Check a lambda's body against the purity of the method it implements - #8111
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smillst wants to merge 11 commits into
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Check a lambda's body against the purity of the method it implements #8111smillst wants to merge 11 commits into
smillst wants to merge 11 commits into
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Two related fixes to purity checking of lambdas. They must land together:
the first is sound only because of the second.
PurityChecker no longer scans the body of a lambda expression, or of a local
or anonymous class, when checking the enclosing method. Evaluating a lambda
or declaring a class does not run the code in it, so the following was a false
positive:
@SideEffectFree Runnable makeRunnable() { return () -> count++; }
The effects occur where the functional method or the class's method is
invoked, and each such invocation is already checked like any other call. An
inner class's methods are checked against their own annotations by
visitMethod, and instantiating one is still checked by visitNewClass.
BaseTypeVisitor.checkLambdaPurity checks a lambda's body against a
@SideEffectFree, @deterministic, or @pure annotation on the functional
interface method that the lambda implements. Nothing did so before, which was
unsound: unlike an overriding method, a lambda does not inherit the
annotation, and unlike a method reference, a lambda has no declaration to
compare against. This was accepted:
@FunctionalInterface interface PureFunc { @SideEffectFree String get(); }
PureFunc f = () -> { count++; return ""; };
That check is what makes it sound to stop scanning lambda bodies.
Also, a lambda that a constructor creates no longer receives the
constructor's permission to assign to fields of its own class. The lambda's
body may run after the constructor has returned, so PurityChecker's
assignmentCheck now uses enclosingMethodOrLambda rather than
TreePathUtil.inConstructor, which walks past a lambda.
The second parameter of BaseTypeVisitor.reportPurityErrors() is widened from
MethodTree to Tree so that a lambda expression can be passed.
Whole-program inference treats a method with no body as vacuously pure, so it
infers @SideEffectFree for the functional method of a user-declared functional
interface without considering the lambdas that implement it. Ajava validation
then reports violations of an annotation that inference itself produced, so
four affected tests are excluded, alongside the existing exclusions for the
same interaction reached through method references.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UsYZWaFr35JX6pakCSztqv
Pass the already-computed function type into checkLambdaPurity, instead of
calling getFunctionTypeFromTree a second time and building the annotated
executable type twice per lambda.
Drop the unused second parameter of reportPurityErrors rather than widening it
from MethodTree to Tree. Nothing in the body read it; every error location
comes from the PurityResult's reasons. This is still a breaking change for a
subclass that overrides the method, so the CHANGELOG entry is reworded rather
than removed.
Build the lambda body's path with `new TreePath(getCurrentPath(), ...)`, which
is exact and O(1), rather than searching from the compilation-unit root. The
`body == null` branch it replaces was dead, and its comment ("not in the
compilation unit that is being processed") was inaccurate: getPath returns null
for artificial trees, and a lambda body is always in the current unit here.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01APsJw8gV5PX2ei8nnEtBAM
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Included review availability: Your plan provides up to 4 included reviews per hour; 3 remain after this review. 📝 WalkthroughWalkthroughThe purity checker now excludes lambda and local or anonymous class method bodies from enclosing-method analysis. Field-assignment exemptions account for lambda boundaries. Suggested reviewers: Priority: ⬇️ Low Change: Bug fix Merge Risk: ⚪ Minimal · up to The lambda purity changes have no verified remaining merge-blocking issue. 🚥 Pre-merge checks | ✅ 2 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (2 passed)
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Inline comments:
In `@checker/build.gradle`:
- Around line 471-474: Add matching delete exclusions for Issue7693.java,
ManyLambda.java, LambdaNestedConstructs.java, and
LambdaTryCheckedExceptions.java to the ainferTestCheckerGenerateStubs task,
preserving the existing paths and exclusion behavior from the related
test-checker task.
In `@framework/tests/flow/PurityLambdaSam.java`:
- Around line 174-175: Add the expected purity.not.sideeffectfree.call
diagnostic alongside the existing purity.not.deterministic.object.creation
expectation for the String(String) construction in visitNewClass’s test case,
while leaving the unconstrained Runnable.run expectation unchanged.
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checker/build.gradledataflow/src/main/java/org/checkerframework/dataflow/util/PurityChecker.javadocs/CHANGELOG.mdframework/src/main/java/org/checkerframework/common/basetype/BaseTypeVisitor.javaframework/tests/flow/PurityLambda.javaframework/tests/flow/PurityLambdaSam.java
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…hecker-framework into purity-checker-lambda-boundary
mernst
approved these changes
Sep 14, 2026
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| ### Changes for type system implementers | ||
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| `BaseTypeVisitor.reportPurityErrors()` no longer takes a `MethodTree` |
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I don't think it is likely that type system implementers are affected by this change.
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Side-effects are allowed in a lambda if the functional method is not annotated with
@SideEffectFreeeven if the lambda is declared in a@SideEffectFreemethod.