feat: add a dedicated Metal backend

crates/khal-std/Cargo.toml

@@ -25,7 +25,7 @@ cuda = []
 unsafe_remove_boundchecks = []
 
 [dependencies]
-glamx = { version = "0.2", default-features = false, features = ["nostd-libm", "bytemuck"] }
+glamx = { version = "0.3", default-features = false, features = ["nostd-libm", "bytemuck", "u32", "i32", "f64"] }
 rayon = { version = "1", optional = true }
 corosensei = { version = "0.3", optional = true }
 spirv-std-macros = "0.10.0-alpha.1"

crates/khal-std/src/build_script.rs

@@ -0,0 +1,40 @@
+//! Build-script helpers for shader crates.
+//!
+//! Host-only module (gated out of GPU-target builds). Intended to be called
+//! from a shader crate's `build.rs` — see [`setup_shader_crate_build`].
+
+/// Standard `build.rs` setup that every shader crate should run.
+///
+/// Does three things:
+///
+/// 1. Emits `cargo::metadata=manifest_dir=<path>` so that host crates
+///    consuming this shader crate via `KhalBuilder::from_dependency`
+///    discover the shader sources both in-workspace and from
+///    `crates.io`-fetched copies.
+/// 2. Declares the `target_arch_is_gpu` cfg via `cargo::rustc-check-cfg`
+///    so `#[cfg(target_arch_is_gpu)]` / `#[cfg(not(target_arch_is_gpu))]`
+///    don't trip the `unexpected_cfgs` lint.
+/// 3. Sets `target_arch_is_gpu` when compiling for any GPU target
+///    (SPIR-V, NVPTX). The host CPU build sees it unset.
+///
+/// Call from `build.rs`:
+///
+/// ```no_run
+/// khal_std::build_script::setup_shader_crate_build();
+/// ```
+///
+/// The shader crate must list `khal-std` as a `[build-dependencies]` entry
+/// (in addition to its regular `[dependencies]` use).
+pub fn setup_shader_crate_build() {
+    let manifest_dir =
+        std::env::var("CARGO_MANIFEST_DIR").expect("CARGO_MANIFEST_DIR not set by cargo");
+    println!("cargo::metadata=manifest_dir={manifest_dir}");
+    println!("cargo:rerun-if-changed=build.rs");
+
+    println!("cargo::rustc-check-cfg=cfg(target_arch_is_gpu)");
+
+    let target_arch = std::env::var("CARGO_CFG_TARGET_ARCH").unwrap_or_default();
+    if matches!(target_arch.as_str(), "spirv" | "nvptx64") {
+        println!("cargo::rustc-cfg=target_arch_is_gpu");
+    }
+}

crates/khal-std/src/lib.rs

@@ -23,6 +23,12 @@ pub mod num_traits;
 /// Synchronization primitives (barriers, atomics).
 pub mod sync;
 
+/// Build-script helpers for shader crates. Host-only.
+#[cfg(not(any(target_arch = "spirv", target_arch = "nvptx64")))]
+pub mod build_script;
+#[cfg(not(any(target_arch = "spirv", target_arch = "nvptx64")))]
+pub use build_script::*;
+
 /// Re-export of the `glamx` math library.
 pub use glamx;
 

crates/khal/Cargo.toml

@@ -13,6 +13,7 @@ derive = ["khal-derive"]
 webgpu = ["khal-derive?/webgpu"]
 cpu = ["khal-derive?/cpu"]
 cuda = ["dep:cudarc", "khal-derive?/cuda"]
+metal = ["dep:metal", "dep:naga", "khal-derive?/webgpu"]
 push_constants = []
 subgroup_ops = []
 
@@ -34,4 +35,8 @@ regex = "1"
 bitflags = "2"
 
 # For test_shader_compilation
-paste = "1"
+paste = "1"
+
+[target.'cfg(target_os = "macos")'.dependencies]
+metal = { version = "0.32", optional = true }
+naga = { version = "29", optional = true, features = ["spv-in", "msl-out"] }