test: add a standalone test for mixing c, zig, threadlocal and build modes.

based on https://github.com/ziglang/zig/issues/8531

test/standalone.zig

@@ -13,6 +13,12 @@ pub fn addCases(cases: *tests.StandaloneContext) void {
     cases.addBuildFile("test/standalone/main_pkg_path/build.zig", .{});
     cases.addBuildFile("test/standalone/shared_library/build.zig", .{});
     cases.addBuildFile("test/standalone/mix_o_files/build.zig", .{});
+    if (std.Target.current.os.tag == .macos) {
+        // TODO zld cannot link llvm-ir object files for LTO yet.
+        cases.addBuildFile("test/standalone/mix_c_files/build.zig", .{ .build_modes = false, .cross_targets = true });
+    } else {
+        cases.addBuildFile("test/standalone/mix_c_files/build.zig", .{ .build_modes = true, .cross_targets = true });
+    }
     cases.addBuildFile("test/standalone/global_linkage/build.zig", .{});
     cases.addBuildFile("test/standalone/static_c_lib/build.zig", .{});
     cases.addBuildFile("test/standalone/link_interdependent_static_c_libs/build.zig", .{});

test/standalone/mix_c_files/build.zig

@@ -0,0 +1,34 @@
+const std = @import("std");
+const Builder = std.build.Builder;
+const CrossTarget = std.zig.CrossTarget;
+
+fn isUnpecifiedTarget(t: CrossTarget) bool {
+    return t.cpu_arch == null and t.abi == null and t.os_tag == null;
+}
+fn isRunnableTarget(t: CrossTarget) bool {
+    if (t.isNative()) return true;
+
+    return (t.getOsTag() == std.Target.current.os.tag and
+        t.getCpuArch() == std.Target.current.cpu.arch);
+}
+
+pub fn build(b: *Builder) void {
+    const mode = b.standardReleaseOptions();
+    const target = b.standardTargetOptions(.{});
+
+    const test_step = b.step("test", "Test the program");
+
+    const exe = b.addExecutable("test", "main.zig");
+    exe.addCSourceFile("test.c", &[_][]const u8{"-std=c11"});
+    exe.setBuildMode(mode);
+    exe.linkLibC();
+    exe.setTarget(target);
+    b.default_step.dependOn(&exe.step);
+
+    if (isRunnableTarget(target)) {
+        const run_cmd = exe.run();
+        test_step.dependOn(&run_cmd.step);
+    } else {
+        test_step.dependOn(&exe.step);
+    }
+}

test/standalone/mix_c_files/main.zig

@@ -0,0 +1,30 @@
+const std = @import("std");
+
+extern fn add_C(x: i32) i32;
+extern fn add_C_zig(x: i32) i32;
+extern threadlocal var C_k: c_int;
+
+export var zig_k: c_int = 1;
+export fn add_zig(x: i32) i32 {
+    return x + zig_k + C_k;
+}
+export fn add_may_panic(x: i32) i32 {
+    if (x < 0) @panic("negative int");
+    return x + zig_k;
+}
+
+pub fn main() anyerror!void {
+    var x: i32 = 0;
+    x = add_zig(x);
+    x = add_C(x);
+    x = add_C_zig(x);
+
+    C_k = 200;
+    zig_k = 2;
+    x = add_zig(x);
+    x = add_C(x);
+    x = add_C_zig(x);
+
+    const u = @intCast(u32, x);
+    try std.testing.expect(u / 100 == u % 100);
+}

test/standalone/mix_c_files/test.c

@@ -0,0 +1,8 @@
+
+extern int zig_k;
+extern int add_may_panic(int);
+
+_Thread_local int  C_k = 100;
+int unused(int x) { return x*x; }
+int add_C(int x) { return x+zig_k+C_k; }
+int add_C_zig(int x) { return add_may_panic(x) + C_k; }