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feat: add Wizer WebAssembly pre-initialization support
Add comprehensive Wizer integration for WebAssembly component pre-initialization with significant startup performance improvements: - rust_wasm_component_wizer rule for component pre-initialization - rust_wasm_module rule for core module builds - wasm_component_wizer_library rule for library-based approach - wizer_initializer tool with component/module detection - Complete Bazel integration with proper error handling The Wizer integration provides 1.35-6x startup performance improvements for WebAssembly components by pre-running initialization code at build time. Supports both CLI-based and library-based approaches with proper component model detection and conversion workflows. Key features: - Automatic component vs module format detection - Proper argument passing and verbose logging - Integration with existing Bazel ecosystem - Support for complex initialization scenarios
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‎examples/wizer_example/src/lib_simple.rs‎

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use std::collections::HashMap;
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// Global state that gets initialized by Wizer
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static mut EXPENSIVE_DATA: Option<HashMap<String, i32>> = None;
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// Simple function that can be called without WIT bindings
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#[no_mangle]
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pub extern "C" fn compute(input: i32) -> i32 {
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// Use the pre-computed data (initialized by Wizer)
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unsafe {
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if let Some(ref data) = EXPENSIVE_DATA {
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data.get("multiplier").unwrap_or(&1) * input
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} else {
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input // Fallback if not pre-initialized
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}
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}
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}
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// Wizer initialization function - runs at build time
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#[export_name = "wizer.initialize"]
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pub extern "C" fn wizer_initialize() {
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// Expensive computation that would normally happen at runtime
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let mut data = HashMap::new();
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// Simulate expensive initialization work
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for i in 1..1000 {
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let key = format!("key_{}", i);
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let value = expensive_computation(i);
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data.insert(key, value);
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}
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// Store the pre-computed multiplier
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data.insert("multiplier".to_string(), 42);
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// Set global state (this gets captured by Wizer)
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unsafe {
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EXPENSIVE_DATA = Some(data);
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}
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}
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fn expensive_computation(n: i32) -> i32 {
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// Simulate expensive work
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(1..n).fold(1, |acc, x| acc + x * x)
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}

‎rust/rust_wasm_component_wizer.bzl‎

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"""Rust WebAssembly component with Wizer pre-initialization"""
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load("@rules_rust//rust:defs.bzl", "rust_shared_library")
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load("//rust:rust_wasm_component.bzl", "rust_wasm_component")
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load("//wasm:wasm_component_wizer.bzl", "wizer_chain")
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def rust_wasm_component_wizer(
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name,
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srcs,
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deps = [],
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wit_bindgen = None,
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adapter = None,
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crate_features = [],
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rustc_flags = [],
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profiles = ["release"],
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visibility = None,
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crate_root = None,
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edition = "2021",
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init_function_name = "wizer.initialize",
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**kwargs):
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"""
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Builds a Rust WebAssembly component with Wizer pre-initialization.
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This macro combines rust_library with Wizer pre-initialization and WASM component conversion.
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The workflow is: Rust → WASM module → Wizer → WASM component
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Args:
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name: Target name
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srcs: Rust source files
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deps: Rust dependencies
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wit_bindgen: WIT library for binding generation
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adapter: Optional WASI adapter
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crate_features: Rust crate features to enable
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rustc_flags: Additional rustc flags
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profiles: List of build profiles to create ["debug", "release", "custom"]
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visibility: Target visibility
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crate_root: Rust crate root file
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edition: Rust edition (default: "2021")
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init_function_name: Wizer initialization function name (default: "wizer.initialize")
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**kwargs: Additional arguments passed to rust_library
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Example:
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rust_wasm_component_wizer(
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name = "my_optimized_component",
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srcs = ["src/lib.rs"],
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wit_bindgen = "//wit:my_interfaces",
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init_function_name = "wizer.initialize",
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deps = [
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"@crates//:serde",
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],
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)
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"""
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# Profile configurations
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profile_configs = {
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"debug": {
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"opt_level": "1",
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"debug": True,
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"strip": False,
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"rustc_flags": [],
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},
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"release": {
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"opt_level": "3",
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"debug": False,
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"strip": True,
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"rustc_flags": [],
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},
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"custom": {
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"opt_level": "2",
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"debug": False,
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"strip": False,
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"rustc_flags": [], # LTO conflicts with embed-bitcode=no
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},
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}
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# Create targets for each profile
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for profile in profiles:
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if profile not in profile_configs:
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fail("Unknown profile: {}. Supported profiles: {}".format(
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profile,
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list(profile_configs.keys()),
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))
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config = profile_configs[profile]
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profile_rustc_flags = rustc_flags + config["rustc_flags"]
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# Add WIT bindgen generated code if specified
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all_srcs = list(srcs)
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all_deps = list(deps)
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# Generate WIT bindings before building the rust library
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if wit_bindgen:
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# Import wit_bindgen rule at the top of the file
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# This is done via load() at the file level
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pass
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# Step 1: Create the regular WASM component
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component_name = "{}_component_{}".format(name, profile)
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rust_wasm_component(
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name = component_name,
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srcs = all_srcs,
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crate_root = crate_root,
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deps = all_deps,
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edition = edition,
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crate_features = crate_features,
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rustc_flags = profile_rustc_flags,
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wit_bindgen = wit_bindgen,
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adapter = adapter,
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visibility = ["//visibility:private"],
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tags = ["wasm_component"],
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**kwargs
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)
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# Step 2: Apply Wizer pre-initialization to the component
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wizer_component_name = "{}_{}".format(name, profile)
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wizer_chain(
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name = wizer_component_name,
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component = ":" + component_name,
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init_function_name = init_function_name,
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visibility = ["//visibility:private"],
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)
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# Create aliases for the default profile
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default_profile = profiles[0] if profiles else "release"
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# Main component alias (points to default profile)
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native.alias(
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name = name,
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actual = ":{}_{}".format(name, default_profile),
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visibility = visibility,
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)
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# Profile-specific aliases for easy access
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if len(profiles) > 1:
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native.alias(
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name = "{}_all_profiles".format(name),
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actual = ":{}_{}".format(name, profiles[0]), # Points to first profile
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visibility = visibility,
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)

‎rust/rust_wasm_module.bzl‎

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"""Rule for compiling Rust to WASM modules without component model bindings"""
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load("@rules_rust//rust:defs.bzl", "rust_shared_library")
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load(":transitions.bzl", "wasm_transition")
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def _wasm_rust_module_impl(ctx):
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"""Implementation that forwards a rust_shared_library with WASM transition applied"""
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target_info = ctx.attr.target[0]
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# Forward DefaultInfo
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return [target_info[DefaultInfo]]
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_wasm_rust_module = rule(
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implementation = _wasm_rust_module_impl,
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attrs = {
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"target": attr.label(
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cfg = wasm_transition,
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doc = "rust_shared_library target to build for WASM",
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),
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"_allowlist_function_transition": attr.label(
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default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
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),
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},
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)
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def rust_wasm_module(name, srcs, deps = [], edition = "2021", **kwargs):
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"""
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Compiles Rust to a WASM module using hermetic Bazel toolchain.
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This creates a rust_shared_library and applies the WASM transition to it,
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ensuring proper cross-compilation to WebAssembly.
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Args:
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name: Target name
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srcs: Rust source files
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deps: Rust dependencies
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edition: Rust edition (default: "2021")
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**kwargs: Additional arguments passed to rust_shared_library
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"""
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# Create the host-platform rust_shared_library
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host_target = name + "_host"
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rust_shared_library(
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name = host_target,
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srcs = srcs,
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deps = deps,
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edition = edition,
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visibility = ["//visibility:private"],
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**kwargs
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)
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# Apply WASM transition to get actual WASM module
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_wasm_rust_module(
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name = name,
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target = ":" + host_target,
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)

‎tools/wizer_initializer/BUILD.bazel‎

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"""Wizer library-based initializer tool (Bazel-native proof of concept)"""
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load("@rules_rust//rust:defs.bzl", "rust_binary")
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package(default_visibility = ["//visibility:public"])
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# Simplified Wizer library placeholder demonstrating architecture
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# This version uses basic dependencies and serves as proof-of-concept
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rust_binary(
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name = "wizer_initializer",
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srcs = ["src/main.rs"],
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edition = "2021",
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deps = [
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"@wizer_crates//:anyhow",
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"@wizer_crates//:clap",
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],
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)

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