Invalid CheckReturnValue suggested fix when a variable must be final or effectively final

[hisener]

CheckReturnValue suggests a fix that causes a non-compilable state for lambda expressions since the variables defined in lambda expressions must be final or effectively final.

You can find a failing test here: PicnicSupermarket@769049b. The test failure:

import com.google.errorprone.annotations.CheckReturnValue;
import java.util.List;

public class TestClass {
  public void test() {
    var variable = new CustomType();
    // BUG: Diagnostic contains: Ignored return value
    List.of(1).forEach(unused -> variable.get());
  }
  static final class CustomType {
    @CheckReturnValue
    public CustomType get() {
      return null;
    }
  }
}


Actual Source:
=================

import com.google.errorprone.annotations.CheckReturnValue;
import java.util.List;

public class TestClass {
  public void test() {
    var variable = new CustomType();
    // BUG: Diagnostic contains: Ignored return value
    List.of(1).forEach(unused -> variable = variable.get());
  }
  static final class CustomType {
    @CheckReturnValue
    public CustomType get() {
      return null;
    }
  }
}

Diffs:
======

Found 1 nodes that differed in expected and actual trees. 

> Difference in expected tree and actual tree.
  Expected node: Line 8 COMPILATION_UNIT->CLASS(TestClass)->METHOD(test)->BLOCK(non-static)->EXPRESSION_STATEMENT->METHOD_INVOCATION->LAMBDA_EXPRESSION->METHOD_INVOCATION->METHOD_INVOCATION
  Actual node: Line 8 COMPILATION_UNIT->CLASS(TestClass)->METHOD(test)->BLOCK(non-static)->EXPRESSION_STATEMENT->METHOD_INVOCATION->LAMBDA_EXPRESSION->ASSIGNMENT->ASSIGNMENT
  Expected node kind to be <METHOD_INVOCATION> but was <ASSIGNMENT>.

I have a naive fix but this would prevent potential fixes for variables defined in lambda expressions.

diff --git a/core/src/main/java/com/google/errorprone/bugpatterns/AbstractReturnValueIgnored.java b/core/src/main/java/com/google/errorprone/bugpatterns/AbstractReturnValueIgnored.java
index d5236902b..0739f866a 100644
--- a/core/src/main/java/com/google/errorprone/bugpatterns/AbstractReturnValueIgnored.java
+++ b/core/src/main/java/com/google/errorprone/bugpatterns/AbstractReturnValueIgnored.java
@@ -223,12 +223,12 @@ public abstract class AbstractReturnValueIgnored extends BugChecker
 
     Fix fix = SuggestedFix.emptyFix();
     Symbol symbol = getSymbol(identifierExpr);
-    if (identifierExpr != null
-        && symbol != null
+    if (symbol != null
         && !symbol.name.contentEquals("this")
+        && state.getPath().getParentPath().getLeaf().getKind() != Kind.LAMBDA_EXPRESSION
         && returnType != null
         && state.getTypes().isAssignable(returnType, identifierType)) {
-      // Fix by assigning the assigning the result of the call to the root receiver reference.
+      // Fix by assigning the result of the call to the root receiver reference.
       fix =
           SuggestedFix.prefixWith(
               methodInvocationTree, state.getSourceForNode(identifierExpr) + " = ");