feat(build.zig): easier cross-complilation

invocation of cross-compilation changed, see news.txt blurb

BUILD.md

@@ -564,6 +564,7 @@ Note that the C++ compiler is explicitly set so that it can be found when the de
    + `zig build functionaltest` to run all functionaltests
    + `zig build functionaltest -- test/functional/autocmd/bufenter_spec.lua` to run the tests in one file
    + `zig build unittest` to run all unittests
+   + `zig build oldtest` to run all oldtests
 
 #### Using system dependencies
 
@@ -575,8 +576,19 @@ See the `prepare` function of [this `PKGBUILD`](https://git.sr.ht/~chinmay/nvim_
 
 ## Cross-compiling
 
-Cross-compilation is not supported, but we collect notes here for reference (improvements welcome).
+Cross-compilation is not fully supported, but we collect notes here for reference (improvements welcome).
 Also relevant for webassembly (WASM) build.
 
-- Set `NVIM_HOST_PRG` so that the docs and tags generation works without
+- cmake: Set `NVIM_HOST_PRG` so that the docs and tags generation works without
   depending on the target binary.
+
+- zig build: often is enabled by just setting -Dtarget, e.g. from a linux host
+
+   zig build -Dtarget=aarch64-macos
+
+  will automatically compilp a host lua for use during build. The
+  "-Dhost={target_string}" option can be used to override the platform for
+  running binaries during compile-time. use "-Dhost=native" to force
+  cross-compiling or "-Dhost=" (empty string) to assume that target binaries
+  can run on the host during the build process (e.g. if target is x86 on a
+  x86_64 system, or if emulation set up with binfmt or similar)

build.zig

@@ -58,7 +58,12 @@ pub fn build(b: *std.Build) !void {
     const modern_unix = is_darwin or os_tag.isBSD() or is_linux;
     const is_wasm = t.cpu.arch == .wasm32;
 
-    const cross_compiling = b.option(bool, "cross", "cross compile") orelse is_wasm;
+    const h = b.graph.host.result;
+    const host = b.option([]const u8, "host", "target for host ") orelse
+        if (target.query.isNative() or h.cpu.arch == t.cpu.arch and h.os.tag == t.os.tag) "" else "native";
+    const cross_compiling = host.len > 0;
+    const target_host = if (cross_compiling) b.resolveTargetQuery(try std.Build.parseTargetQuery(.{ .arch_os_abi = host })) else target;
+
     const emscripten_sysroot = b.option([]const u8, "emscripten-sysroot", "path to emscripten sysroot");
     const emscripten_include = if (emscripten_sysroot) |s|
         std.Build.LazyPath{ .cwd_relative = b.pathJoin(&.{ s, "include" }) }
@@ -77,9 +82,6 @@ pub fn build(b: *std.Build) !void {
         break :blk null;
     } else null;
 
-    // TODO(bfredl): option to set nlua0 target explicitly when cross compiling?
-    const target_host = if (cross_compiling) b.graph.host else target;
-
     // without cross_compiling we like to reuse libluv etc at the same optimize level
     const optimize_host = if (cross_compiling) .ReleaseSafe else optimize;
 
@@ -282,13 +284,12 @@ pub fn build(b: *std.Build) !void {
 
     const version_lua = gen_config.add("nvim_version.lua", lua_version_info(b));
 
-    var config_str = b.fmt("zig build -Doptimize={s}", .{@tagName(optimize)});
+    // Note: these represent the actually resolved target and host platforms, which are often
+    // more verbose than what needs to be passed in (often -Dhost=native is auto-decteted)
+    // TODO(bfredl): we could include stuff like specific cpu features and os version but eh
+    var config_str = b.fmt("zig build -Doptimize={s} -Dtarget={s}", .{ @tagName(optimize), try t.linuxTriple(b.graph.arena) });
     if (cross_compiling) {
-        config_str = b.fmt("{s} -Dcross -Dtarget={s} (host: {s})", .{
-            config_str,
-            try t.linuxTriple(b.allocator),
-            try b.graph.host.result.linuxTriple(b.allocator),
-        });
+        config_str = b.fmt("{s} -Dhost={s}", .{ config_str, try target_host.result.linuxTriple(b.graph.arena) });
     }
 
     const versiondef_step = b.addConfigHeader(.{
@@ -463,6 +464,7 @@ pub fn build(b: *std.Build) !void {
         &api_headers,
         versiondef_git,
         version_lua,
+        !cross_compiling,
     );
 
     const test_config_step = b.addWriteFiles();

runtime/doc/news.txt

@@ -131,6 +131,19 @@ BUILD
 
 • Building using "zig build" requires zig 0.16.x.
 
+• zig build: "-Dcross" option was removed. Often cross-compilation is now
+detected, so e.g. from a linux host
+
+   zig build -Dtarget=aarch64-macos
+
+will automatically compile a host Lua for use during build.
+
+The new "-Dhost={target_string}" option can be used to override the used host.
+use "-Dhost=native" to force cross-compiling or "-Dhost=" (empty string) to
+assume that target binaries can run on the host during the build process (e.g.
+if target is x86 on a x86_64 system, or if emulation set up with binfmt or
+similar)
+
 DEFAULTS
 
 • todo

src/gen/gen_steps.zig

@@ -11,6 +11,7 @@ pub fn nvim_gen_sources(
     api_headers: *std.ArrayList(LazyPath),
     versiondef_git: LazyPath,
     version_lua: LazyPath,
+    precompile: bool,
 ) !struct { *std.Build.Step.WriteFile, LazyPath } {
     const gen_headers = b.addWriteFiles();
 
@@ -64,7 +65,7 @@ pub fn nvim_gen_sources(
     {
         const gen_step = b.addRunArtifact(nlua0);
         gen_step.addFileArg(b.path("src/gen/gen_char_blob.lua"));
-        gen_step.addArg("-c");
+        if (precompile) gen_step.addArg("-c");
         _ = gen_header(b, gen_step, "lua/vim_module.generated.h", gen_headers);
         // NB: vim._init_packages and vim.inspect must be be first and second ones
         // respectively, otherwise --luamod-dev won't work properly.