Improve error messages for the asserts in the various tools

[jsturtevant]

I mis-configured wit-bingen macro and got the error message:

proc macro panicked
message: assertion failed: prev.is_none()

When trying to debug it I tried using cargo expand and cargo docs to track down what I did wrong but the gave the same error.

After some digging and forking the various repositories, I eventually tracked the error message to this repo which has a bunch of asserts: https://github.com/search?q=repo%3Abytecodealliance%2Fwasm-tools%20%20prev.is_none&type=code

As a developer it would be nice to have a better error message for these asserts to improve the debugging experience.

[alexcrichton]

In theory asserts should never happen and they're only intended for developers of the tool itself, any other error should be signaled through a Result or some sort of other first-class mechanism. In that sense I think the "true bug" here lies in whatever configuration you fed to wit-bindgen and it panicked. The macro itself should never panic ideally.

Although naturally internal assertions do trip (e.g. this) and I don't disagree that having better error messages would be helpful. That being said though I wouldn't want to improve things to expect all users to see them because the true bug is having this trip at all.

[jsturtevant]

The mis-configuration came from using the macro in a way that was intended for testing purposes only (https://docs.rs/wit-bindgen/latest/wit_bindgen/macro.generate.html#options-to-generate)

// Usually used in testing, our test suite may want to generate code
// from wit files located in multiple paths within a single mod, and we
// don't want to copy these files again.

The configuration was bringing in two references to the wasi-io:

wit_bindgen::generate!({
    inline: "
        package wasmtime:test;

        world test {
            include wasi:cli/[email protected];
            include wasi:http/[email protected];
            include wasi:config/[email protected];
            include wasi:keyvalue/[email protected];
            include wasi:tls/[email protected];  #added line
        }
    ",
    path: [
        "../wasi-http/wit", // this also has a deps folder to io
        "../wasi-config/wit",
        "../wasi-keyvalue/wit",
        "../wasi-tls/wit/",  // this has a deps folder to io
    ],
    world: "wasmtime:test/test",
    features: ["cli-exit-with-code", "tls"],
    generate_all,
});

The two wasi-io deps collided with the error "prev.is_none()`. Technically I guess this is an error and not an assert?

[alexcrichton]

Oh dear that looks like something that should be expected to work. That documentation is a bit outdated and the array-of-paths situation is a bit more expected to work nowadays as well.

Would you be able to make a reproduction? If it's still using wit-bindgen that's ok, I can work on reducing it as a reproduction for this repository.

[jsturtevant]

I believe this is a simplified reproduction: https://github.com/jsturtevant/wasi-cli/tree/prev-is-none

I am getting:

cargo build 
  Compiling wit-bindgen-rust-macro v0.39.0
   Compiling wit-bindgen v0.39.0
   Compiling path-repo v0.1.0 (/home/jstur/projects/wit/wasi-cli)
error: proc macro panicked
  --> src/main.rs:2:5
   |
2  | /     wit_bindgen::generate!({
3  | |         inline: "
4  | |             package wasmtime:test;
...  |
17 | |         generate_all,
18 | |     });
   | |______^
   |
   = help: message: assertion failed: prev.is_none()

I wasn't able to figure out how to load multiple deps via just the wasm-tools component wit command