prefer_const_constructors

Use 'const' with the constructor to improve performance.

The analyzer produces this diagnostic when an invocation of a const constructor isn't either preceded by const or in a constant context.

The following code produces this diagnostic because the invocation of the const constructor is neither prefixed by const nor in a constant context:

class C {
  const C();
}

C c = C();

If the context can be made a constant context, then do so:

class C {
  const C();
}

const C c = C();

If the context can't be made a constant context, then add const before the constructor invocation:

class C {
  const C();
}

C c = const C();

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