Allow setting working directory for compile steps

Current working directory affects the linking behavior when `-L` is a
relative path.

We need this to add a test case to `test/link/elf.zig` for checking
`-l`/`-L` behavior compatibility with Clang/GCC for ELF.

Author: a-khabarov

lib/std/Build/Step.zig

@@ -370,6 +370,7 @@ 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,
@@ -411,8 +412,8 @@ 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;
@@ -421,6 +422,7 @@ 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("unable to spawn {s}: {s}", .{
         argv[0], @errorName(err),

lib/std/Build/Step/Compile.zig

@@ -1,5 +1,6 @@
 const builtin = @import("builtin");
 const std = @import("std");
+const Build = std.Build;
 const mem = std.mem;
 const fs = std.fs;
 const assert = std.debug.assert;
@@ -226,6 +227,9 @@ zig_process: ?*Step.ZigProcess,
 /// 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,
@@ -1778,9 +1782,11 @@ 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_bin_path = step.evalZigProcess(
+        cwd,
         zig_args,
         options.progress_node,
         (b.graph.incremental == true) and options.watch,
@@ -2042,3 +2048,8 @@ fn moduleNeedsCliArg(mod: *const Module) bool {
         else => continue,
     } else false;
 }
+
+pub fn setCwd(self: *Compile, cwd: Build.LazyPath) void {
+    cwd.addStepDependencies(&self.step);
+    self.cwd = cwd;
+}

lib/std/Build/Step/ObjCopy.zig

@@ -159,7 +159,7 @@ 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

@@ -155,7 +155,7 @@ fn make(step: *Step, options: Step.MakeOptions) !void {
 
     try argv_list.append(translate_c.source.getPath2(b, step));
 
-    const output_path = try step.evalZigProcess(argv_list.items, prog_node, false);
+    const output_path = try step.evalZigProcess(null, argv_list.items, prog_node, false);
 
     translate_c.out_basename = fs.path.basename(output_path.?);
     const output_dir = fs.path.dirname(output_path.?).?;