Avoid builds-without-the-bytes bug on Windows (#2919)

After #2826 was merged, I
started seeing flaky builds on Windows related to build script
executables
(#2891 (comment)).
This appears to be related to
bazelbuild/bazel#21747 so to avoid the issue,
this change ensures that Windows build script executables are copied
instead of symlinked.

cargo/private/BUILD.bazel

@@ -1,4 +1,12 @@
 load("@bazel_skylib//:bzl_library.bzl", "bzl_library")
+load("//rust:defs.bzl", "rust_binary")
+
+rust_binary(
+    name = "copy_file",
+    srcs = ["copy_file.rs"],
+    edition = "2021",
+    visibility = ["//visibility:public"],
+)
 
 bzl_library(
     name = "bzl_lib",

cargo/private/cargo_build_script.bzl

@@ -40,11 +40,26 @@ def _cargo_build_script_runfiles_impl(ctx):
 
     is_windows = script.extension == "exe"
     exe = ctx.actions.declare_file("{}{}".format(ctx.label.name, ".exe" if is_windows else ""))
-    ctx.actions.symlink(
-        output = exe,
-        target_file = script,
-        is_executable = True,
-    )
+
+    # Avoid the following issue on Windows when using builds-without-the-bytes.
+    # https://github.com/bazelbuild/bazel/issues/21747
+    if is_windows:
+        args = ctx.actions.args()
+        args.add(script)
+        args.add(exe)
+
+        ctx.actions.run(
+            executable = ctx.executable._copy_file,
+            arguments = [args],
+            inputs = [script],
+            outputs = [exe],
+        )
+    else:
+        ctx.actions.symlink(
+            output = exe,
+            target_file = script,
+            is_executable = True,
+        )
 
     # Tools are ommitted here because they should be within the `script`
     # attribute's runfiles.
@@ -95,6 +110,11 @@ https://github.com/bazelbuild/bazel/issues/15486
             allow_files = True,
             cfg = "exec",
         ),
+        "_copy_file": attr.label(
+            cfg = "exec",
+            executable = True,
+            default = Label("//cargo/private:copy_file"),
+        ),
     },
     executable = True,
 )

cargo/private/copy_file.rs

@@ -0,0 +1,20 @@
+//! A tool for copying files and avoiding
+//! https://github.com/bazelbuild/bazel/issues/21747
+
+use std::env;
+use std::fs;
+use std::path::PathBuf;
+
+fn main() {
+    let src = PathBuf::from(std::env::args().nth(1).expect("No source file provided"));
+    let dest = PathBuf::from(env::args().nth(2).expect("No destination provided"));
+
+    fs::copy(&src, &dest).unwrap_or_else(|e| {
+        panic!(
+            "Failed to copy file `{} -> {}`\n{:?}",
+            src.display(),
+            dest.display(),
+            e
+        )
+    });
+}