Expand cross compilation docs and add wasi example

Also refactored current examples.

.gitignore

@@ -1,3 +1,4 @@
+target
 result
 result-*
 *.tmp

docs/cross_compilation.md

@@ -1,4 +1,60 @@
 # Cross compilation with `rust-overlay`
 
-There an example for cross compilation to `aarch64-linux` in
-[`example/cross-aarch64`](../examples/cross-aarch64).
+There are examples for cross compilation in [`example` directory](../examples).
+To try examples,
+1. `cd` into `example/cross-aarch64` (or other directory).
+2. `nix-shell` to enter the development environment.
+3. `make run` to build and run the program in an emulater.
+
+The structure of `shell.nix` should like this,
+```nix
+# Import from global `<nixpkgs>`. Can also be from flake input if you like.
+(import <nixpkgs> {
+  # Target triple in nixpkgs format, can be abbreviated.
+  # Some special targets, like `wasm32-wasi`, need special configurations here, check
+  # `examples/cross-wasi/shell.nix` for details.
+  crossSystem = "aarch64-linux";
+  # Apply `rust-overlay`.
+  overlays = [ (import ../..) ];
+
+# Workaround: we need `callPackage` to enable auto package-splicing, thus we don't need to manually prefix
+# everything in `nativeBuildInputs` with `buildPackages.`.
+# See: https://github.com/NixOS/nixpkgs/issues/49526
+}).callPackage (
+{ mkShell, stdenv, rust-bin, pkg-config, openssl, qemu }:
+mkShell {
+  # Build-time dependencies. build = host = your-machine, target = aarch64
+  # Typically contains,
+  # - Configure-related: cmake, pkg-config
+  # - Compiler-related: gcc, rustc, binutils
+  # - Code generators run at build time: yacc, bision
+  nativeBuildInputs = [
+    # `minimal` is enough for basic building and running.
+    # Can also be `default` to bring other components like `rustfmt`, `clippy`, and etc.
+    rust-bin.stable.latest.minimal
+    pkg-config
+  ];
+
+  # Build-time tools which are target agnostic. build = host = target = your-machine.
+  # Emulaters should essentially also go `nativeBuildInputs`. But with some packaging issue,
+  # currently it would cause some rebuild.
+  # We put them here just for a workaround.
+  # See: https://github.com/NixOS/nixpkgs/pull/146583
+  depsBuildBuild = [ qemu ];
+
+  # Run-time dependencies. build = your-matchine, host = target = aarch64
+  # Usually are libraries to be linked.
+  buildInputs = [ openssl ];
+
+  # Tell cargo about the linker and an optional emulater. So they can be used in `cargo build`
+  # and `cargo run`.
+  # Environment variables are in format `CARGO_TARGET_<UPPERCASE_UNDERSCORE_RUST_TRIPLE>_LINKER`.
+  # They are also be set in `.cargo/config.toml` instead.
+  # See: https://doc.rust-lang.org/cargo/reference/config.html#target
+  CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER = "${stdenv.cc.targetPrefix}cc";
+  CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_RUNNER = "qemu-aarch64";
+}) {}
+```
+
+For more details about these different kinds of dependencies,
+see also [Nix Wiki - Cross Compiling](https://nixos.wiki/wiki/Cross_Compiling#How_to_specify_dependencies).

examples/cross-aarch64/.cargo/config

@@ -1,6 +0,0 @@
-[build]
-target-dir = "target"
-
-[target.aarch64-unknown-linux-gnu]
-linker = "aarch64-unknown-linux-gnu-gcc"
-runner = "qemu-aarch64"

examples/cross-aarch64/Makefile

@@ -1,9 +1,11 @@
-.PHONY: all test
+# This Makefile is expected to be run inside nix-shell.
 
-test: all
-	nix-shell ./shell.nix --command \
-		"cargo test --target aarch64-unknown-linux-gnu"
+CARGO_FLAGS := --target aarch64-unknown-linux-gnu
 
+.PHONY: all
 all: Cargo.toml Cargo.lock src/main.rs
-	nix-shell ./shell.nix --command \
-		"cargo test --target aarch64-unknown-linux-gnu --no-run"
+	cargo build $(CARGO_FLAGS)
+
+.PHONY: test
+run: all
+	cargo run $(CARGO_FLAGS)

examples/cross-aarch64/shell.nix

@@ -1,22 +1,19 @@
+# See docs/cross_compilation.md for details.
 (import <nixpkgs> {
   crossSystem = "aarch64-linux";
   overlays = [ (import ../..) ];
 }).callPackage (
-{ mkShell, rust-bin, pkg-config, openssl, pkgsBuildBuild }:
+{ mkShell, stdenv, rust-bin, pkg-config, openssl, qemu }:
 mkShell {
   nativeBuildInputs = [
-    # Manual `buildPackages` is required here. See: https://github.com/NixOS/nixpkgs/issues/49526
-    # build = host = x86_64, target = aarch64
     rust-bin.stable.latest.minimal
     pkg-config
+  ];
 
-    # build = host = target = x86_64
-    # qemu itself is multi-platform and `target` doesn't matter for it.
-    # Use build system's to avoid rebuild.
-    pkgsBuildBuild.qemu
-  ];
-  buildInputs = [
-    # build = x86_64, host = target = aarch64
-    openssl
-  ];
+  depsBuildBuild = [ qemu ];
+
+  buildInputs = [ openssl ];
+
+  CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_LINKER = "${stdenv.cc.targetPrefix}cc";
+  CARGO_TARGET_AARCH64_UNKNOWN_LINUX_GNU_RUNNER = "qemu-aarch64";
 }) {}

examples/cross-aarch64/src/main.rs

@@ -1,7 +1,6 @@
-#[test]
-fn test_openssl_version() {
+fn main() {
+    // Test linking against OpenSSL.
     openssl::init();
     assert!(openssl::version::version().starts_with("OpenSSL "));
+    println!("Hello, world!");
 }
-
-fn main() {}

examples/cross-wasi/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 3
+
+[[package]]
+name = "cross-wasi"
+version = "0.1.0"

examples/cross-wasi/Cargo.toml

@@ -0,0 +1,8 @@
+[package]
+name = "cross-wasi"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]

examples/cross-wasi/Makefile

@@ -0,0 +1,11 @@
+# This Makefile is expected to be run inside nix-shell.
+
+CARGO_FLAGS := --target wasm32-wasi
+
+.PHONY: all
+all: Cargo.toml Cargo.lock src/main.rs
+	cargo build $(CARGO_FLAGS)
+
+.PHONY: test
+run: all
+	cargo run $(CARGO_FLAGS)

examples/cross-wasi/shell.nix

@@ -0,0 +1,21 @@
+# See docs/cross_compilation.md for details.
+(import <nixpkgs> {
+  crossSystem = {
+    config = "wasm32-wasi";
+    # Nixpkgs currently only supports LLVM lld linker for wasm32-wasi.
+    useLLVM = true;
+  };
+  overlays = [ (import ../..) ];
+}).callPackage (
+{ mkShell, stdenv, rust-bin, wasmtime }:
+mkShell {
+  nativeBuildInputs = [ rust-bin.stable.latest.minimal ];
+
+  depsBuildBuild = [ wasmtime ];
+
+  # This is optional for wasm32-like targets, since rustc will automatically use
+  # the bundled `lld` for linking.
+  # CARGO_TARGET_WASM32_WASI_LINKER = "${stdenv.cc.targetPrefix}cc";
+  CARGO_TARGET_WASM32_WASI_RUNNER = "wasmtime";
+}) {}
+

examples/cross-wasi/src/main.rs

@@ -0,0 +1,3 @@
+fn main() {
+    println!("Hello, world!");
+}