[Suggestion] Compiler Flag to treat types as immutable

TypeScript Version: 2.3.4

Code

I'm aware of #10725, but I want to take it one step further and add a compiler flag (e.g. --immutable) that will cause the compiler to infer Readonly, ReadonlyArray, ReadonlyMap, and ReadonlySet (and any other data structures I've missed) everywhere. The flag will also force you to use const. Thus eliminating mutability from the language.

// tsconfig.json

{
  "compilerOptions": {
    "immutable": true
  }
}

// demo.ts

// band is automatically inferred as:
// Readonly<{
//   name: string;
//   members: ReadonlyArray<{
//     name: string;
//     instruments: ReadonlyArray<string>;
//   }>;
// }>
const band = {
  name: 'U2',
  members: [
    {
      name: 'Bono',
      instruments: ['vocals', 'rhythm guitar', 'harmonic']
    },
    {
      name: 'The Edge',
      instruments: ['lead guitar', 'keyboards', 'backing vocals']
    },
    // ...
  ]
};

// The following will cause compiler errors:
// band.name = 'Green Day';
// band.members.push({ name: 'Billie Joe Armstrong', instruments: ['lead vocals', 'guitar'] });

// ----------------------------------------------------------------------

// numberMap is automatically inferred to be ReadonlyMap<number, string>
const numberMap = new Map<number, string>([
  [1, 'one'],
  [2, 'two']
]);

// Compiler error:
// numberMap.set(3, 'three');

// letterSet is is automatically inferred to be ReadonlySet<string>
const letterSet = new Set<string>(['A', 'B', 'C', 'D']);

// Compiler error:
// letterSet.add('E');

Inizia esaminando la proposta, l’issue referenziata #10725 e la guida collegata di TypeScript sulle design proposals. Definisci il comportamento dell’opzione del compilatore per const enforcement e readonly inference negli esempi e nelle altre strutture dati; l’issue non fornisce file o test da eseguire, quindi il completamento richiederebbe un design concordato e la copertura corrispondente.