[compiler] Null out source locations where not explicitly preserved

[josephsavona]

Stack from ghstack (oldest at bottom):

• [compiler] Improve source map coverage for variable declarations/reassignments #33060
• -> [compiler] Null out source locations where not explicitly preserved #33051

Inspired by #32950. Specifically from #32950 (comment), it sounds like Babel by default emits source map information for all nodes, even when they don't have a loc property set. Code coverage tools then pick up the synthesized source location information for this, leading to the issue described.

It wasn't super clear how to omit nodes from the source map. A few google searches didn't turn up any documented way to do so: the user can do so via a function supplied to babel, but plugins can't. Entire files can be opted out, but not nodes. I asked an LLM which answered that explicitly setting node.loc = null omits the node from source maps, but that didn't work.

What did work was explicitly setting the loc property to an object that follows the shape of a source location, but with all properties nulled out. With that, we get the desired result:

I'm going to feature-flag this since setting some of these properties in this way might break in other versions of Babel or have other issues.

[josephsavona]

From the preview deployment this doesn't seem to work, at least in the source map viewer.

edit: addressed, see updated description

[josephsavona]

At least on the version of Babel we're using for tests, just setting loc: null wasn't enough to exclude the node from source maps. But setting all of this was enough to exclude them.

[mofeiZ]

Makes sense, I wonder how this affects setting debugger breakpoints with source maps enabled -- missing / greyed out breakpoints are probably less confusing than misleading ones, so let's try it

On a side note, hopefully this is a no-op for fbt enums which rely on sourcemaps

[josephsavona]

This is blocked on playground being broken. It looks like the changes may be breaking the source maps display, which means that the way i'm nulling out source locations may not be safe. Unfortunately the way Babel treats loc: null is just broken — filling in a default even though you told it to null the field — so it isn't really clear what to do here.