"too many locals" on rustc generated code

[osa1]

I don't expect this to be fixed, and I understand that the Wasm spec allows implementations to have their own limits, but I thought it would be good to document this at least, in case others come across the same issue.

Curently rustc can generate more locals than wasmparser supports in debug mode, making it difficult to quickly recompile and debug Wasm apps when using wasm-pack.

wasm-pack uses wasm-bindgen-cli, which uses walrus for some Wasm transformations (no idea why?), which uses wasmparser to decode the rustc-generated Wasm binaries.

Note that binaryen's wasm-opt can decode the same file.

To repro:

• Clone github.com/fir-lang/fir
• Build with cargo build --lib --target=wasm32-unknown-unknown
• Try to decode the generated file with wasmparser

You'll see that it hits this line:

wasm-tools/crates/wasmparser/src/readers/core/code.rs

Line 117 in bc0a785

None => bail!(self.reader.original_position(), "too many locals"),

I suspect the function in question is probably a lalrpop-generated code, I can't imagine anything else generating so many locals in debug mode without inlining.

[alexcrichton]

The limits in wasmparser are intended to match the limits found in other implementations like web browsers, so if it doesn't load in wasmparser it's unlikely to load in other runtimes (or at least that's the intention). Having an opt-in way to not apply limits I think would be reasonable though.