ensure recursion guard is always used as a stack

[davidhewitt]

Change Summary

Introduces RecursionGuard as a type which goes in a function frame and releases on drop.

This fixes the issue seen updating to 2.16.0.

Because RecursionGuard takes ownership of the state, it guarantees that two guards can't overlap their usage of the state.

Related issue number

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1166 will improve performances by 25.14%

_{Comparing dh/ser-rec-guard (670fd51) with main (4da7192)}

Summary

⚡ 2 improvements
✅ 144 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	dh/ser-rec-guard	Change
⚡	test_core_future_str	39.4 µs	31.5 µs	+25.14%
⚡	test_to_json_list_of_lists	10.2 ms	8.4 ms	+21.5%

[codecov]

Codecov Report

Merging #1166 (670fd51) into main (4da7192) will increase coverage by 0.02%.
The diff coverage is 98.76%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1166      +/-   ##
==========================================
+ Coverage   89.97%   90.00%   +0.02%     
==========================================
  Files         106      106              
  Lines       16618    16656      +38     
  Branches       36       36              
==========================================
+ Hits        14952    14991      +39     
+ Misses       1659     1658       -1     
  Partials        7        7              

Files	Coverage Δ
src/recursion_guard.rs	100.00% <100.00%> (ø)
src/serializers/extra.rs	99.08% <100.00%> (+0.01%)	⬆️
src/serializers/infer.rs	95.23% <100.00%> (+0.32%)	⬆️
src/serializers/mod.rs	99.53% <100.00%> (ø)
src/serializers/type_serializers/definitions.rs	98.46% <100.00%> (-0.11%)	⬇️
src/validators/generator.rs	92.85% <ø> (ø)
src/validators/mod.rs	95.37% <100.00%> (ø)
src/validators/validation_state.rs	100.00% <100.00%> (ø)
src/validators/definitions.rs	95.38% <83.33%> (+0.57%)	⬆️

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[samuelcolvin]

LGTM.