Support DWARF in wasm-bindgen

[mitsuhiko]

Motivation

Chrome, Firefox and now also Sentry support DWARF for WebAssembly as debug format. In particularly for crash reporting systems like Sentry DWARF is a very important tool to create accurate stack traces for production builds of web assembly applications.

This issue was previously brought up in #1981.

Proposed Solution

Most likely the bulk of the implementation will be in walrus (see rustwasm/walrus#67).

[Cypher1]

Just hoping for an update as this would be really handy for debugging WASM performance issues. Thx

[michael8090]

After some experiments, I find it's possible to show Rust code in the Chrome DevTools with the following steps:

1. add a --keep-debug in wasm-bindgen when building the wasm file, i.e. wasm-bindgen --keep-debug --out-dir ...
2. install a C++ chrome extension and enable DWARF support feature in chrome dev tools: https://developer.chrome.com/blog/wasm-debugging-2020/?utm_source=devtools

Then the rust code can show up in the dev tools, but other functionalities such as setting breakpoints and evaluating rust code, are broken:

It seems the DWARF info is kept when using wasm-bindgen with --keep-debug, but chrome doesn't fully support the dwarf debugging features for rust code.

[CryZe]

It seems more like the debug info is "broken" after wasm-bindgen processes it, as in enough to make the source show up, but not enough for Chrome to handle it.

[michael8090]

It seems more like the debug info is "broken" after wasm-bindgen processes it, as in enough to make the source show up, but not enough for Chrome to handle it.

The extension is for C++ only, and you can see when I try to evaluate ElementsManager::get_static() in console, it takes the string as C++ code and says it's an invalid one. My guess is that maybe the dwarf info is correct, but it's the C++ extension that cannot understand rust code. I hope maybe someone inside the extension project or wasm-bindgen can give some concrete information about it.

[CryZe]

Yeah, calling methods and stuff almost definitely won't work, but local variables and their fields, and breakpoints are all things that should work and last I tested DO work... if you don't use wasm-bindgen. So that still seems like a strong indication that the dwarf is broken.

[stan-irl]

@CryZe did breakpoints work for you?? The devtools find my rust code but none of the standard library or cargo packages. They are all prefixed like /cargo/... or /rustc/...

[RReverser]

but none of the standard library or cargo packages. They are all prefixed like /cargo/... or /rustc/...

It's expected and happens because standard library in Rust is prebuilt on CI with those paths hardcoded. You can remap them to either github or your local rust-src paths via the extension settings.

But, either way, the breakpoints won't work at the moment because, as mentioned above, DWARF remapping is not implemented in wasm-bindgen.

[stan-irl]

got it! thankyou for the quick response 🙏

[RReverser]

But, either way, the breakpoints won't work at the moment because, as mentioned above, DWARF remapping is not implemented in wasm-bindgen.

Should probably clarify that this is only if you use wasm-bindgen. If you use raw Wasm, it will work (but that's uncommon on the web).

[stan-irl]

ah i got. because wasm-bindgen is actually changing the wasm when it generates the bindings but its not changing the DWARF so nothing matches up.

Regular wasm doesnt do that so its not a problem

[71]

Note: with a few changes, I managed to successfully step through wasm-bindgen'd Rust code in Chrome using @nokotan's fork of walrus. See rustwasm/walrus#231 (comment).

However, that fix is very much non-production ready. As I mentioned in the comment linked above, it requires several forks of existing libraries, including wit-walrus which is not maintained anymore (since WIT no longer seem active). wasm-bindgen-cli-support would need a lot of work to drop the dependencies on WIT-related crates, so for now that dependency needs to stay. Once that's done, we'd need to make sure the PR I linked above would be merged. Since DWARF doesn't seem to work well with Chrome (at least on Windows), we'd also need make our own version of wasm-sourcemap.py (since a Python dependency on top of Rust and JS would be a bit much).

Note: I'd like to get more involved with wasm-bindgen (and its dependencies), so if any maintainer sees this and can point me to tasks that would help make this possible, please do!