Fix UnusedVariable false positives for private record parameters

Fixes #2713

It looks like the tree emitted by the compiler is incomplete for the canonical constructor of a record, which is why the parameters were erroneously flagged as unused. Apparently this only affected `private` record classes because for them the implicit canonical constructor is `private` as well, and the `UnusedVariable` check treats parameters of `private` methods and constructors differently.

#2713 (comment) showed an example where this also affected record fields, however I was unable to reproduce that. Maybe they were using an older Error Prone version which does not include 47da3af, their sample code is incomplete, or my test setup was incorrect.
Edit: It looks like they were using an older Error Prone version, see #2713 (comment).

Fixes #3837

FUTURE_COPYBARA_INTEGRATE_REVIEW=#3837 from Marcono1234:record-unused-param-false-positives 4535778
PiperOrigin-RevId: 557197001

core/src/main/java/com/google/errorprone/bugpatterns/UnusedVariable.java

@@ -623,6 +623,13 @@ && exemptedFieldBySuperType(getType(variableTree), state)) {
           if (variableTree.getName().contentEquals("this")) {
             return null;
           }
+          // Ignore if parameter is part of canonical record constructor; tree does not seem
+          // to contain usage in that case, but parameter is always used implicitly
+          // For compact canonical constructor parameters don't have record flag so need to
+          // check constructor flags (`symbol.owner`) instead
+          if (hasRecordFlag(symbol.owner)) {
+            return null;
+          }
           unusedElements.put(symbol, getCurrentPath());
           if (!isParameterSubjectToAnalysis(symbol)) {
             onlyCheckForReassignments.add(symbol);
@@ -645,14 +652,18 @@ private boolean isFieldEligibleForChecking(VariableTree variableTree, VarSymbol
           && ASTHelpers.hasDirectAnnotationWithSimpleName(variableTree, "Inject")) {
         return true;
       }
-      if ((symbol.flags() & RECORD_FLAG) == RECORD_FLAG) {
+      if (hasRecordFlag(symbol)) {
         return false;
       }
       return canBeRemoved(symbol) && !SPECIAL_FIELDS.contains(symbol.getSimpleName().toString());
     }
 
     private static final long RECORD_FLAG = 1L << 61;
 
+    private boolean hasRecordFlag(Symbol symbol) {
+      return (symbol.flags() & RECORD_FLAG) == RECORD_FLAG;
+    }
+
     /** Returns whether {@code sym} can be removed without updating call sites in other files. */
     private boolean isParameterSubjectToAnalysis(Symbol sym) {
       checkArgument(sym.getKind() == ElementKind.PARAMETER);

core/src/test/java/com/google/errorprone/bugpatterns/UnusedVariableTest.java

@@ -1398,12 +1398,105 @@ public void nestedRecord() {
         .addSourceLines(
             "SimpleClass.java",
             "public class SimpleClass {",
-            "   public record SimpleRecord (Integer foo, Long bar) {}",
+            "  public record SimpleRecord (Integer foo, Long bar) {}",
             "}")
         .expectNoDiagnostics()
         .doTest();
   }
 
+  @Test
+  public void recordWithStaticFields() {
+    assumeTrue(RuntimeVersion.isAtLeast16());
+    helper
+        .addSourceLines(
+            "SimpleClass.java",
+            "public class SimpleClass {",
+            "  public record MyRecord (int foo) {",
+            "    private static int a = 1;",
+            "    private static int b = 1;",
+            "    // BUG: Diagnostic contains: is never read",
+            "    private static int c = 1;",
+            "    ",
+            "    public MyRecord {",
+            "      foo = Math.max(a, foo);",
+            "    }",
+            "  }",
+            "",
+            "  public int b() {",
+            "    return MyRecord.b;",
+            "  }",
+            "}")
+        .doTest();
+  }
+
+  // Implicit canonical constructor has same access as record
+  // (https://docs.oracle.com/javase/specs/jls/se17/html/jls-8.html#jls-8.10.4)
+  // Therefore this case is important to test because UnusedVariable treats parameters of private
+  // methods differently
+  @Test
+  public void nestedPrivateRecord() {
+    assumeTrue(RuntimeVersion.isAtLeast16());
+    helper
+        .addSourceLines(
+            "SimpleClass.java",
+            "public class SimpleClass {",
+            "  private record SimpleRecord (Integer foo, Long bar) {}",
+            "}")
+        .expectNoDiagnostics()
+        .doTest();
+  }
+
+  @Test
+  public void nestedPrivateRecordCompactCanonicalConstructor() {
+    assumeTrue(RuntimeVersion.isAtLeast16());
+    helper
+        .addSourceLines(
+            "SimpleClass.java",
+            "public class SimpleClass {",
+            "  private record SimpleRecord (Integer foo, Long bar) {",
+            // Compact canonical constructor implicitly assigns field values at end
+            "    private SimpleRecord {",
+            "      System.out.println(foo);",
+            "    }",
+            "  }",
+            "}")
+        .expectNoDiagnostics()
+        .doTest();
+  }
+
+  @Test
+  public void nestedPrivateRecordNormalCanonicalConstructor() {
+    assumeTrue(RuntimeVersion.isAtLeast16());
+    helper
+        .addSourceLines(
+            "SimpleClass.java",
+            "public class SimpleClass {",
+            "  private record SimpleRecord (Integer foo, Long bar) {",
+            "    private SimpleRecord(Integer foo, Long bar) {",
+            "      this.foo = foo;",
+            "      this.bar = bar;",
+            "    }",
+            "  }",
+            "}")
+        .expectNoDiagnostics()
+        .doTest();
+  }
+
+  @Test
+  public void unusedRecordConstructorParameter() {
+    assumeTrue(RuntimeVersion.isAtLeast16());
+    helper
+        .addSourceLines(
+            "SimpleRecord.java",
+            "public record SimpleRecord (int x) {",
+            "  // BUG: Diagnostic contains: The parameter 'b' is never read",
+            "  private SimpleRecord(int a, int b) {",
+            "    this(a);",
+            "  }",
+            "}")
+        .doTest();
+  }
+
   @Test
   public void unusedInRecord() {
     assumeTrue(RuntimeVersion.isAtLeast16());