file:// URLs should be treated as absolute when populating sourcemap sources

[Rich-Harris]

Describe the bug
If you pass filename and outputFilename options to svelte.compile, the sources property of the resulting sourcemap will contain a relative path from outputFilename to filename (and likewise for cssOutputFilename). This is the only thing outputFilename is used for.

sources is used inconsistently in the wild; it's often unclear whether it's supposed to be describing filepaths or URLs. We treat it as a an array of filepaths, but that sometimes yields unwanted results: FredKSchott/snowpack#1941 (reply in thread).

In the case where to is absolute, relative(from, to) === to should be true. file:// URLs are absolute, but we don't treat them as such.

To Reproduce

const { pathToFileURL } = require('url');
const { compile } = require('svelte/compiler');
const assert = require('assert');

const source = `<h1>Hello world!</h1>`;

const filename = pathToFileURL('index.svelte').href;
const outputFilename = '/components/index.svelte';

const result = compile(source, {
  filename,
  outputFilename
});

assert.deepEqual(
  result.js.map.sources,
  [filename]
);

Expected behavior
outputFilename should be disregarded, sources should equal [filename]. Instead we get ../../../file:///path/to/project/index.svelte.

Severity
Not critical — possible to workaround.

[benmccann]

CC'ing our recent source mapping experts @dmitrage @milahu 😄

[milahu]

could be easy as master...milahu:patch-2

[dmitrage]

Is it how preprocess is expected to work then?

• We continue passing filename (not file:// url) to preprocess
• It generates source map with sources relative to component source file location (basename for component itself)
• When combining (apply_preprocessor_sourcemap), we resolve sources to file:// form relative to passed url

Example:
preprocess map sources: ["index.svelte", "external.css"]
compile input filename: file:///path/to/project/index.svelte
result map sources: ["file:///path/to/project/index.svelte", "file:///path/to/project/external.css"]

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.