[Bug]: sourcemap paths are not relative

[alexeagle]

What happened?

When we changed to swcx we changed the sourcemaps:

https://github.com/aspect-build/rules_swc/blob/main/examples/rc/src/expected.js.map
"sources":["examples/rc/src/in.ts"]
https://github.com/aspect-build/rules_swc/blob/v0.20.1/examples/rc/src/expected.js.map
"sources":["in.ts"]

Version

It changed in the commit where we switched from @swc/cli (the nodejs wrapper) to swcx
https://github.com/aspect-build/rules_swc/commit/6358e06ac2a8d6aca2c2ea0cd6ea9dc7b9217a27#diff-90f27955a28ad20f91303a[…]4c55c9ef8b0ccb3255a1bdd1f726

[alexeagle]

Note, this is pretty typical under Bazel since tools are run in a working directory at the repository root.

Options:

1. change swcx to relativize source paths in the sourcemap output
2. compose with another action to post-process the output of swcx
3. wrap swcx to run it from a different working directory than Bazel spawn the action

[alexeagle]

@realtimetodie any ideas on whether option 1 is feasible?

[alexeagle]

Maybe it's just that we need source_root which is listed as missing on swc-project/swc#4017

[realtimetodie]

Yes, the source_root option is not implemented in the Rust-based SWC cli. In the Node.js @swc/cli the entire source map is reparsed as JSON again, see swc-project/swc#1388. I mentioned this bug in #154 (comment).

https://github.com/swc-project/cli/blob/461231917181133fd2e98b954fce07b599711502/src/swc/compile.ts#L20

https://github.com/swc-project/cli/blob/461231917181133fd2e98b954fce07b599711502/src/swc/util.ts#L58-L59

I will look into it this weekend

[alexeagle]

Ping @realtimetodie have any time to look? If not maybe @titanous who contributed another swc fix?

[realtimetodie]

Yes, I'm about to complete it. I have some more time tomorrow.

[realtimetodie]

The changes were merged upstream. Here is an example https://github.com/realtimetodie/rules_swc/tree/bug-166

[alexeagle]

Cool, I guess we're waiting on an swc release before we can upgrade here to land the fix.

[alexeagle]

@realtimetodie user question on this that I'm not sure how to answer: https://bazelbuild.slack.com/archives/CEZUUKQ6P/p1677624490464619?thread_ts=1675968328.558779&cid=CEZUUKQ6P

[realtimetodie]

@alexeagle So I went back in the history to test the behaviour when rules_swc used the Node.js @swc/cli.

In general, the sourceRoot value is not set by default. However, in rules_swc we set a path by default now. I think this is wrong. There should be no value set by default. We can't possibly assume the right path. Every project is different and the value could also be an URL.

Instead, the user should to set the value explicitly by either a) by using the --source-root option as a custom argument, or b) we define a new optional variable in the rule source_root. This would follow the default swc behaviour.

Example

swc(
    name = "compile",
    source_maps = True,
    source_root = "./path/to",
)

https://github.com/swc-project/cli/blob/df9b9ec04201ac353c3e794fed6fc29152fc4cb0/src/swc/options.ts#L221

[realtimetodie]

The same goes for the source file name, which is the first entry in the sources JSON array using the --source-file-name option.

Example

swc(
    name = "compile",
    source_maps = True,
    source_file_name = "custom.ts",
)

https://github.com/swc-project/cli/blob/df9b9ec04201ac353c3e794fed6fc29152fc4cb0/src/swc/options.ts#L220

[alexeagle]

TAL at #176 which aligns the behavior with ts_project

[alexeagle]

Seems like the mistake was just that the source-root should have been relative to the rootDir.