[Tree Shaking] Automatically mark known globals as pure.

[hyrious]

Maybe tree-shaking can do more with this feature:

var a = new Map() // same as..
var a = /*@__PURE__*/new Map()

There's a list in rollup describing the info.

[evanw]

Treating Map as pure is incorrect when map takes an argument. Rollup generates incorrect code by following the rules you linked to. For example, the following code is incorrectly deleted by Rollup even though it has side effects:

const x = new Map({
  [Symbol.iterator]() {
    return {
      next() {
        console.log('side effect')
        return { done: true }
      }
    }
  }
})

Here's a link to this code in Rollup's REPL.

Edit: Filed as a bug with Rollup here: rollup/rollup#4282.