Metal Rules for Bazel

Provides the following rules for organizing Metal builds:

• metal_library for reusable targets, akin to cc_library. Can be depended on by metal_binary and metal_library targets. Can include .h, .hpp headers and .metal sources.
• metal_binary for producing a final .metallib file to be loaded at runtime. Build structure-wise, it is akin to cc_binary, the root of a Metal dependency graph. Can depend on metal_library. Can include .h, .hpp headers and .metal sources.

Example

# Produces the final executable
cc_binary(
    name = "app",
    srcs = [
        "vertex_types.h",
        "main1.cc",
    ],
    # Bundles shaders.metallib into a runfile for the executable
    data = [":shaders"],
)

# Produces shaders.metallib
metal_binary(
    name = "shaders",
    srcs = [
        "vertex_function.metal",
        "fragment_function.metal",
    ],
    deps = [":vertex_types"],
)

# A Metal library target usable by other Metal targets
metal_library(
    name = "vertex_types",
    hdrs = ["vertex_types.h"],
)