This is now a proper compile-time constant:

import { IS_DEV } from './other'
if (IS_DEV) { // statically evaluates to `true`
  console.log('x')
} else {
  require("library") // not bundled
}
console.log(IS_DEV); // is replaced with console.log(true);

// other.ts
export const SOME_VALUE = 'x'

const node_env = process.env.NODE_ENV
const development_ent = 'development'
export const IS_DEV = node_env === development_ent

You can use code to compute constants just fine, and use any existing constants such as process.env.NODE_ENV.
Currently, you can't use imports to other constants modules, but we can add that later on.

We can't perform this constants check for every single import, so you need to either

• have UPPER_CASE import names as in the example above
• or use import { lower } from './other' with { turbopackConstants: 'true' }

Then that referenced module will be analyzed for constants, and if the referenced export is a constant, it will participate in constant inlining just as process.env.FOO.

This also works fine with barrel imports, you can still do import { IS_DEV } from './barrel.js' and it will find the constants.js file which in itself will indeed only have constants exports.

Because it's not always opt-in at the import site, we can't automatically make non-constant exports an error. For that you have to add 'use turbopack constants' at the top of the module, which will make it an error if any constant import references that module, and the module has any non-constant exports:

error - [analysis] /turbopack/crates/turbopack-tests/tests/snapshot/comptime/cross-module/input/other.constants.js:8:7 
Export NO_CONSTANT is not a constant
  
       4 | const development_ent = 'development'
       5 | 
       6 | export const IS_DEV = node_env === development_ent
       7 | 
         +        v--------------------------------v
       8 + export const NO_CONSTANT = globalThis.foo
         +        ^--------------------------------^
       9 | 
  
  It was analyzed to be FreeVar(globalThis)["foo"]
  
  
  Import trace:
    test:
      ./turbopack/crates/turbopack-tests/tests/snapshot/comptime/cross-module/input/other.constants.js
      ./turbopack/crates/turbopack-tests/tests/snapshot/comptime/cross-module/input/index.js

Some prior art: https://rspack.rs/blog/announcing-1-5#const-inline-optimization, https://rspack.rs/config/optimization#optimizationinlineexports

No compile-time impact on a big app:

canary dfbc3dc6b7: 
438.11s user, 68.35s system, 801% cpu, 1:03.22 total
439.44s user, 71.47s system, 756% cpu, 1:07.56 total
440.32s user, 68.57s system, 750% cpu, 1:07.81 total

constants 2d7eb8220d298133212813b7267a5847eed03a22
433.22s user, 65.44s system, 800% cpu, 1:02.32 total
440.52s user, 67.68s system, 770% cpu, 1:05.95 total
433.13s user, 69.47s system, 776% cpu, 1:04.74 total