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Fix bad hollow-pipelining implementation #12

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Merged
dzbarsky merged 2 commits into from
Apr 10, 2026
Merged

Fix bad hollow-pipelining implementation #12

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43ccab5
Revert "Switch pipelining metadata action to hollow rlib (-Zno-codegen)"
walter-zeromatter Apr 10, 2026
6b800e4
Switch pipelined compilation from --rustc-quit-on-rmeta to -Zno-codegen
walter-zeromatter Apr 10, 2026
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4 changes: 2 additions & 2 deletions extensions/prost/private/prost.bzl
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Original file line number Diff line number Diff line change
Expand Up @@ -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)
Expand All @@ -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)
Expand Down
2 changes: 1 addition & 1 deletion rust/private/clippy.bzl
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Original file line number Diff line number Diff line change
Expand Up @@ -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,
Expand Down
14 changes: 5 additions & 9 deletions rust/private/rust.bzl
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Original file line number Diff line number Diff line change
Expand Up @@ -180,15 +180,11 @@ def _rust_library_common(ctx, crate_type):
crate_type,
disable_pipelining = getattr(ctx.attr, "disable_pipelining", False),
):
# The hollow rlib uses .rlib extension (not .rmeta) so rustc reads it as an
# rlib archive containing lib.rmeta with optimized MIR. It is placed in a
# "_hollow/" subdirectory so the full rlib and hollow rlib never appear in the
# same -Ldependency= search directory (which would cause E0463).
rust_metadata = ctx.actions.declare_file(
"_hollow/" + rust_lib_name[:-len(".rlib")] + "-hollow.rlib",
paths.replace_extension(rust_lib_name, "_meta.rlib"),
sibling = rust_lib,
)
rustc_rmeta_output = generate_output_diagnostics(ctx, rust_metadata)

metadata_supports_pipelining = (
can_use_metadata_for_pipelining(toolchain, crate_type) and
not ctx.attr.disable_pipelining
Expand Down Expand Up @@ -258,7 +254,7 @@ 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)
Expand Down Expand Up @@ -360,7 +356,7 @@ 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)
Expand Down Expand Up @@ -430,7 +426,7 @@ 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)
Expand Down
152 changes: 25 additions & 127 deletions rust/private/rustc.bzl
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Original file line number Diff line number Diff line change
Expand Up @@ -663,7 +663,7 @@ def _disambiguate_libs(actions, toolchain, crate_info, dep_info, use_pic):
visited_libs[name] = artifact
return ambiguous_libs

def _depend_on_metadata(crate_info, force_depend_on_objects, experimental_use_cc_common_link = False):
def _depend_on_metadata(crate_info, force_depend_on_objects):
"""Determines if we can depend on metadata for this crate.

By default (when pipelining is disabled or when the crate type needs to link against
Expand All @@ -673,34 +673,16 @@ def _depend_on_metadata(crate_info, force_depend_on_objects, experimental_use_cc
In some rare cases, even if both of those conditions are true, we still want to
depend on objects. This is what force_depend_on_objects is.

When experimental_use_cc_common_link is True, bin/cdylib crates also use hollow
rlib deps. The rustc step only emits .o files (no rustc linking), so SVH chain
consistency is sufficient; the actual linking is done by cc_common.link, which
does not check SVH.

Callers are responsible for zeroing out experimental_use_cc_common_link for
exec-platform builds before calling this function (see rustc_compile_action).
Exec-platform binaries (build scripts) must use full rlib deps because their
CcInfo linking contexts may lack a CC toolchain.

Args:
crate_info (CrateInfo): The Crate to determine this for.
force_depend_on_objects (bool): if set we will not depend on metadata.
experimental_use_cc_common_link (bool): if set, bin/cdylib crates also use
hollow rlib deps for SVH consistency. Must already be False for
exec-platform builds when this function is called.

Returns:
Whether we can depend on metadata for this crate.
"""
if force_depend_on_objects:
return False

if experimental_use_cc_common_link and crate_info.type in ("bin", "cdylib"):
# cc_common.link: rustc only emits .o files, so hollow rlib deps are safe and
# keep the SVH chain consistent (avoiding E0460 from nondeterministic proc macros).
return True

return crate_info.type in ("rlib", "lib")

def collect_inputs(
Expand Down Expand Up @@ -788,7 +770,7 @@ def collect_inputs(
linkstamp_outs = []

transitive_crate_outputs = dep_info.transitive_crate_outputs
if _depend_on_metadata(crate_info, force_depend_on_objects, experimental_use_cc_common_link):
if _depend_on_metadata(crate_info, force_depend_on_objects):
transitive_crate_outputs = dep_info.transitive_metadata_outputs

nolinkstamp_compile_direct_inputs = []
Expand Down Expand Up @@ -824,12 +806,6 @@ def collect_inputs(
transitive = [
crate_info.srcs,
transitive_crate_outputs,
# Always include hollow rlibs so they are present in the sandbox for
# -Ldependency= resolution. Binaries and proc-macros compile against full
# rlib --extern deps but need hollow rlibs available for transitive
# dependency resolution when those rlibs were themselves compiled against
# hollow deps. For rlib/lib crates this is a no-op (already included above).
dep_info.transitive_metadata_outputs,
crate_info.compile_data,
dep_info.transitive_proc_macro_data,
toolchain.all_files,
Expand Down Expand Up @@ -925,7 +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,
Expand All @@ -934,7 +910,6 @@ def construct_arguments(
use_json_output = False,
build_metadata = False,
force_depend_on_objects = False,
experimental_use_cc_common_link = False,
skip_expanding_rustc_env = False,
require_explicit_unstable_features = False,
always_use_param_file = False,
Expand Down Expand Up @@ -1070,14 +1045,11 @@ def construct_arguments(
error_format = "json"

if build_metadata:
if crate_info.type in ("rlib", "lib"):
# Hollow rlib approach (Buck2-style): rustc runs to completion with -Zno-codegen,
# producing a hollow .rlib (metadata only, no object code) via --emit=link=<path>.
# No need to kill rustc — -Zno-codegen skips codegen entirely and exits quickly.
rustc_flags.add("-Zno-codegen")

# else: IDE-only metadata for non-rlib types (bin, proc-macro, etc.): rustc exits
# naturally after writing .rmeta via --emit=dep-info,metadata (no kill needed).
# 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:
Expand Down Expand Up @@ -1110,12 +1082,10 @@ def construct_arguments(

emit_without_paths = []
for kind in emit:
if kind == "link" and build_metadata and crate_info.type in ("rlib", "lib") and crate_info.metadata:
# Hollow rlib: direct rustc's link output to the -hollow.rlib path.
# The file has .rlib extension so rustc reads it as an rlib archive
# (with optimized MIR in lib.rmeta). Using a .rmeta path would cause
# E0786 "found invalid metadata files" because rustc parses .rmeta files
# as raw metadata blobs, not rlib archives.
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")
Expand Down Expand Up @@ -1192,7 +1162,7 @@ def construct_arguments(
include_link_flags = include_link_flags,
)

use_metadata = _depend_on_metadata(crate_info, force_depend_on_objects, experimental_use_cc_common_link)
use_metadata = _depend_on_metadata(crate_info, force_depend_on_objects)

# These always need to be added, even if not linking this crate.
add_crate_link_flags(rustc_flags, dep_info, force_all_deps_direct, use_metadata)
Expand Down Expand Up @@ -1361,13 +1331,6 @@ def rustc_compile_action(
rustc_output = crate_info.rustc_output
rustc_rmeta_output = crate_info.rustc_rmeta_output

# Use the hollow rlib approach (Buck2-style) for rlib/lib crate types when a metadata
# action is being created. This always applies for rlib/lib regardless of whether
# pipelining is globally enabled — the hollow rlib is simpler than killing rustc.
# Non-rlib types (bin, proc-macro, etc.) use --emit=dep-info,metadata instead
# (rustc exits naturally after writing .rmeta, no process-wrapper kill needed).
use_hollow_rlib = bool(build_metadata) and crate_info.type in ("rlib", "lib")

# Determine whether to use cc_common.link:
# * either if experimental_use_cc_common_link is 1,
# * or if experimental_use_cc_common_link is -1 and
Expand All @@ -1381,12 +1344,6 @@ def rustc_compile_action(
elif ctx.attr.experimental_use_cc_common_link == -1:
experimental_use_cc_common_link = toolchain._experimental_use_cc_common_link

# Exec-platform binaries (build scripts) skip cc_common.link: exec-configuration
# rlib deps may lack a CC toolchain, causing empty CcInfo linking contexts. They
# use standard rustc linking with full rlib deps instead.
if experimental_use_cc_common_link and is_exec_configuration(ctx):
experimental_use_cc_common_link = False

dep_info, build_info, linkstamps = collect_deps(
deps = deps,
proc_macro_deps = proc_macro_deps,
Expand Down Expand Up @@ -1425,34 +1382,12 @@ def rustc_compile_action(
experimental_use_cc_common_link = experimental_use_cc_common_link,
)

# The main Rustc action uses FULL rlib deps so the full rlib it produces records
# full-rlib SVHs. A downstream binary links against full rlibs; if the Rustc action
# had used hollow rlib deps instead, nondeterministic proc macros could produce
# different SVHs for the hollow vs full rlib, causing E0460 in the binary build.
# The RustcMetadata action still uses hollow rlibs (compile_inputs_for_metadata)
# so it can start before full codegen of its deps completes.
compile_inputs_for_metadata = compile_inputs
if use_hollow_rlib:
compile_inputs, _, _, _, _, _ = collect_inputs(
ctx = ctx,
file = ctx.file,
files = ctx.files,
linkstamps = linkstamps,
toolchain = toolchain,
cc_toolchain = cc_toolchain,
feature_configuration = feature_configuration,
crate_info = crate_info,
dep_info = dep_info,
build_info = build_info,
lint_files = lint_files,
stamp = stamp,
force_depend_on_objects = True,
experimental_use_cc_common_link = experimental_use_cc_common_link,
)
compile_inputs_metadata = compile_inputs

# The main Rustc action emits dep-info and link (the full rlib/binary/cdylib).
# When cc_common linking is enabled, emit a `.o` file instead.
emit = ["dep-info", "link"]
# The types of rustc outputs to emit.
# When cc_common linking is enabled, emit a `.o` file, which is later
# passed to the cc_common.link action.
emit = ["link"]
if experimental_use_cc_common_link:
emit = ["obj"]

Expand Down Expand Up @@ -1487,33 +1422,21 @@ def rustc_compile_action(
force_all_deps_direct = force_all_deps_direct,
stamp = stamp,
use_json_output = bool(build_metadata) or bool(rustc_output) or bool(rustc_rmeta_output),
# Force full rlib --extern deps so the full rlib records full-rlib SVHs.
force_depend_on_objects = use_hollow_rlib,
experimental_use_cc_common_link = experimental_use_cc_common_link,
skip_expanding_rustc_env = skip_expanding_rustc_env,
require_explicit_unstable_features = require_explicit_unstable_features,
always_use_param_file = not ctx.executable._process_wrapper,
)

args_metadata = None
if build_metadata:
if use_hollow_rlib:
# Hollow rlib: emit dep-info and link (directed to the -hollow.rlib path via
# -Zno-codegen). dep-info must be included: it affects the SVH stored in the
# rlib, so both actions must include it to keep SVHs consistent.
metadata_emit = ["dep-info", "link"]
else:
# IDE-only metadata for non-rlib types (bin, proc-macro, etc.): rustc exits
# naturally after writing .rmeta with --emit=dep-info,metadata.
metadata_emit = ["dep-info", "metadata"]
args_metadata, _ = construct_arguments(
ctx = ctx,
attr = attr,
file = ctx.file,
toolchain = toolchain,
tool_path = toolchain.rustc.path,
cc_toolchain = cc_toolchain,
emit = metadata_emit,
emit = ["link"],
feature_configuration = feature_configuration,
crate_info = crate_info,
dep_info = dep_info,
Expand All @@ -1528,7 +1451,6 @@ def rustc_compile_action(
stamp = stamp,
use_json_output = True,
build_metadata = True,
experimental_use_cc_common_link = experimental_use_cc_common_link,
require_explicit_unstable_features = require_explicit_unstable_features,
)

Expand All @@ -1537,11 +1459,10 @@ def rustc_compile_action(
# this is the final list of env vars
env.update(env_from_args)

if use_hollow_rlib:
# Both the metadata action and the full Rustc action must have RUSTC_BOOTSTRAP=1
# for SVH compatibility. RUSTC_BOOTSTRAP=1 changes the crate hash — setting it
# on only one action would cause SVH mismatch even for deterministic crates.
# This enables -Zno-codegen on stable Rust compilers for the metadata action.
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":
Expand Down Expand Up @@ -1614,7 +1535,7 @@ def rustc_compile_action(
if args_metadata:
ctx.actions.run(
executable = ctx.executable._process_wrapper,
inputs = compile_inputs_for_metadata,
inputs = compile_inputs_metadata,
outputs = [build_metadata] + [x for x in [rustc_rmeta_output] if x],
env = env,
arguments = args_metadata.all,
Expand Down Expand Up @@ -2228,14 +2149,9 @@ def add_crate_link_flags(args, dep_info, force_all_deps_direct = False, use_meta
crate_to_link_flags = _crate_to_link_flag_metadata if use_metadata else _crate_to_link_flag
args.add_all(direct_crates, uniquify = True, map_each = crate_to_link_flags)

# Use hollow rlib directories for -Ldependency= when use_metadata=True (rlib/lib)
# so that both --extern= and -Ldependency= point to the same hollow rlib file.
# When use_metadata=False (bins, proc-macros), use full rlib directories; pointing
# to hollow dirs alongside full --extern= args would cause E0463 (ambiguous crate).
get_dirname = _get_crate_dirname_pipelined if use_metadata else _get_crate_dirname
args.add_all(
dep_info.transitive_crates,
map_each = get_dirname,
map_each = _get_crate_dirname,
uniquify = True,
format_each = "-Ldependency=%s",
)
Expand Down Expand Up @@ -2293,24 +2209,6 @@ def _get_crate_dirname(crate):
"""
return crate.output.dirname

def _get_crate_dirname_pipelined(crate):
"""For pipelined compilation: returns the _hollow/ directory for pipelined crates

When a crate supports pipelining and has a hollow rlib in its _hollow/ subdirectory,
pointing -Ldependency= to that subdirectory lets rustc find the hollow rlib (which has
the correct SVH matching downstream metadata). Pointing to the parent directory instead
would expose the full rlib (compiled separately, with a different SVH), causing E0460.

Args:
crate (CrateInfo): A CrateInfo provider from the current rule

Returns:
str: The directory to use for -Ldependency= search.
"""
if crate.metadata and crate.metadata_supports_pipelining:
return crate.metadata.dirname
return crate.output.dirname

def _portable_link_flags(lib, use_pic, ambiguous_libs, get_lib_name, for_windows = False, for_darwin = False, flavor_msvc = False):
artifact = get_preferred_artifact(lib, use_pic)
if ambiguous_libs and artifact.path in ambiguous_libs:
Expand Down
2 changes: 1 addition & 1 deletion rust/private/unpretty.bzl
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Original file line number Diff line number Diff line change
Expand Up @@ -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,
)

Expand Down
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