__stdout

cargo build --target=wasm32-unknown-unknown --release
    Finished release [optimized] target(s) in 0.0 secscp target/wasm32-unknown-unknown/release/semver_steve.wasm .
ls -lah *wasm
-rw-r--r--  1 pascal  staff    78K Nov 29 17:42 semver_steve.wasm
wasm-opt -Oz semver_steve.wasm -o semver_steve.wasm
ls -lah *wasm
-rw-r--r--  1 pascal  staff    73K Nov 29 17:42 semver_steve.wasm
wasm-gc semver_steve.wasm semver_steve.wasm
ls -lah *wasm
-rw-r--r--  1 pascal  staff    59K Nov 29 17:42 semver_steve.wasm

Cargo.lock

[[package]]
name = "semver"
version = "0.9.0"
source = "registry+https://github.com/rust-lang/crates.io-index"
dependencies = [
 "semver-parser 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)",
]

[[package]]
name = "semver-parser"
version = "0.7.0"
source = "registry+https://github.com/rust-lang/crates.io-index"

[[package]]
name = "semver-steve"
version = "0.1.0"
dependencies = [
 "semver 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)",
]

[metadata]
"checksum semver 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)" = "1d7eb9ef2c18661902cc47e535f9bc51b78acd254da71d375c2f6720d9a40403"
"checksum semver-parser 0.7.0 (registry+https://github.com/rust-lang/crates.io-index)" = "388a1df253eca08550bef6c72392cfe7c30914bf41df5269b68cbd6ff8f570a3"

Cargo.toml

[package]
authors = ["Pascal Hertleif <killercup@gmail.com>"]
name = "semver-steve"
version = "0.1.0"
homepage = "https://www.steveklabnik.com/wasm/demos/semver.html"

[lib]
path = "lib.rs"
crate-type = ["cdylib"]

[dependencies]
semver = "0.9.0"

[profile.release]
opt-level = "s"

lib.rs

extern crate semver;

use semver::Version;
use semver::VersionReq;

use std::mem;
use std::ffi::CStr;
use std::os::raw::{c_char, c_void};

macro_rules! check {
    ($expr:expr) => (match $expr {
        Ok(val) => val,
        Err(_) => return false,
    })
}

#[no_mangle]
pub extern fn is_match(v: *mut c_char, r: *mut c_char) -> bool {
    unsafe {
        let v = CStr::from_ptr(v);
        let v = check!(v.to_str());

        let r = CStr::from_ptr(r);
        let r = check!(r.to_str());
        
        let v = check!(Version::parse(v));

        let r = check!(VersionReq::parse(r));

        r.matches(&v)
    }
}

// this shenanigans is due to it being tough to deal with JS strings into wasm,
// see the JS code for more.

#[no_mangle]
pub extern "C" fn alloc(size: usize) -> *mut c_void {
    let mut buf = Vec::with_capacity(size);
    let ptr = buf.as_mut_ptr();
    mem::forget(buf); // This is JS' responsibility now
    return ptr as *mut c_void;
}

#[no_mangle]
pub extern "C" fn dealloc(ptr: *mut c_void, cap: usize) {
    unsafe  {
        let _buf = Vec::from_raw_parts(ptr, 0, cap);
    }
}

#[cfg(test)]
mod tests {
    use std::ffi::CString;
    use super::is_match;

    #[test]
    fn matches() {
        // these are going to leak but it's two tests so I don't care
        let v = CString::new("1.0.0").unwrap().into_raw();
        let r = CString::new(">= 1.0.0").unwrap().into_raw();

        assert!(is_match(v, r));
    }

    #[test]
    fn does_not_match() {
        // these are going to leak but it's two tests so I don't care
        let v = CString::new("0.1.0").unwrap().into_raw();
        let r = CString::new(">= 1.0.0").unwrap().into_raw();

        assert!(!is_match(v, r));
    }
}

Makefile

build:
	cargo build --target=wasm32-unknown-unknown --release
	cp target/wasm32-unknown-unknown/release/semver_steve.wasm .
	ls -lah *wasm
	wasm-opt -Oz semver_steve.wasm -o semver_steve.wasm
	ls -lah *wasm
	wasm-gc semver_steve.wasm semver_steve.wasm
	ls -lah *wasm