Ah, so the interaction between the Rust code and the javascript is as follows:

- each `[src]` link gets an id with its nodeid
- `[src]` links of inlined doc items point to `page?gotosrc=<id>` where `page` is the page for the type/function/whatever in its original crate
- this JS picks up the `<id>` and uses it to find the correct local `[src]` and then follows that link.