Zhu]

I have addressed the comments in the new commits. Thanks!

[codecov-commenter]

Codecov Report

Merging #720 (1332b1a) into main (b3eda50) will increase coverage by 0.00%.
The diff coverage is 95.34%.

@@           Coverage Diff           @@
##             main     #720   +/-   ##
=======================================
  Coverage   94.80%   94.81%           
=======================================
  Files         247      247           
  Lines       25731    25762   +31     
=======================================
+ Hits        24395    24425   +30     
- Misses       1336     1337    +1     

Impacted Files	Coverage Δ
libcst/metadata/base_provider.py	84.78% <77.77%> (-0.94%)	⬇️
libcst/_metadata_dependent.py	94.11% <100.00%> (+1.80%)	⬆️
libcst/matchers/_matcher_base.py	91.44% <100.00%> (+0.06%)	⬆️
libcst/metadata/name_provider.py	98.64% <100.00%> (ø)
libcst/metadata/tests/test_name_provider.py	100.00% <100.00%> (ø)

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[zsol]

Looks great overall, we just gotta figure out this type here. I'll look at it tomorrow a bit more closely

[Chenguang-Zhu]

Sure, thanks a lot!

[zsol]

I've fixed the typing issues and added LazyValue support to the base metadata provider in my latest commit. We still need:

• unit tests that cover all the new logic in MetadataDependent
• unit tests that cover the new logic in BaseMetadataProvider
• unit tests that cover the new logic in BaseMatcher

[Chenguang-Zhu]

I've fixed the typing issues and added LazyValue support to the base metadata provider in my latest commit. We still need:

• unit tests that cover all the new logic in MetadataDependent
• unit tests that cover the new logic in BaseMetadataProvider
• unit tests that cover the new logic in BaseMatcher

The tests MatchersMetadataTest.test_lambda_metadata_matcher, MatchersMetadataTest.test_lambda_metadata_matcher_with_no_metadata, and MatchersMetadataTest.test_lambda_metadata_matcher_operators already cover the new logic via QualifiedNameProvider.

[zsol]

Nice!