Wast encoder

[ahuoguo]

Hi! I'm trying to get rid of the named labels, custom sections, inline export, and folded exprs for all the spec test files as our interpreter only deals with plain wat files with numbers. It looks like the wast crate already provides support for parsing and a representation for Wast files, but there's only encoding to binary for QuoteWat. It will be ideal if the Wast can be prettyprinted in this case but it doesn't seem to be supported.

Related Issue #327

[alexcrichton]

Would something like wasm-tools json-from-wast fit your use case? That converts to *.wasm but that could be converted back to text with wasm-tools print. Although now that I type this out I realize that wasm-tools print will still print $foo names, but a flag could possibly be added to skip that.

Or perhaps put another way: would you prefer to keep parsing the text format as opposed to parsing the binary format? If you'd prefer binary then wasm-tools json-from-wast might be a good fit?

[ahuoguo]

We thought about that. This means we should write a parse the JSON file, load the corresponding wasm, and perform the test.

We thought it would be better if we can just manipulate the wast file directly. Like in the wasm reference interpreter, it has ./wasm -d memory.wast -o memory.bin.wast that encodes the module in place into its binary format (but we want to replace the the binary format to plain wat).

[alexcrichton]

That's true yeah, using json-from-wast would require parsing JSON.

Otherwise though there's no equivalent to what the reference is doing here in the wast crate. If you'd like though I don't think it would be unreasonable to add support for that to the subcommands of wasm-tools. For example it might be reasonable to add *.wast support to wasm-tools print and then add a flag to print either binary modules (e.g. *.bin.wast) or avoid printing all names (which I think is what you want here?)

[ahuoguo]

Yep! That's what we have in mind.

But to get wasm-tools printworking on wast. There should be some pretty printing on the WastDirective struct which I think only supports parsing for now. This should work somewhat like JsonBuilder in json-from-wast.

[alexcrichton]

I'm not sure where this logic would necessarily best live, but I think starting in print.rs for the print subcommand might be reasonable? That could do some detection if the input is actually *.wast and delegate internally to a different printing routine to implement the pretty-printing. That would complicate the parsing from what it currently is, but that's ok.