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use-existing-variable

Group: Style
Path: lint/style/use-existing-variable
Default: Off, opt in
Origin: Falcon
Rule: 489 of 512

Reuse a local for stable SDK arithmetic and String.length/int parity getters. Calls, user getters, allocations and overloaded operators are excluded. Whole-scope mutation facts deliberately reject even later/closure writes.

Invalid

example.dartdart

int a() {
  final x = 2;
  final cached = x + 1;
  return x + 1;
}
int b() {
  final x = 3;
  final cached = x * 2;
  return (x * 2);
}
int c() {
  final x = 4;
  final cached = x - 1;
  final next = x - 1;
  return next;
}
int d() {
  final x = 5;
  final cached = (x + 1) * 2;
  return (x + 1) * 2;
}
int e() {
  final cached = 2 + 3;
  return 2 + 3;
}
void sdkGetter(String value) {
  final cached = value.length.isOdd;
  print(value.length.isOdd);
}

Reuse the existing local instead of repeating this computation.

3 final cached = x + 1;

4 return x + 1;

∙

Reuse the existing local instead of repeating this computation.

8 final cached = x * 2;

9 return (x * 2);

∙

Reuse the existing local instead of repeating this computation.

13 final cached = x - 1;

14 final next = x - 1;

∙

Reuse the existing local instead of repeating this computation.

19 final cached = (x + 1) * 2;

20 return (x + 1) * 2;

∙

Reuse the existing local instead of repeating this computation.

23 final cached = 2 + 3;

24 return 2 + 3;

∙

Reuse the existing local instead of repeating this computation.

27 final cached = value.length.isOdd;

28 print(value.length.isOdd);

∙

Valid

example.dartdart

int mutation() {
  var x = 1;
  final cached = x + 2;
  x = 2;
  return x + 2;
}
int captured() {
  var x = 1;
  final cached = x + 2;
  void change() { x++; }
  return x + 2;
}
Object allocate() {
  final cached = Object();
  return Object();
}
int calls(int Function() next) {
  final cached = next() + 1;
  return next() + 1;
}
int shadow() {
  final x = 1;
  final cached = x + 1;
  { final x = 2; return x + 1; }
}
int branch(bool flag) {
  final x = 1;
  if (flag) { final cached = x + 1; }
  return x + 1;
}
int division() {
  final x = 1;
  final cached = x ~/ 2;
  return x ~/ 2;
}
class Counter {
  Counter operator +(int other) => Counter();
}
Counter overloaded() {
  final x = Counter();
  final cached = x + 1;
  return x + 1;
}
int mutatedCache() {
  final x = 1;
  var cached = x + 1;
  cached = 3;
  return x + 1;
}
int constantContext() {
  final cached = 2 + 3;
  const next = 2 + 3;
  return next;
}
int lateDependency() {
  late final x = 1;
  final cached = x + 1;
  return x + 1;
}
class Unstable {
  int get length => DateTime.now().second;
}
bool customGetter(Unstable value) {
  final cached = value.length.isOdd;
  return value.length.isOdd;
}

Configure use-existing-variable

Set its severity in falcon.json. See the configuration guide.

falcon.jsonjson

{
  "linter": {
    "rules": {
      "style": {
        "use-existing-variable": "error"
      }
    }
  }
}

Or silence one occurrence with a suppression comment; the reason is required.

example.dartdart

// falcon-ignore lint/style/use-existing-variable: <reason>

use-full-hex-values-for-flutter-colorsuse_full_hex_values_for_flutter_colorsrecommendedflutter
Require an eight-digit hexadecimal integer for the canonical Flutter Color default constructor.
use-function-type-syntax-for-parametersuse_function_type_syntax_for_parametersrecommended
Flags an old-style function-typed parameter written with inline parameter syntax.
use-if-null-to-convert-nulls-to-boolsuse_if_null_to_convert_nulls_to_boolsrecommended
Prefers ?? to comparing a nullable bool against a boolean literal.
use-late-for-private-fields-and-variablesuse_late_for_private_fields_and_variables
Flags a private nullable field or top-level variable that is only ever assigned non-null values and read through !, so it could be late with a non-nullable type.
use-named-constantsuse_named_constantsrecommended
Prefers a class's predefined static constant to constructing an equal constant.
use-raw-stringsuse_raw_strings
Flags string literals whose only escapes are \\ and \$, which a raw string would express without any backslash.
use-setters-to-change-propertiesuse_setters_to_change_properties
Flags one-parameter void methods that only assign that parameter to a field.
use-spacer-as-expanded-childuse_spacer_as_expanded_childrecommendedflutter
Flags an Expanded wrapping an empty SizedBox or Container that should be a Spacer.