lib: allow setting current directory for compile steps

We need this to implement the new test in
ziglang#19818

This commits adds:

* `setCwd` for `Build.Step.Compile`.
  Allows setting current directory for compile steps.
* `opt_cwd` for `evalZigProcess` in `Build.Step`.
  Allows running `evalZigProcess` in a specific directory.
  Needed for setting current directory for compile steps.

`--zig-lib-dir` is now appended differently in `Build.Step.Compile`.
This is needed for compatibility with `setCwd`.

Author: a-khabarov

lib/std/Build/Step.zig

@@ -371,6 +371,7 @@ pub const ZigProcess = struct {
 /// is the zig compiler - the same version that compiled the build runner.
 pub fn evalZigProcess(
     s: *Step,
+    opt_cwd: ?[]const u8,
     argv: []const []const u8,
     prog_node: std.Progress.Node,
     watch: bool,
@@ -401,7 +402,7 @@ pub fn evalZigProcess(
             };
             s.result_peak_rss = zp.child.resource_usage_statistics.getMaxRss() orelse 0;
             s.clearZigProcess();
-            try handleChildProcessTerm(s, term, null, argv);
+            try handleChildProcessTerm(s, term, opt_cwd, argv);
             return error.MakeFailed;
         }
 
@@ -412,8 +413,8 @@ pub fn evalZigProcess(
     const arena = b.allocator;
     const gpa = arena;
 
-    try handleChildProcUnsupported(s, null, argv);
-    try handleVerbose(s.owner, null, argv);
+    try handleChildProcUnsupported(s, opt_cwd, argv);
+    try handleVerbose(s.owner, opt_cwd, argv);
 
     var child = std.process.Child.init(argv, arena);
     child.env_map = &b.graph.env_map;
@@ -422,6 +423,7 @@ pub fn evalZigProcess(
     child.stderr_behavior = .Pipe;
     child.request_resource_usage_statistics = true;
     child.progress_node = prog_node;
+    child.cwd = opt_cwd;
 
     child.spawn() catch |err| return s.fail("failed to spawn zig compiler {s}: {s}", .{
         argv[0], @errorName(err),
@@ -463,15 +465,15 @@ pub fn evalZigProcess(
             else => {},
         };
 
-        try handleChildProcessTerm(s, term, null, argv);
+        try handleChildProcessTerm(s, term, opt_cwd, argv);
     }
 
     // This is intentionally printed for failure on the first build but not for
     // subsequent rebuilds.
     if (s.result_error_bundle.errorMessageCount() > 0) {
         return s.fail("the following command failed with {d} compilation errors:\n{s}", .{
             s.result_error_bundle.errorMessageCount(),
-            try allocPrintCmd(arena, null, argv),
+            try allocPrintCmd(arena, opt_cwd, argv),
         });
     }
 

lib/std/Build/Step/Compile.zig

@@ -5,6 +5,7 @@ const fs = std.fs;
 const assert = std.debug.assert;
 const panic = std.debug.panic;
 const ArrayList = std.ArrayList;
+const Build = std.Build;
 const StringHashMap = std.StringHashMap;
 const Sha256 = std.crypto.hash.sha2.Sha256;
 const Allocator = mem.Allocator;
@@ -243,6 +244,9 @@ zig_process: ?*Step.ZigProcess,
 /// builtin fuzzer, see the `fuzz` flag in `Module`.
 sanitize_coverage_trace_pc_guard: ?bool = null,
 
+/// Use `setCwd` to set the initial current working directory
+cwd: ?Build.LazyPath = null,
+
 pub const ExpectedCompileErrors = union(enum) {
     contains: []const u8,
     exact: []const []const u8,
@@ -1705,7 +1709,7 @@ fn getZigArgs(compile: *Compile, fuzz: bool) ![][]const u8 {
 
     if (opt_zig_lib_dir) |zig_lib_dir| {
         try zig_args.append("--zig-lib-dir");
-        try zig_args.append(zig_lib_dir);
+        try zig_args.append(try fs.cwd().realpathAlloc(arena, zig_lib_dir));
     }
 
     try addFlag(&zig_args, "PIE", compile.pie);
@@ -1803,9 +1807,11 @@ fn make(step: *Step, options: Step.MakeOptions) !void {
     const b = step.owner;
     const compile: *Compile = @fieldParentPtr("step", step);
 
+    const cwd: ?[]const u8 = if (compile.cwd) |lazy_cwd| lazy_cwd.getPath(b) else null;
     const zig_args = try getZigArgs(compile, false);
 
     const maybe_output_dir = step.evalZigProcess(
+        cwd,
         zig_args,
         options.progress_node,
         (b.graph.incremental == true) and options.watch,
@@ -1888,8 +1894,9 @@ pub fn rebuildInFuzzMode(c: *Compile, progress_node: std.Progress.Node) !Path {
     c.step.result_error_bundle.deinit(gpa);
     c.step.result_error_bundle = std.zig.ErrorBundle.empty;
 
+    const cwd: ?[]const u8 = if (c.cwd) |lazy_cwd| lazy_cwd.getPath(c.step.owner) else null;
     const zig_args = try getZigArgs(c, true);
-    const maybe_output_bin_path = try c.step.evalZigProcess(zig_args, progress_node, false);
+    const maybe_output_bin_path = try c.step.evalZigProcess(cwd, zig_args, progress_node, false);
     return maybe_output_bin_path.?;
 }
 
@@ -2130,3 +2137,8 @@ pub fn getCompileDependencies(start: *Compile, chase_dynamic: bool) []const *Com
 
     return compiles.keys();
 }
+
+pub fn setCwd(self: *Compile, cwd: Build.LazyPath) void {
+    cwd.addStepDependencies(&self.step);
+    self.cwd = cwd;
+}

lib/std/Build/Step/ObjCopy.zig

@@ -236,7 +236,7 @@ fn make(step: *Step, options: Step.MakeOptions) !void {
     try argv.appendSlice(&.{ full_src_path, full_dest_path });
 
     try argv.append("--listen=-");
-    _ = try step.evalZigProcess(argv.items, prog_node, false);
+    _ = try step.evalZigProcess(null, argv.items, prog_node, false);
 
     objcopy.output_file.path = full_dest_path;
     if (objcopy.output_file_debug) |*file| file.path = full_dest_path_debug;

lib/std/Build/Step/TranslateC.zig

@@ -200,7 +200,7 @@ fn make(step: *Step, options: Step.MakeOptions) !void {
     const c_source_path = translate_c.source.getPath2(b, step);
     try argv_list.append(c_source_path);
 
-    const output_dir = try step.evalZigProcess(argv_list.items, prog_node, false);
+    const output_dir = try step.evalZigProcess(null, argv_list.items, prog_node, false);
 
     const basename = std.fs.path.stem(std.fs.path.basename(c_source_path));
     translate_c.out_basename = b.fmt("{s}.zig", .{basename});