WebAssembly Base Project

build.rs

fn main() {
    if matches!(std::env::var("CARGO_CFG_TARGET_FAMILY").ok().as_deref(), Some("wasm")) {
        std::env::set_var("CARGO_ENCODED_RUSTFLAGS", "-Clink-arg=-zstack-size=65536\x1f-Clink-arg=--import-memory\x1f-Ctarget-cpu=mvp");
    }
}

build.sh

#!/bin/bash
set -e

####################################################################
# STEP 1: Build the project with the wasm32-unknown-unknown target #
####################################################################

# Set the stack size to 64KB
STACK_SIZE=65536

# List of custom flags to pass to all compiler invocations that Cargo performs.
RUST_WASM_FLAGS=(
    # Max wasm stack size
    "-Clink-arg=-zstack-size=$STACK_SIZE"
    # Configure the wasm target to import instead of export memory
    '-Clink-arg=--import-memory'
    # Doesn't optimize this build for any specific CPU
    '-Ctarget-cpu=mvp'
)

# Separated flags by 0x1f (ASCII Unit Separator)
# Reference: https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-reads
RUST_WASM_FLAGS="$(printf '%s\x1f%s\x1f%s' ${RUST_WASM_FLAGS[@]})"
echo "$RUST_WASM_FLAGS"

# Build the project with the wasm32-unknown-unknown target
CARGO_ENCODED_RUSTFLAGS="$RUST_WASM_FLAGS" cargo build --release --target=wasm32-unknown-unknown

######################################################
# Remove unnecessary code and optimize the wasm file #
######################################################
# Run `wasm-opt --help` to see all available options
WASM_OPT_OPTIONS=(
    -O3
    --dce
    --precompute
    --precompute-propagate
    --optimize-instructions
    --optimize-casts
    --low-memory-unused
    --optimize-added-constants
    --optimize-added-constants-propagate
    --simplify-globals-optimizing
    --inlining-optimizing
    --once-reduction
    --merge-locals
    --merge-similar-functions
    --strip
    --strip-debug
    # --remove-memory
    --remove-unused-names
    --remove-unused-types
    --remove-unused-module-elements
    --duplicate-function-elimination
    --duplicate-import-elimination
    --reorder-functions
    --abstract-type-refining
    --alignment-lowering
    --avoid-reinterprets
    # --zero-filled-memory
    --disable-simd
    --disable-threads
    --disable-gc
    --disable-multivalue
    --disable-reference-types
    --disable-exception-handling
    --optimize-stack-ir
    --vacuum
    --unsubtyping
)

# Create the `optimized.wasm` file (binary format)
wasm-opt \
    "${WASM_OPT_OPTIONS[@]}" \
    --output ./optimized.wasm \
    ./target/wasm32-unknown-unknown/release/rust_wasm_minimal.wasm

# Create the `optimized.wat` file (text format)
wasm-opt \
    "${WASM_OPT_OPTIONS[@]}" \
    --emit-text \
    --output ./optimized.wat \
    ./target/wasm32-unknown-unknown/release/rust_wasm_minimal.wasm

# Print the `optimized.wat` file
cat ./optimized.wat

Cargo.toml

[package]
name = "rust-wasm-minimal"
version = "0.1.0"
edition = "2021"

[lib]
name = "rust_wasm_minimal"
crate-type = ["cdylib"]

[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
rand = { version = "0.8.4" }

[profile.release]
opt-level = 3
lto = "fat"
codegen-units = 1
panic = "abort"
strip = "symbols"
debug = false
overflow-checks = false
debug-assertions = false
incremental = false

lib.rs

// Disable `std` library and `main` entrypoint, because it's not available in WebAssembly.
// OBS: `std` and `main` are only available when running tests.
#![cfg_attr(all(target_arch = "wasm32", not(test)), no_std, no_main)]

// Override the default panic handler when compilling to WebAssembly.
// Reference: https://doc.rust-lang.org/nomicon/panic-handler.html
#[cfg(target_arch = "wasm32")]
#[panic_handler]
unsafe fn panic(_info: &core::panic::PanicInfo) -> ! {
    core::arch::wasm32::unreachable()
}

/// Adds two numbers.
#[no_mangle]
pub const extern "C" fn add(a: u32, b: u32) -> u32 {
    a + b
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn it_works() {
        let result = add(2, 2);
        assert_eq!(result, 4);
    }
}