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Add renaming to _ as a suggested fix in UnusedVariable #5442

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Merged
copybara-service merged 1 commit into from
Jan 14, 2026
Merged

Add renaming to _ as a suggested fix in UnusedVariable #5442

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59 changes: 49 additions & 10 deletions core/src/main/java/com/google/errorprone/bugpatterns/UnusedVariable.java
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Original file line number Diff line number Diff line change
Expand Up @@ -64,6 +64,7 @@
import com.google.errorprone.suppliers.Supplier;
import com.google.errorprone.suppliers.Suppliers;
import com.google.errorprone.util.ASTHelpers;
import com.google.errorprone.util.SourceVersion;
import com.sun.source.tree.ArrayAccessTree;
import com.sun.source.tree.AssignmentTree;
import com.sun.source.tree.BindingPatternTree;
Expand Down Expand Up @@ -247,21 +248,27 @@ public Description matchCompilationUnit(CompilationUnitTree tree, VisitorState s
if (!unusedElements.containsKey(unusedSymbol)) {
isEverUsed.add(unusedSymbol);
}
boolean suggestUnderscore =
suggestUnderscore(state, isEverUsed, unusedSymbol, specs, allUsageSites);
SuggestedFix makeFirstAssignmentDeclaration =
makeAssignmentDeclaration(unusedSymbol, specs, allUsageSites, state);
// Don't complain if this is a public method and we only overwrote it once.
if (onlyCheckForReassignments.contains(unusedSymbol) && specs.size() <= 1) {
if ((onlyCheckForReassignments.contains(unusedSymbol) && specs.size() <= 1)
&& !suggestUnderscore) {
continue;
}
Tree unused = specs.iterator().next().assignmentPath().getLeaf();
VarSymbol symbol = (VarSymbol) unusedSymbol;
ImmutableList<SuggestedFix> fixes;
ImmutableList.Builder<SuggestedFix> fixes = ImmutableList.builder();
if (symbol.getKind() == ElementKind.PARAMETER
&& !onlyCheckForReassignments.contains(unusedSymbol)
&& !isEverUsed.contains(unusedSymbol)) {
fixes = buildUnusedParameterFixes(symbol, allUsageSites, state);
fixes.addAll(buildUnusedParameterFixes(symbol, allUsageSites, state));
} else {
fixes = buildUnusedVarFixes(symbol, allUsageSites, state);
fixes.addAll(buildUnusedVarFixes(symbol, allUsageSites, state));
}
if (suggestUnderscore) {
fixes.add(SuggestedFixes.renameVariable((VariableTree) unused, "_", state));
}
state.reportMatch(
buildDescription(unused)
Expand All @@ -272,14 +279,49 @@ public Description matchCompilationUnit(CompilationUnitTree tree, VisitorState s
describeVariable(symbol),
symbol.name))
.addAllFixes(
fixes.stream()
fixes.build().stream()
.map(f -> SuggestedFix.merge(makeFirstAssignmentDeclaration, f))
.collect(toImmutableList()))
.build());
}
return Description.NO_MATCH;
}

private static boolean suggestUnderscore(
VisitorState state,
Set<Symbol> isEverUsed,
Symbol symbol,
Collection<UnusedSpec> specs,
ImmutableList<TreePath> allUsageSites) {
TreePath unusedPath = specs.iterator().next().assignmentPath();
Tree unused = unusedPath.getLeaf();
if (!(unused instanceof VariableTree variableTree)) {
return false;
}
if (isEverUsed.contains(symbol) || specs.size() != 1 || allUsageSites.size() > 1) {
return false;
}
if (!SourceVersion.supportsUnnamedVariablesAndPatterns(state.context)) {
return false;
}
if (!allowsUnderscore((VarSymbol) symbol, unusedPath)) {
return false;
}
if (symbol.getKind().equals(ElementKind.LOCAL_VARIABLE)
&& variableTree.getInitializer() == null) {
return false;
}
return true;
}

private static boolean allowsUnderscore(VarSymbol symbol, TreePath path) {
return switch (symbol.getKind()) {
case LOCAL_VARIABLE -> true;
case PARAMETER -> path.getParentPath().getLeaf() instanceof LambdaExpressionTree;
default -> false;
};
}

private static SuggestedFix makeAssignmentDeclaration(
Symbol unusedSymbol,
Collection<UnusedSpec> specs,
Expand Down Expand Up @@ -615,11 +657,8 @@ private void handleVariable(VariableTree variableTree) {
VarSymbol symbol = getSymbol(variableTree);
var parent = getCurrentPath().getParentPath().getLeaf();
if (parent instanceof LambdaExpressionTree) {
if (FUNCTIONAL_INTERFACE_TYPES_TO_CHECK.stream()
.anyMatch(t -> isSubtype(getType(parent), state.getTypeFromString(t), state))) {
unusedElements.put(symbol, getCurrentPath());
usageSites.put(symbol, getCurrentPath());
}
unusedElements.put(symbol, getCurrentPath());
usageSites.put(symbol, getCurrentPath());
return;
}
if (symbol.getKind() == ElementKind.FIELD
Expand Down
52 changes: 51 additions & 1 deletion core/src/test/java/com/google/errorprone/bugpatterns/UnusedVariableTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -15,8 +15,10 @@
package com.google.errorprone.bugpatterns;

import static com.google.common.truth.TruthJUnit.assume;
import static com.google.errorprone.BugCheckerRefactoringTestHelper.TestMode.TEXT_MATCH;

import com.google.common.collect.ImmutableList;
import com.google.common.collect.Iterables;
import com.google.errorprone.BugCheckerRefactoringTestHelper;
import com.google.errorprone.BugCheckerRefactoringTestHelper.FixChoosers;
import com.google.errorprone.BugCheckerRefactoringTestHelper.TestMode;
Expand Down Expand Up @@ -1956,6 +1958,7 @@ void test() {

@Test
public void unusedFunctionalInterfaceParameter() {
assume().that(Runtime.version().feature()).isAtLeast(22);
helper
.addSourceLines(
"Test.java",
Expand Down Expand Up @@ -2010,6 +2013,7 @@ public void test(List<Integer> xs) {

@Test
public void unusedFunctionalInterfaceParameter_noFix() {
assume().that(Runtime.version().feature()).isAtLeast(22);
refactoringHelper
.addInputLines(
"Test.java",
Expand All @@ -2020,7 +2024,7 @@ public void unusedFunctionalInterfaceParameter_noFix() {

class Test {
public void test(List<Integer> xs) {
Collections.sort(xs, (a, b) -> a > a ? 1 : 0);
Collections.sort(xs, (a, unused) -> a > a ? 1 : 0);
Collections.sort(xs, (a, unused) -> a > a ? 1 : 0);
Collections.sort(
xs,
Expand Down Expand Up @@ -2141,4 +2145,50 @@ private boolean eq(Object o) {
.expectUnchanged()
.doTest();
}

@Test
public void suggestUnderscoreVariable() {
assume().that(Runtime.version().feature()).isAtLeast(22);
refactoringHelper
.addInputLines(
"Test.java",
"""
import java.util.function.Function;

class Test {

static void sink(Object... o) {}

public static void main(String[] args) {
int x;
var foo = new Object();
Function<String, Integer> f = s -> 1;
Function<String, Integer> g = (String s) -> 1;
Function<String, Integer> h = (s) -> 1;
sink(f, g, h);
}
}
""")
.addOutputLines(
"Test.java",
"""
import java.util.function.Function;

class Test {

static void sink(Object... o) {}

public static void main(String[] args) {

var _ = new Object();
Function<String, Integer> f = _ -> 1;
Function<String, Integer> g = (String _) -> 1;
Function<String, Integer> h = (_) -> 1;
sink(f, g, h);
}
}
""")
.setFixChooser(Iterables::getLast)
.doTest(TEXT_MATCH);
}
}
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