wasmtime: Surprisingly slow module compilation

Thanks for opening a bug report! Please answer the questions below if they’re relevant and delete this text before submitting.

• What are the steps to reproduce the issue?

I have pushed a repository at https://github.com/wycats/wand/tree/str-wasmtime. The branch str-wasmtime is self-contained.

The script ./script/all.sh runs all of the build steps. The final step, ./script/run.sh, runs an executable, which is a Rust executable that uses the wasmtime embedding API. Its source is located in ./crates/wand-cli. The Rust source for the wasm is located in ./src.

The build scripts use wasm-pack to compile and optimize the wasm. They copy the .wasm file into a data directory in the wand-cli crate, which uses include_bytes! to get the bytes.

The executable has a bunch of println!s that list out the execution steps and how much time has elapsed.

• What do you expect to happen? What does actually happen? Does it panic, and if so, with which assertion?

I expect the Module::new step to be relatively fast. Instead, it takes several seconds on my machine.

• Which Wasmtime version / commit hash / branch are you using?

0.22.

• If relevant, can you include some extra information about your environment? (Rust version, operating system, architecture…)

$ rustc --version
rustc 1.52.0-nightly (d1206f950 2021-02-15)
$ cargo --version
cargo 1.51.0-nightly (ab64d1393 2021-02-10)
$ uname -a
Linux XPS-13-2019 4.19.84-microsoft-standard #1 SMP Wed Nov 13 11:44:37 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 20.04.2 LTS
Release:        20.04
Codename:       focal

I’m on Ubuntu 20.04 inside of WSL2 on Windows 10.