Do we support Multi-level mapping source map lookups for final source map consumption

[littledan]

For multilevel source maps, This solution is proposed in the v3 source map spec, in the "Multi-level Mapping Notes" section for maps between languages which don't have built-in syntactic support (which is just JS and CSS, now with Wasm added). Should we do this?

[tromey]

This bug is referring to the text here: https://docs.google.com/document/d/1U1RGAehQwRypUTovF1KRlpiOFze0b-_2gc6fAH0KY0k/edit#heading=h.e8hx254xu4sa

[jridgewell]

I maintain @ampproject/remapping, which traces multi-level source maps to generate a flattened output. It has a (bad) test suite of fixture outputs from build tools, and a few unit tests.

There are a few complexities when remapping, eg:

• What happens if the output map points to an unmapped region in the transitive map?
• What happens if the output map points to a "generated" mapping in the transitive map?
• What happens if the output map has a named mapping, but the transitive map doesn't?
• What happens if the output map has a source mapping, but the transitive map has a named mapping?

[petebacondarwin]

I also spent some time building a similar tool for Angular.

https://github.com/angular/angular/tree/1eda1bdfccc09426ea8d8dd8cfbd8f521cc15f14/packages/compiler-cli/src/ngtsc/sourcemaps

I wrote a doc on what we found and built.

https://docs.google.com/document/d/1x1laiDVFgJtpiw49ozwr91LG7hqohytWDiptJ93nJcE/edit#heading=h.9gb628qxj3gl

In case this is useful here.

[Jack-Works]

Is a multi-level source map ever supported in any debugger? e.g. Chrome doesn't support it (https://bugs.chromium.org/p/chromium/issues/detail?id=1086380)

I think this feature is useful when there is multiple-level transformation of the code.

[jkup]

Talked about this at the Munich source maps hackathon. I think the direction we want to go in is to fully disallow source map references in sources content from being fetched. It seems to open up some security and performance concerns as well as concerns about it being surprising for some users.

We should move to explicitly state we do not support this in the spec. If anyone has strong opinions on keeping it, please update this thread.

[Jack-Works]

I can think of the use case of a multi-level source map. Although no implementation supported this, we should not ban it in the spec.

[nicolo-ribaudo]

Although no implementation supported this, we should not ban it in the spec.

We should either enforce that all implementations support it, or ban it. If there are use cases, it can always be added as a new proposal.