ESM config doesn't work on Windows

[jods4]

Describe the bug

Try to write an ESM config with vite.config.mjs or simply vite.config.js in a package with type: "module".
Vite tries to import it but fails with

failed to load config from C:\****\vite.config.mjs
error when starting dev server:
Error [ERR_UNSUPPORTED_ESM_URL_SCHEME]: Only file and data URLs are supported by the default ESM loader. On Windows, absolute paths must be valid file:// URLs. Received protocol 'c:'
    at new NodeError (node:internal/errors:278:15)
    at Loader.defaultResolve [as _resolve] (node:internal/modules/esm/resolve:825:11)
    at Loader.resolve (node:internal/modules/esm/loader:86:40)
    at Loader.getModuleJob (node:internal/modules/esm/loader:230:28)
    at Loader.import (node:internal/modules/esm/loader:165:28)
    at importModuleDynamically (node:internal/modules/cjs/loader:1051:27)
    at exports.importModuleDynamicallyCallback (node:internal/process/esm_loader:30:14)
    at eval (eval at loadConfigFromFile (C:\****\node_modules\vite\dist\node\chunks\dep-661a710c.js:46795:33), <anonymous>:1:21)
    at loadConfigFromFile (C:\****\node_modules\vite\dist\node\chunks\dep-661a710c.js:46795:33)      
    at resolveConfig (C:\****\node_modules\vite\dist\node\chunks\dep-661a710c.js:46584:34)

The root cause is here:

vite/packages/vite/src/node/config.ts

Line 415 in 1ea0168

userConfig = (await eval(`import(resolvedPath + '?t=${Date.now()}')`))

You can't import("C:\..."), you need to add the file:/// protocol.
Node suggests that you use pathToFileUrl for that as escaping can sometimes be tricky.

Suggested fix:

({ default: userConfig } = await import(`${url.pathToFileURL(resolvedPath)}?t=${Date.now()}`)

✔️ Works on my machine.

Notice that I removed the eval. I am no Rollup expert but I believe rollup can't handle dynamic imports that aren't constant. There's a plugin that handles interpolation but it also gives up if the interpolation doesn't begin with . or .. and ends up with a file extension. So I don't think the ugly eval is required here, I might be wrong.

Bonus suggested improvement: that code doesn't look for a possible .cjs config, which would be commonjs inside a esm-by-default package.

I don't have time for that additional bug: when using Typescript vite.config.ts the result seems to always be interpreted as a CJS module, even inside a package that is ESM by default. This blew up on me when one of my modules had a top-level await, which is only allowed in ESM mode in Node 14.3+. Just FYI, I don't have time to go into this bug.

System Info

• vite version: 2.0.0-beta.21
• Operating System: win 10
• Node version: 15
• Package manager (npm/yarn/pnpm) and version: npm 7

[yyx990803]

The eval is still necessary during development because TS will compile all import into sync requires when targeting commonjs.

[jods4]

You went for a totally different file path encoding, which is all fine by me, but it leaves me wondering how it works if you reference stuff on a different drive altogether.

The /path/to/config.js syntax is relative to the root of the current drive, and your code blindly strips any drive in the absolute path.
That might be a super rare use-case, but wouldn't that fail if I use an absolute path to another drive in my inline config?

EDIT: I'm not doing that and I don't really care if it doesn't work, just noticing.

[yyx990803]

Vite's resolve plugin and most official Rollup plugins normalize windows paths this way, so the whole thing likely won't work with any cross drive imports anyway.