new URL(url, import.meta.url) should not be fatal in SSR mode when its never called

[doomnoodles]

Describe the bug

Temporary fix

Relevant line in Vite source that throws the described error
Relevant vite-plugin-rsw issue
Relevant SvelteKit issue

vite-plugin-rsw, is a plugin that lets one use Rust on the web with all the good things that come with Vite(HMR, SSR, ...).
That plugin uses wasm-pack under the hood, which creates javascript glue code and types to easily interface with the wasm file.
The glue code contains the following segment, which seems to create an error in Vite when SSR is enabled in SvelteKit:

async function init(input) {
    if (typeof input === 'undefined') { // not executed if input exists
         input = new URL('example-rust-crate-rs_bg.wasm', import.meta.url); // Error: 'new URL(url, import.meta.url)' is not supported in SSR.
    }
    const imports = {};


    if (typeof input === 'string' || (typeof Request === 'function' && input instanceof Request) || (typeof URL === 'function' && input instanceof URL)) {
        input = fetch(input);
    }



    const { instance, module } = await load(await input, imports);

    wasm = instance.exports;
    init.__wbindgen_wasm_module = module;

    return wasm;
}

If the init function is called with a path to the .wasm file then that line never gets executed but Vite still throws the error. If the line is deleted then Vite does not complain and actually renders the .wasm file in SSR context, but that has to be done by hand.

There should be an ability to ignore such lines and not throw a fatal error at compile time, and only throw the error at run time, if that line is somehow reached, because its demonstrated that the code actually works and its Vite being too harsh.

Reproduction

1. Download sveltekit-rust-ssr-template and its dependencies
2. npm install
3. npm run dev and exit
4. npm run build

Interestingly when trying SSR with npm run dev then it works sometimes but sluggishly.

System Info

System:
    OS: Windows 10 10.0.19042
    CPU: (8) x64 AMD Ryzen 7 4700U with Radeon Graphics
    Memory: 2.71 GB / 7.37 GB
  Binaries:
    Node: 16.0.0 - C:\Program Files\nodejs\node.EXE
    npm: 7.20.5 - C:\Program Files\nodejs\npm.CMD
  Browsers:
    Chrome: 94.0.4606.61
    Edge: Spartan (44.19041.1023.0), Chromium (92.0.902.78)

Used Package Manager

npm

Logs

No response

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• Make sure this is a Vite issue and not a framework-specific issue. For example, if it's a Vue SFC related bug, it should likely be reported to https://github.com/vuejs/vue-next instead.
• Check that this is a concrete bug. For Q&A open a GitHub Discussion or join our Discord Chat Server.
• The provided reproduction is a minimal reproducible example of the bug.

[sh-tiye]

Also from wasm-pack nshen/test-npm-crate#1 .

[bluwy]

Fixed by #7219