Add @__NO_SIDE_EFFECTS__ or @__PURE__ annotations

[tetsuharuohzeki]

Motivation

This helps to provide more information to a minifier for code reduction.

Implementation

1. Add @__NO_SIDE_EFFECTS__ annotation for function definitions
2. Add @__PURE__ annotation for callee site

Unresolved Questions

1. We need to add them carefully.
   • If we supply them, a minifier trusts their information for code reduction. This means that this effort would lead a bug. It might be hard to reproduce or detect because an user would run minifier only in a production/release build.
2. Testing.

See also

• feat: support #__NO_SIDE_EFFECTS__ annotation for function declaration  rollup/rollup#5024
• Support #__NO_SIDE_EFFECTS__ comment from Rollup evanw/esbuild#3149
• Support __NO_SIDE_EFFECTS__ in the minifier swc-project/swc#7525
• Extending the pure annotation for functions mishoo/UglifyJS#5234

[tetsuharuohzeki]

At least, we cannot mark map() method as @__NO_SIDE_EFFECTS__. We think and provide map() as that its transformer argument does not have any side effect ordinally. But an user probably pass any functions that do something having a side effect wildly and we don't have any way to detect and disallow them.

[tetsuharuohzeki]

If there are some demands from user, we may have a benefit to do this.

But I suspect that we need to wait and see the progress about their annotations (e.g. specification, or inventing a lint rule and push into ESLint).

Furthermore, our implementation uses a function declaration syntax for all to define APIs. Of course, this does not help to provide a complete information for code reduction with less computation but their syntax is easier to reduce than function expression + variable declaratio like const bar = (val) => { ...} , I think.