Add cross compilation

Cargo.toml

@@ -14,6 +14,7 @@ rix-eval = { path = "rix-eval" }
 usage = { package = "usage-rs", version = "6.9.1" }
 thiserror = "2.0.20"
 rootcause = "0.13.0"
+annotate-snippets = "0.12.16"
 rowan = "0.17.0"
 serde = { version = "1.0.229", features = ["derive"] }
 serde_json = "1.0.151"
@@ -35,6 +36,7 @@ reqwest = { version = "0.13", default-features = false, features = ["blocking",
 flate2 = { version = "1.1.10", features = ["zlib-rs"], default-features = false }
 tar = "0.4"
 colored = "2.2"
+parking_lot = "0.12"
 
 [profile.dev.package."*"]
 opt-level = 3

README.md

@@ -8,3 +8,46 @@ This project is based on:
 
 - [Tom's nix-eval crate](https://github.com/Industrial/nixos-dotfiles/tree/main/rust/tools/nix-eval)
 - [rnix parser](https://github.com/nix-community/rnix-parser)
+
+## Cross compilation (work in progress)
+
+Rix tracks two platform triples (a `cpu-os` pair such as `x86_64-linux` or
+`x86_64-darwin`) while building:
+
+- the *build* platform performs the build (the machine running `rix`), and
+- the *host* platform runs the produced package.
+
+They are configured with global flags:
+
+```console
+$ rix build -A xnu --system x86_64-linux --cross-system x86_64-darwin
+```
+
+- `--system` sets the build platform and what `builtins.currentSystem`
+  reports. It defaults to the running machine.
+- `--cross-system` sets the host platform. When it differs from the build
+  platform the build is a cross build.
+- `--runner` gives the command used to execute host-platform binaries during
+  the build (for example `--runner darling` for Darwin binaries on Linux, or
+  `--runner wine` for Windows).
+
+The platforms are exposed to build scripts as `$buildPlatform`,
+`$hostPlatform` and `$system` (the host platform), and to Nix expressions as
+
+- `builtins.currentSystem` / `builtins.currentBuildPlatform`,
+- `builtins.currentHostPlatform`, and
+- `builtins.isCrossCompiling`.
+
+When a runner is configured, Rix installs a `rix-run-host` helper on `PATH`
+(its path is also exported as `$RIX_TARGET_RUNNER`) that executes a
+host-platform binary through the runner:
+
+```sh
+rix-run-host ./target-binary --some-flag
+```
+
+Without an explicit runner, same-OS foreign-CPU builds (for example
+`x86_64-linux` -> `aarch64-linux`) fall back to a `qemu-<arch>-static` style
+emulator, and other combinations only warn that host binaries cannot run.
+Rix deliberately does not guess a runner from `PATH` so that a misconfigured
+cross build fails loudly rather than silently doing the wrong thing.

rix-cli/Cargo.toml

@@ -14,3 +14,4 @@ rix-eval.workspace = true
 serde_json.workspace = true
 colored.workspace = true
 rootcause.workspace = true
+annotate-snippets.workspace = true