Add -includes option to zig libc

Prints the detected libc include paths for the target and exits

src/Compilation.zig

@@ -4887,7 +4887,7 @@ fn getZigShippedLibCIncludeDirsDarwin(arena: Allocator, zig_lib_dir: []const u8,
     };
 }
 
-fn detectLibCIncludeDirs(
+pub fn detectLibCIncludeDirs(
     arena: Allocator,
     zig_lib_dir: []const u8,
     target: Target,

src/main.zig

@@ -4260,12 +4260,14 @@ pub const usage_libc =
     \\Options:
     \\  -h, --help             Print this help and exit
     \\  -target [name]         <arch><sub>-<os>-<abi> see the targets command
+    \\  -includes              Print the libc include directories for the target
     \\
 ;
 
 pub fn cmdLibC(gpa: Allocator, args: []const []const u8) !void {
     var input_file: ?[]const u8 = null;
     var target_arch_os_abi: []const u8 = "native";
+    var print_includes: bool = false;
     {
         var i: usize = 0;
         while (i < args.len) : (i += 1) {
@@ -4279,6 +4281,8 @@ pub fn cmdLibC(gpa: Allocator, args: []const []const u8) !void {
                     if (i + 1 >= args.len) fatal("expected parameter after {s}", .{arg});
                     i += 1;
                     target_arch_os_abi = args[i];
+                } else if (mem.eql(u8, arg, "-includes")) {
+                    print_includes = true;
                 } else {
                     fatal("unrecognized parameter: '{s}'", .{arg});
                 }
@@ -4294,6 +4298,59 @@ pub fn cmdLibC(gpa: Allocator, args: []const []const u8) !void {
         .arch_os_abi = target_arch_os_abi,
     });
 
+    if (print_includes) {
+        var arena_state = std.heap.ArenaAllocator.init(gpa);
+        defer arena_state.deinit();
+        const arena = arena_state.allocator();
+
+        const libc_installation: ?*LibCInstallation = libc: {
+            if (input_file) |libc_file| {
+                var libc = try arena.create(LibCInstallation);
+                libc.* = LibCInstallation.parse(arena, libc_file, cross_target) catch |err| {
+                    fatal("unable to parse libc file at path {s}: {s}", .{ libc_file, @errorName(err) });
+                };
+                break :libc libc;
+            } else {
+                break :libc null;
+            }
+        };
+
+        const self_exe_path = try introspect.findZigExePath(arena);
+        var zig_lib_directory = introspect.findZigLibDirFromSelfExe(arena, self_exe_path) catch |err| {
+            fatal("unable to find zig installation directory: {s}\n", .{@errorName(err)});
+        };
+        defer zig_lib_directory.handle.close();
+
+        const target = cross_target.toTarget();
+        const is_native_abi = cross_target.isNativeAbi();
+
+        var libc_dirs = Compilation.detectLibCIncludeDirs(
+            arena,
+            zig_lib_directory.path.?,
+            target,
+            is_native_abi,
+            true,
+            libc_installation,
+        ) catch |err| {
+            const zig_target = try target.zigTriple(arena);
+            fatal("unable to detect libc for target {s}: {s}", .{ zig_target, @errorName(err) });
+        };
+
+        if (libc_dirs.libc_include_dir_list.len == 0) {
+            const zig_target = try target.zigTriple(arena);
+            fatal("no include dirs detected for target {s}", .{zig_target});
+        }
+
+        var bw = io.bufferedWriter(io.getStdOut().writer());
+        var writer = bw.writer();
+        for (libc_dirs.libc_include_dir_list) |include_dir| {
+            try writer.writeAll(include_dir);
+            try writer.writeByte('\n');
+        }
+        try bw.flush();
+        return cleanExit();
+    }
+
     if (input_file) |libc_file| {
         var libc = LibCInstallation.parse(gpa, libc_file, cross_target) catch |err| {
             fatal("unable to parse libc file at path {s}: {s}", .{ libc_file, @errorName(err) });