microsoft / TypeScript

TypeScript is a superset of JavaScript that compiles to clean JavaScript output.
https://www.typescriptlang.org
Apache License 2.0
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Modifier for bitflag enums #42521

Open DanielRosenwasser opened 3 years ago

DanielRosenwasser commented 3 years ago

Consider the following code

export enum ScriptKind {
  Unknown = 0,
  JS = 1,
  JSX = 2,
  TS = 3,
  TSX = 4,
  External = 5,
  JSON = 6,
}

If a user tries to pass along the expression ScriptKind.JSX | ScriptKind.TSX to a parameter that expects a ScriptKind, then will end up constructing a value equal to ScriptKind.JSON.

Really, the user should have been told that this enum wasn't intended to be used in bitwise operations! But unfortunately, TypeScript doesn't catch this because you can assign any number to any numeric enum.

One could imagine a mode that prevents both the construction and the assignment for certain enums. In this mode, an enum has to be declared with the bitflags modifier if it wants to opt in to these types of bitwise operations, and to allow number to be assignable to that enum.

bitflags enum SymbolFlags {
    // ...
}

Alternatively, this could be something more akin to /** @deprecated */ or /** @internal */.

/// @bitflags
enum SymbolFlags {
    // ...
}

This is in some part inspired by Rust's bitflags macro.

MartinJohns commented 3 years ago

This sounds like #21598.

DanielRosenwasser commented 3 years ago

It is pretty similar! The difference is that this modifier is really just a hint for some sort of strictness flag. It has no runtime impact.

ShuiRuTian commented 3 years ago

I like this! I would try to give a prototype.