GROOVY-12143: lambda hoisting (GEP-27)

[paulk-asert]

A statically compiled lambda targeting a functional interface currently generates a per-lambda class extending groovy.lang.Closure, even though its runtime value is a LambdaMetafactory-produced functional-interface instance. For a capturing lambda the class also holds the captured state (Reference-wrapped) and is instantiated per lambda.

Like the JVM does for Java lambdas, hoist the lambda body onto the enclosing class as a private static method and bootstrap LambdaMetafactory directly against it, so no per-lambda class is generated:

• Non-capturing: re-home the already-static doCall onto the enclosing class; the metafactory produces the same zero-allocation singleton.
• Read-only capturing: emit a static method taking the captured values as leading parameters and have the metafactory capture those values directly - eliminating the lambda class, its instance, and the Reference wrapping. Captured Reference-field reads in the doCall are repointed to the value parameters (codegen then loads the value directly).

Only lambdas sitting directly in a real method are hoisted. Serializable, nested, instance- accessing, and mutated-capture lambdas (which need the shared Reference) are unchanged and keep their generated class. Behaviour and metadata are preserved (verified: Predicate/ BiFunction, multi-capture, mixed capture/SAM types, lambdas in static methods, qualified outer static calls, serializable round-trips, singleton identity for non-capturing, mutated- capture mutation across invocations, parameter type annotations on the hoisted method); stack traces now name a $lambda$ method on the enclosing class (Java-like).

Gated by an opt-in system property, groovy.target.lambda.hoist (default off), read per lambda so it can be toggled without a JVM restart, while IDE/tooling compatibility with the changed generated-class shape is verified. Default off keeps existing behaviour unchanged.

Testing:

• LambdaHoistTest covers the opt-in path (non-capturing and read-only-capturing hoist, singleton identity, multi-capture behaviour) and the fall-back cases (mutated capture, instance-access, serializable, nested, default-off).
• Existing tests that assert the pre-hoist generated-class structure (LambdaTest.NonCapturingLambdaOptimizationTest, LambdaTest.NativeLambdaBytecodeTest, LambdaTest.testLambdaClassIsntSynthetic, TypeAnnotationsTest.testTypeAnnotationsForLambda) are annotated @DisabledIfSystemProperty so they skip when the flag is on; the build forwards -P/-Dgroovy.target.lambda.hoist to the test JVM.
• The full core test suite passes with the flag on (0 failures; only the pre-hoist bytecode-shape tests skip) and is unchanged with it off.

See GEP-27 (Compact Closure and Lambda Compilation) for the broader design.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 85.55556% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.7123%. Comparing base (07adcb3) to head (5abdf5a).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
...roovy/classgen/asm/sc/StaticTypesLambdaWriter.java	85.5556%	5 Missing and 8 partials ⚠️

Additional details and impacted files

@@                Coverage Diff                 @@
##               master      #2680        +/-   ##
==================================================
+ Coverage     68.6965%   68.7123%   +0.0158%     
- Complexity      34121      34143        +22     
==================================================
  Files            1536       1536                
  Lines          128912     129006        +94     
  Branches        23368      23387        +19     
==================================================
+ Hits            88558      88643        +85     
- Misses          32523      32527         +4     
- Partials         7831       7836         +5     

Files with missing lines	Coverage Δ
...roovy/classgen/asm/sc/StaticTypesLambdaWriter.java	90.2256% <85.5556%> (-2.4711%)	⬇️

... and 4 files with indirect coverage changes

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[testlens-app]

✅ All tests passed ✅

🏷️ Commit: 5abdf5a
▶️ Tests: 94451 executed
⚪️ Checks: 31/31 completed

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