Strong coupling between flake systems and bazel platforms

[layus]

Using flakes has a lot of potential. Within a flake, we can list the set of available packages, packages paths are prefixed with the target system, and evaluation can be cached.

With rules_nixpkgs, we could have:

## MODULES.bazel

nix_repo = use_extension("@rules_nixpkgs_core//extensions:repository.bzl", "nix_repo")
nix_repo.flake(
	path = "build/nix",
	prefix = "build.nix.",
	platform_mappings = {
		"x86_64-linux.buildPackages": [exec transition constraints],
		"x86_64-linux": [list of platform constraints],
		"aarch64-linux": [list of platform constraints],
	},
) # use a subfolder as the whole flake


nix_pkg = use_extension("@rules_nixpkgs_core//extensions:package.bzl", "nix_pkg")
nix_pkg.attr(attr = "jq", toolchains = ["jq"], )

use_repo(nix_repo, "use_nix_deps")
use_nix_deps() # unwraps to
# nix_pkg = use_extension("@rules_nixpkgs_core//extensions:package.bzl", "nix_pkg")
# nix_pkg.attr(attr = "jq", name = "build.nix.jq") # Unwraps to
# # build.nix.jq repo with select w.r.t platform constraints
# # build.nix.x86_64-linux.jq repo
# # build.nix.aarch64-linux.jq repo
# # register toolchains with correct platform info
# nix_pkg.attr(attr = "gawk", name = "build.nix.gawk") # idem
# use_repo(nix_pkg, "build.nix.jq", "build.nix.gawk", ...)

## BUILD.bazel
 
some_rule(deps = ["@build.nix.jq"])

## somewhere.bzl

another_rule = rule(
	...
	toolchains = ["@build.nix.jq://toolchains/jq:toolchain_type"],
)