`UnnecessaryStringBuilder` misses the split declaration/assignment form

[Chordrain]

Error Prone version

Error Prone 2.49.0 (javac plugin) — javac 21.0.8, source/target 21.

Check name

UnnecessaryStringBuilder

Description

UnnecessaryStringBuilder flags a StringBuilder local that is created and not meaningfully used (replaceable by plain string handling). The matcher keys on the combined declarator-with-initializer node StringBuilder x = new StringBuilder(...). Splitting it into a separate declaration and assignment — StringBuilder x; x = new StringBuilder(...); — is a semantics-preserving refactor (the variable is still initialized once before any use), yet the rule stops firing entirely.

Reproducer

// BEFORE — 2 findings (on the two StringBuilder locals)
class S {
  public void f() {
    StringBuffer foo = new java.lang.StringBuffer('x');
    StringBuilder foo2 = new StringBuilder('x');  // flagged
    StringBuffer foo3 = new StringBuffer("x");
    StringBuilder foo4 = new StringBuilder("x");  // flagged
  }
}

// AFTER — equivalent rewrite, 0 findings
class S {
  public void f() {
    StringBuffer foo;
    foo = new java.lang.StringBuffer('x');
    StringBuilder foo2;
    foo2 = new StringBuilder('x');
    StringBuffer foo3;
    foo3 = new StringBuffer("x");
    StringBuilder foo4;
    foo4 = new StringBuilder("x");
  }
}

Expected behavior

UnnecessaryStringBuilder should report the same findings on BEFORE and AFTER, since T x = init; and T x; x = init; are observationally equivalent.

Actual behavior

• BEFORE: 2 findings (Prefer string concatenation over explicitly using StringBuilder#append ...).
• AFTER: 0 findings.

The matcher only recognizes the declarator-with-initializer form and does not follow the variable when the initializer is a separate assignment statement.