Switch pipelined compilation from --rustc-quit-on-rmeta to -Zno-codegen

Replace the process wrapper's rmeta-interception approach (kill rustc
after metadata emission) with rustc's -Zno-codegen flag, which produces
a hollow rlib containing metadata and MIR but no object code. This is
the same approach used by Buck2.

Key changes:
- Metadata action uses `rustc -Zno-codegen --emit=link=<path>` to
  produce a hollow rlib (_meta.rlib) instead of raw .rmeta files
- Remove --rustc-quit-on-rmeta flag, LineOutput::Terminate, and all
  associated kill logic from the process wrapper
- Full action emits only --emit=link (no longer includes metadata)
- Set RUSTC_BOOTSTRAP=1 on both metadata and full actions for SVH
  compatibility (required for the unstable -Zno-codegen flag)

Author: walter-zeromatter

cargo/Cargo.lock

@@ -180,7 +180,7 @@ def _compile_rust(
         prefix = "lib",
         name = crate_name,
         lib_hash = output_hash,
-        extension = ".rmeta",
+        extension = "_meta.rlib",
     )
 
     lib = ctx.actions.declare_file(lib_name)
@@ -193,7 +193,7 @@ def _compile_rust(
             prefix = "lib",
             name = crate_name,
             lib_hash = output_hash,
-            extension = ".rmeta",
+            extension = "_meta.rlib",
         )
         rmeta = ctx.actions.declare_file(rmeta_name)
         rustc_rmeta_output = generate_output_diagnostics(ctx, rmeta)

extensions/prost/private/prost.bzl

@@ -177,7 +177,7 @@ def rust_clippy_action(ctx, clippy_executable, process_wrapper, crate_info, conf
         out_dir = out_dir,
         build_env_files = build_env_files,
         build_flags_files = build_flags_files,
-        emit = ["dep-info", "metadata"],
+        emit = ["metadata"],
         skip_expanding_rustc_env = True,
         use_json_output = bool(clippy_diagnostics_file),
         error_format = error_format,

rust/private/clippy.bzl

@@ -206,7 +206,7 @@ def _rust_library_common(ctx, crate_type):
         disable_pipelining = getattr(ctx.attr, "disable_pipelining", False),
     ):
         rust_metadata = ctx.actions.declare_file(
-            paths.replace_extension(rust_lib_name, ".rmeta"),
+            paths.replace_extension(rust_lib_name, "_meta.rlib"),
             sibling = rust_lib,
         )
         rustc_rmeta_output = generate_output_diagnostics(ctx, rust_metadata)
@@ -279,7 +279,7 @@ def _rust_binary_impl(ctx):
     rustc_rmeta_output = None
     if can_build_metadata(toolchain, ctx, ctx.attr.crate_type):
         rust_metadata = ctx.actions.declare_file(
-            paths.replace_extension("lib" + crate_name, ".rmeta"),
+            paths.replace_extension("lib" + crate_name, "_meta.rlib"),
             sibling = output,
         )
         rustc_rmeta_output = generate_output_diagnostics(ctx, rust_metadata)
@@ -381,7 +381,7 @@ def _rust_test_impl(ctx):
         rustc_rmeta_output = None
         if can_build_metadata(toolchain, ctx, crate_type):
             rust_metadata = ctx.actions.declare_file(
-                paths.replace_extension("lib" + crate_name, ".rmeta"),
+                paths.replace_extension("lib" + crate_name, "_meta.rlib"),
                 sibling = output,
             )
             rustc_rmeta_output = generate_output_diagnostics(ctx, rust_metadata)
@@ -451,7 +451,7 @@ def _rust_test_impl(ctx):
         rustc_rmeta_output = None
         if can_build_metadata(toolchain, ctx, crate_type):
             rust_metadata = ctx.actions.declare_file(
-                paths.replace_extension("lib" + crate_name, ".rmeta"),
+                paths.replace_extension("lib" + crate_name, "_meta.rlib"),
                 sibling = output,
             )
             rustc_rmeta_output = generate_output_diagnostics(ctx, rust_metadata)

rust/private/rust.bzl

@@ -901,7 +901,7 @@ def construct_arguments(
         out_dir,
         build_env_files,
         build_flags_files,
-        emit = ["dep-info", "link"],
+        emit = ["link"],
         force_all_deps_direct = False,
         add_flags_for_binary = False,
         include_link_flags = True,
@@ -1045,8 +1045,11 @@ def construct_arguments(
         error_format = "json"
 
     if build_metadata:
-        # Configure process_wrapper to terminate rustc when metadata are emitted
-        process_wrapper_flags.add("--rustc-quit-on-rmeta", "true")
+        # Build a hollow rlib (metadata-full, Buck2 equivalent) using -Zno-codegen.
+        # This produces an rlib with metadata but no object code, allowing downstream
+        # crates to start compiling without waiting for codegen.
+        # RUSTC_BOOTSTRAP=1 must be set in the action env for this unstable flag.
+        rustc_flags.add("-Zno-codegen")
         if crate_info.rustc_rmeta_output:
             process_wrapper_flags.add("--output-file", crate_info.rustc_rmeta_output.path)
     elif crate_info.rustc_output:
@@ -1079,7 +1082,12 @@ def construct_arguments(
 
     emit_without_paths = []
     for kind in emit:
-        if kind == "link" and crate_info.type == "bin" and crate_info.output != None:
+        if kind == "link" and build_metadata and crate_info.metadata != None:
+            # Redirect hollow rlib output to the declared metadata file path,
+            # since -Zno-codegen --emit=link would otherwise write lib<name>.rlib
+            # which collides with the full action's output.
+            rustc_flags.add(crate_info.metadata, format = "--emit=link=%s")
+        elif kind == "link" and crate_info.type == "bin" and crate_info.output != None:
             rustc_flags.add(crate_info.output, format = "--emit=link=%s")
         else:
             emit_without_paths.append(kind)
@@ -1377,17 +1385,12 @@ def rustc_compile_action(
         experimental_use_cc_common_link = experimental_use_cc_common_link,
     )
 
+    compile_inputs_metadata = compile_inputs
+
     # The types of rustc outputs to emit.
-    # If we build metadata, we need to keep the command line of the two invocations
-    # (rlib and rmeta) as similar as possible, otherwise rustc rejects the rmeta as
-    # a candidate.
-    # Because of that we need to add emit=metadata to both the rlib and rmeta invocation.
-    #
     # When cc_common linking is enabled, emit a `.o` file, which is later
     # passed to the cc_common.link action.
-    emit = ["dep-info", "link"]
-    if build_metadata:
-        emit.append("metadata")
+    emit = ["link"]
     if experimental_use_cc_common_link:
         emit = ["obj"]
 
@@ -1435,7 +1438,7 @@ def rustc_compile_action(
             toolchain = toolchain,
             tool_path = toolchain.rustc.path,
             cc_toolchain = cc_toolchain,
-            emit = emit,
+            emit = ["link"],
             feature_configuration = feature_configuration,
             crate_info = crate_info,
             dep_info = dep_info,
@@ -1458,6 +1461,12 @@ def rustc_compile_action(
     # this is the final list of env vars
     env.update(env_from_args)
 
+    if build_metadata:
+        # RUSTC_BOOTSTRAP=1 is required for -Zno-codegen on stable rustc, and must
+        # be set on both the metadata and full actions for SVH compatibility (since
+        # RUSTC_BOOTSTRAP affects the crate hash).
+        env["RUSTC_BOOTSTRAP"] = "1"
+
     if hasattr(attr, "version") and attr.version != "0.0.0":
         formatted_version = " v{}".format(attr.version)
     else:
@@ -1529,7 +1538,7 @@ def rustc_compile_action(
         if args_metadata:
             ctx.actions.run(
                 executable = ctx.executable._process_wrapper,
-                inputs = compile_inputs,
+                inputs = compile_inputs_metadata,
                 outputs = [build_metadata] + [x for x in [rustc_rmeta_output] if x],
                 env = env,
                 arguments = args_metadata.all,

rust/private/rustc.bzl

@@ -202,7 +202,7 @@ def _rust_unpretty_aspect_impl(target, ctx):
             out_dir = out_dir,
             build_env_files = build_env_files,
             build_flags_files = build_flags_files,
-            emit = ["dep-info", "metadata"],
+            emit = ["metadata"],
             skip_expanding_rustc_env = True,
         )
 

rust/private/unpretty.bzl

@@ -112,10 +112,15 @@ def use_real_import_macro():
     )
 
 def pipelined_compilation():
-    """When set, this flag causes rustc to emit `*.rmeta` files and use them for `rlib -> rlib` dependencies.
+    """When set, this flag enables pipelined compilation for `rlib -> rlib` dependencies.
 
-    While this involves one extra (short) rustc invocation to build the rmeta file,
-    it allows library dependencies to be unlocked much sooner, increasing parallelism during compilation.
+    For each library target, a second RustcMetadata action is created that runs rustc with
+    `-Zno-codegen --emit=link` to produce a hollow rlib (metadata & MIR, no object code).
+    Downstream library compilations can start as soon as this hollow rlib is available,
+    increasing parallelism during compilation.
+
+    Requires RUSTC_BOOTSTRAP=1, which is set automatically on both the metadata and full
+    actions for pipelined targets.
     """
     bool_flag(
         name = "pipelined_compilation",

rust/settings/settings.bzl

@@ -157,8 +157,8 @@ rust_binary(
 )
 
 rust_test(
-    name = "rustc_quit_on_rmeta",
-    srcs = ["rustc_quit_on_rmeta.rs"],
+    name = "rustc_output_format",
+    srcs = ["rustc_output_format.rs"],
     data = [
         ":fake_rustc",
         "//util/process_wrapper",