Improve source map handling

[marvinhagemeister]

The source map handling in WMR can be a lot better. Namely:

• Ensure --sourcemap generates sourcemaps Add groundwork for source map support #715
• Merge source maps in container.transform()
• Ensure acorn-traverse generates correct mappings
• Always include sourcesContent, so that the original code is shown in browser devtools
• Ensure npm plugin supports source maps (might need a rewrite) - out of scope for now. The rewrite will happen either way and we can do the source maps in one go
• Add source map support to .sass (iirc they have different file mappings)
• Decide on path format ./foo.js vs foo.js
• Improve performance of source map generation (check if it can be made so fast, that we can turn this always on)

[Inviz]

Huge plus one on this. Currently debugging things in builds is a pretty sizable PITA. On my app, chrome code formatted just gives up, and Sentry error reports also are pretty damn useless.

[Inviz]

In my app what i'm seeing is that .js file has a sourcemap file that goes like this:

{"version":3,"file":"index.31a59432.js","sources":["source:///components/formField/style.module.scss","source:///components/combobox/style.module.scss","source:///components/form/style.module.scss","source:///components/button/style.module.scss"....

The issue is that sources array only has scss files, and no javascript files at all.

[Inviz]

And there's these messages as well:

Sourcemap is likely to be incorrect: a plugin (default-loaders) was used to transform files, but didn't generate a sourcemap for the transformation.```

[Inviz]

If defaultLoaders plugin is commented out from wmr, all sourcemaps start working

[marvinhagemeister]

Had a go at this over the weekend and there are various places which are not aware of sourcemaps yet. Those generate wrong line and column mappings or miss source information completely. The default-laoders plugin is one of those places. Then the other question is about how to merge all the maps from the generated stages. This part is a little trickier and something other tools seem to have trouble with too.

One reason for column mismatches seems to be that something in our stack interprets column mappings wrong when a new line begins. The spec clearly says that all column positions of the original file are relative to the previous one, but something resets it to 0 with each new line.

Also found various ways to speed up source map generation and parsing in itself by eliminating all cases of allocations. The underlying library of magic-string that does the soucemapping puts a lot of pressure on the GC.

Looking forward to dive deeper into this!

[Inviz]

Sounds super exciting!