falcon

Rules / Complexity / unnecessary-const

unnecessary-const

lint/complexity/unnecessary-const
Same as unnecessary_const · Dart lints

Flags a redundant const keyword inside an already-const context.

Once an expression sits in a constant context — a const collection, the arguments of a const constructor invocation, a const variable initializer, or another const expression — every nested const is inferred and the keyword is just noise; removing it leaves behavior unchanged. The rule tracks const depth as it descends through const variable, field, and local declarations and const expressions, flagging any explicit const marker (on a new, list, map, set, or dot-shorthand) found at depth greater than zero.

Invalid

example.dartdart
// Bad: a `const` keyword used where the context is already constant.
class Point {
  final int x;
  final int y;
  const Point(this.x, this.y);
}

const outerList = const [1, 2, 3];
const outerMap = const {'a': 1};
const outerSet = const {1, 2, 3};
const point = const Point(1, 2);
const nestedList = [const [1, 2]];
const wrapped = [const Point(1, 2)];
Unnecessary `const` keyword inside an already-const context.
7
8const outerList = const [1, 2, 3];
Unnecessary `const` keyword inside an already-const context.
8const outerList = const [1, 2, 3];
9const outerMap = const {'a': 1};
Unnecessary `const` keyword inside an already-const context.
9const outerMap = const {'a': 1};
10const outerSet = const {1, 2, 3};
Unnecessary `const` keyword inside an already-const context.
10const outerSet = const {1, 2, 3};
11const point = const Point(1, 2);
Unnecessary `const` keyword inside an already-const context.
11const point = const Point(1, 2);
12const nestedList = [const [1, 2]];
Unnecessary `const` keyword inside an already-const context.
12const nestedList = [const [1, 2]];
13const wrapped = [const Point(1, 2)];

Valid

example.dartdart
// Good: `const` only where it is actually required to establish a constant.
class Point {
  final int x;
  final int y;
  const Point(this.x, this.y);
}

const outerList = [1, 2, 3]; // implicitly const, no keyword needed
const point = Point(1, 2); // implicitly const
const nested = [
  [1],
  [2],
]; // all implicit
final runtime = const [1, 2, 3]; // final var: `const` needed to make it constant
var mutable = const <int>[1]; // `const` needed (context is not constant)

void f() {
  const local = [1, 2, 3]; // implicitly const initializer
  print(const [1, 2, 3]); // `const` needed: argument context is not constant
  print(local);
}

How to configure

Set the severity of unnecessary-const in your falcon.json:

falcon.jsonjson
{
  "linter": {
    "rules": {
      "complexity": {
        "unnecessary-const": "error"
      }
    }
  }
}

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