feat(cli): Add Deno.mainModule

[Alhadis]

This PR exposes the path/URL of the original script that was executed on the command-line (previously available as location.pathname; removed in #5034). It's the moral equivalent of Node's process.argv[1] value, except that it's a URL string instead of an ordinary path. This is because deno run can execute remote files directly, making it necessary to distinguish between /foo/bar and https://deno.land/foo/bar.

Be aware that for deno eval, Deno.mainModule will contain some fabricated location ending in __$deno$eval.ts. I would have set this property to null in such cases, but this is literally the first time I've ever touched Rust, and my attempts were met with confusing and frustrating errors (I guess 15 minutes isn't enough to wrap one's head around the basics of Rust…)

[CLAassistant]

All committers have signed the CLA.

[ry]

All for this addition but it should be unstable first.

[nayeemrmn]

We should probably put this behind a blind read permission check like CWD and exec path. This will in a lot of cases leak the user's username, for example.

[Alhadis]

Okay, done. Hopefully I did that correctly.

[Alhadis]

@ry FYI, the tests pass when I run them locally. The CI failure seems to have been caused by a connection timeout.

[ry]

LGTM - thanks!

[ghost]

@Alhadis would you mind explaining what the difference is to import.meta.url?

Put in a script file, these two outputs are equivalent:

console.log(Deno.mainModule)
console.log(import.meta.url)

Or is it meant to have a more concise wording?

[kitsonk]

It isn't when that module imports another module. They are only equivalent in the main module.

[ghost]

Right, thanks! I just had the usual case in mind, that the script entry point triggers all main-file related code actions, so import.meta.url would be equivalent here. Deno.mainModule certainly is more flexible and a good addition.