Improve diagnostic message for unused parameters that are reassigned.

PiperOrigin-RevId: 864792550

Author: graememorgan

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

@@ -267,14 +267,21 @@ public Description matchCompilationUnit(CompilationUnitTree tree, VisitorState s
       if (suggestUnderscore(state, isEverUsed, unusedSymbol, specs, allUsageSites)) {
         fixes.add(SuggestedFixes.renameVariable((VariableTree) unused, "_", state));
       }
+      String message;
+      if (!isEverUsed.contains(symbol)) {
+        message =
+            String.format("The %s '%s' is never read.", describeVariable(symbol), symbol.name);
+      } else if (unused instanceof VariableTree && symbol.getKind() == ElementKind.PARAMETER) {
+        message = String.format("The parameter '%s' is reassigned before being read.", symbol.name);
+      } else {
+        message =
+            String.format(
+                "This assignment to the %s '%s' is never read.",
+                describeVariable(symbol), symbol.name);
+      }
       state.reportMatch(
           buildDescription(unused)
-              .setMessage(
-                  String.format(
-                      "%s %s '%s' is never read.",
-                      isEverUsed.contains(symbol) ? "This assignment to the" : "The",
-                      describeVariable(symbol),
-                      symbol.name))
+              .setMessage(message)
               .addAllFixes(
                   fixes.build().stream()
                       .map(f -> SuggestedFix.merge(makeFirstAssignmentDeclaration, f))

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

@@ -1324,6 +1324,23 @@ public int test() {
         .doTest();
   }
 
+  @Test
+  public void unusedParameterImplicitAssignment() {
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            class Test {
+              // BUG: Diagnostic contains: The parameter 'a' is reassigned before being read
+              private int test(int a) {
+                a = 1;
+                return a;
+              }
+            }
+            """)
+        .doTest();
+  }
+
   @Test
   public void unusedAssignment_nulledOut_noWarning() {
     helper