build: restore compatibility support again

Commit e214c44 (build: update ZiglingStep to the new API) broke again
the compatibility support for old compilers, due to the use of the
multi-object for loop syntax.

Move the Zig IPC support code to src/ipc.zig.

Use the while statement, instead of the for statement.

build.zig

@@ -1,6 +1,7 @@
 const std = @import("std");
 const builtin = @import("builtin");
 const compat = @import("src/compat.zig");
+const ipc = @import("src/ipc.zig");
 const tests = @import("test/tests.zig");
 
 const Build = compat.Build;
@@ -878,8 +879,8 @@ const ZiglingStep = struct {
         });
         defer poller.deinit();
 
-        try sendMessage(child.stdin.?, .update);
+        try ipc.sendMessage(child.stdin.?, .update);
-        try sendMessage(child.stdin.?, .exit);
+        try ipc.sendMessage(child.stdin.?, .exit);
 
         const Header = std.zig.Server.Message.Header;
         var result: ?[]const u8 = null;
@@ -907,21 +908,7 @@ const ZiglingStep = struct {
                         return error.ZigVersionMismatch;
                 },
                 .error_bundle => {
-                    const EbHdr = std.zig.Server.Message.ErrorBundle;
+                    const error_bundle = try ipc.parseErrorBundle(allocator, body);
-                    const eb_hdr = @ptrCast(*align(1) const EbHdr, body);
-                    const extra_bytes =
-                        body[@sizeOf(EbHdr)..][0 .. @sizeOf(u32) * eb_hdr.extra_len];
-                    const string_bytes =
-                        body[@sizeOf(EbHdr) + extra_bytes.len ..][0..eb_hdr.string_bytes_len];
-                    // TODO: use @ptrCast when the compiler supports it
-                    const unaligned_extra = std.mem.bytesAsSlice(u32, extra_bytes);
-                    const extra_array = try allocator.alloc(u32, unaligned_extra.len);
-                    // TODO: use @memcpy when it supports slices
-                    for (extra_array, unaligned_extra) |*dst, src| dst.* = src;
-                    const error_bundle: std.zig.ErrorBundle = .{
-                        .string_bytes = try allocator.dupe(u8, string_bytes),
-                        .extra = extra_array,
-                    };
 
                     // Print the compiler error bundle now.
                     // TODO: use the same ttyconf from the builder.
@@ -939,13 +926,8 @@ const ZiglingStep = struct {
                     sub_prog_node.setName(node_name.items);
                 },
                 .emit_bin_path => {
-                    const EbpHdr = std.zig.Server.Message.EmitBinPath;
+                    const emit_bin = try ipc.parseEmitBinPath(allocator, body);
-
+                    result = emit_bin.path;
-                    // TODO: add cache support?
-                    //const ebp_hdr = @ptrCast(*align(1) const EbpHdr, body);
-                    //s.result_cached = ebp_hdr.flags.cache_hit;
-
-                    result = try allocator.dupe(u8, body[@sizeOf(EbpHdr)..]);
                 },
                 else => {}, // ignore other messages
             }
@@ -985,14 +967,6 @@ const ZiglingStep = struct {
     }
 };
 
-fn sendMessage(file: std.fs.File, tag: std.zig.Client.Message.Tag) !void {
-    const header: std.zig.Client.Message.Header = .{
-        .tag = tag,
-        .bytes_len = 0,
-    };
-    try file.writeAll(std.mem.asBytes(&header));
-}
-
 // Print a message to stderr.
 const PrintStep = struct {
     step: Step,

src/ipc.zig

@@ -0,0 +1,68 @@
+/// Client side support for Zig IPC.
+const std = @import("std");
+const debug = std.debug;
+const fs = std.fs;
+const mem = std.mem;
+
+const Allocator = mem.Allocator;
+const Client = std.zig.Client;
+const ErrorBundle = std.zig.ErrorBundle;
+const Server = std.zig.Server;
+
+/// This data structure must be kept in sync with zig.Server.Message.EmitBinPath.
+const EmitBinPath = struct {
+    flags: Flags,
+    path: []const u8,
+
+    pub const Flags = Server.Message.EmitBinPath.Flags;
+
+    pub fn deinit(self: *EmitBinPath, allocator: Allocator) void {
+        allocator.free(self.path);
+        self.* = undefined;
+    }
+};
+
+pub fn parseErrorBundle(allocator: Allocator, data: []const u8) !ErrorBundle {
+    const EbHdr = Server.Message.ErrorBundle;
+    const eb_hdr = @ptrCast(*align(1) const EbHdr, data);
+    const extra_bytes =
+        data[@sizeOf(EbHdr)..][0 .. @sizeOf(u32) * eb_hdr.extra_len];
+    const string_bytes =
+        data[@sizeOf(EbHdr) + extra_bytes.len ..][0..eb_hdr.string_bytes_len];
+
+    // TODO: use @ptrCast when the compiler supports it
+    const unaligned_extra = std.mem.bytesAsSlice(u32, extra_bytes);
+    const extra_array = try allocator.alloc(u32, unaligned_extra.len);
+    // TODO: use @memcpy when it supports slices
+    //
+    // Don't use the "multi-object for loop" syntax, in
+    // order to avoid a syntax error with old Zig compilers.
+    var i: usize = 0;
+    while (i < extra_array.len) : (i += 1) {
+        extra_array[i] = unaligned_extra[i];
+    }
+
+    return .{
+        .string_bytes = try allocator.dupe(u8, string_bytes),
+        .extra = extra_array,
+    };
+}
+
+pub fn parseEmitBinPath(allocator: Allocator, data: []const u8) !EmitBinPath {
+    const EbpHdr = Server.Message.EmitBinPath;
+    const ebp_hdr = @ptrCast(*align(1) const EbpHdr, data);
+    const path = try allocator.dupe(u8, data[@sizeOf(EbpHdr)..]);
+
+    return .{
+        .flags = ebp_hdr.flags,
+        .path = path,
+    };
+}
+
+pub fn sendMessage(file: fs.File, tag: Client.Message.Tag) !void {
+    const header: Client.Message.Header = .{
+        .tag = tag,
+        .bytes_len = 0,
+    };
+    try file.writeAll(mem.asBytes(&header));
+}