ref(inject): Inject code snippet at the start

[loewenheim]

This injects the code snippet as early in the file as possible, which is to say, after an initial block of "use …"; statements, if there is one. Since those statements must itself be at the start of the file (except for empty lines and comments), we skip empty lines and comments and then any number of "use …"; statements before injecting. For example, the file

// a
// comment
// block

// another
// comment
// block

'use strict';
"use server";
function t(t) {
  return '[object Object]' === Object.prototype.toString.call(t);
}
//# sourceMappingURL=/path/to/sourcemap

would be modified to

// a
// comment
// block

// another
// comment
// block

'use strict';
"use server";
<CODE_SNIPPET>[<debug_id>]
function t(t) {
  return '[object Object]' === Object.prototype.toString.call(t);
}
//# debugId=<debug_id>
//# sourceMappingURL=/path/to/sourcemap

Modifying the minified source file in this way requires us to fiddle with the sourcemap as well since every line number below the insertion point has now increased by one. Thus, we simply add ; at the start of the sourcemap's mapping string to denote the code snippet's empty line mapping.

Question: Is it sound to just always add the empty mapping at the beginning? If any of the lines before the injection point had mappings, they would get scrambled by this change, but since they're only comments and "use …";, can they even have nontrivial mappings?

[loewenheim]

As it turns out, this proposal has a serious flaw. Injecting text at the beginning of a file will throw off all offsets in the sourcemap. To make this work, we'd have to fix those up after the fact.

Edit: sourcemaps are now getting adjusted as required.

[github-actions]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀

[loewenheim]

Picking this up again because it turned out to be necessary in an internal test.

[kamilogorek]

Looks good to me overall.

[loewenheim]

One thing I definitely still need to handle is embedded sourcemaps.

[kamilogorek]

TODO: Handle when no sourcemap file is available locally. In this case, we should append at the end of the file instead, as sourcemap cannot have its mappings fixed up.

[loewenheim]

This now also handles the edge case of a sourcemap not being available, by essentially falling back to the status quo behavior. I also refactored inject_debug_ids some; the double loop is no longer necessary.