fix: copy Rust updater library to build output dir for Ninja tracking

The GN action for building the Rust updater via cargo declared only a
stamp file as output. The library path referenced by libs[] was in the
source tree, so Ninja couldn't map it to any build rule — causing
"missing and no known rule to make it" for targets where the pre-built
library didn't already exist (e.g. host_debug on macOS).

Fix: the build_rust_updater.py script now copies the cargo-built
library from the source tree to target_gen_dir, which is declared as
an action output. This lets Ninja properly order the link step after
the cargo build.

Author: bdero

engine/src/flutter/shell/common/shorebird/BUILD.gn

@@ -17,21 +17,30 @@ source_set("snapshots_data_handle") {
 }
 
 if (shorebird_updater_supported) {
+  # The library is copied from cargo's source-tree output into the build
+  # output dir so GN/Ninja can track it as an action output and properly
+  # order the link step after the cargo build.
+  _updater_output_lib = "$target_gen_dir/$shorebird_updater_lib_name"
+
   action("build_rust_updater") {
     script = "//flutter/shell/common/shorebird/build_rust_updater.py"
 
-    # The stamp file is the declared output for Ninja dependency tracking.
     _stamp =
         "$target_gen_dir/rust_updater_${shorebird_updater_rust_target}.stamp"
-    outputs = [ _stamp ]
+    outputs = [
+      _stamp,
+      _updater_output_lib,
+    ]
 
     args = [
       "--rust-target",
       shorebird_updater_rust_target,
       "--manifest-dir",
       rebase_path("$shorebird_updater_dir", root_build_dir),
+      "--cargo-output-lib",
+      rebase_path("$shorebird_updater_cargo_lib", root_build_dir),
       "--output-lib",
-      rebase_path("$shorebird_updater_output_lib", root_build_dir),
+      rebase_path("$_updater_output_lib", root_build_dir),
       "--stamp",
       rebase_path(_stamp, root_build_dir),
     ]
@@ -73,7 +82,7 @@ source_set("updater") {
 
   if (shorebird_updater_supported) {
     deps += [ ":build_rust_updater" ]
-    libs = [ shorebird_updater_output_lib ]
+    libs = [ _updater_output_lib ]
 
     if (is_win) {
       libs += [ "userenv.lib" ]

engine/src/flutter/shell/common/shorebird/build_rust_updater.gni

@@ -60,7 +60,12 @@ if (shorebird_updater_supported) {
     _updater_lib_name = "libupdater.a"
   }
 
-  shorebird_updater_output_lib = "$shorebird_updater_dir/target/$shorebird_updater_rust_target/release/$_updater_lib_name"
+  # Path where cargo produces the library (in the source tree).
+  shorebird_updater_cargo_lib = "$shorebird_updater_dir/target/$shorebird_updater_rust_target/release/$_updater_lib_name"
+
+  # The library filename, used by BUILD.gn to construct the output path
+  # in target_gen_dir (which is only available inside BUILD.gn targets).
+  shorebird_updater_lib_name = _updater_lib_name
 
   # Glob all .rs source files so the input list stays in sync automatically.
   shorebird_updater_rs_sources =

engine/src/flutter/shell/common/shorebird/build_rust_updater.py

@@ -19,7 +19,14 @@ def main():
   parser.add_argument(
       '--manifest-dir', required=True, help='Directory containing the workspace Cargo.toml'
   )
-  parser.add_argument('--output-lib', required=True, help='Expected output library path')
+  parser.add_argument(
+      '--cargo-output-lib',
+      required=True,
+      help='Path where cargo places the built library (in the source tree)'
+  )
+  parser.add_argument(
+      '--output-lib', required=True, help='Path to copy the library to (in the build output dir)'
+  )
   parser.add_argument('--stamp', required=True, help='Stamp file to write on success')
   parser.add_argument('--ndk-path', help='Path to the Android NDK (required for Android targets)')
   parser.add_argument(
@@ -45,6 +52,7 @@ def main():
   # Ninja runs the action). Resolve them to absolute paths so they work
   # regardless of cargo's working directory.
   manifest_path = os.path.abspath(os.path.join(args.manifest_dir, 'Cargo.toml'))
+  cargo_output_lib = os.path.abspath(args.cargo_output_lib)
   output_lib = os.path.abspath(args.output_lib)
 
   cmd = [
@@ -65,10 +73,16 @@ def main():
     print(f'ERROR: cargo build failed with exit code {result.returncode}', file=sys.stderr)
     return result.returncode
 
-  if not os.path.exists(output_lib):
-    print(f'ERROR: Expected output library not found: {output_lib}', file=sys.stderr)
+  if not os.path.exists(cargo_output_lib):
+    print(f'ERROR: Cargo output library not found: {cargo_output_lib}', file=sys.stderr)
     return 1
 
+  # Copy the library from the cargo output (source tree) to the build output
+  # dir so GN/Ninja can track it as an action output.
+  import shutil
+  os.makedirs(os.path.dirname(output_lib), exist_ok=True)
+  shutil.copy2(cargo_output_lib, output_lib)
+
   # Write stamp file to signal success to Ninja.
   with open(args.stamp, 'w') as f:
     f.write('')