feat: per-crate rustc flag configuration trimming

crate_universe/src/rendering.rs

@@ -798,6 +798,10 @@ impl Renderer {
                 ),
                 platforms,
             ),
+            // Enable configuration trimming for third-party crates to improve cache hit rates.
+            // This ensures that per_crate_rustc_flag settings (which don't affect third-party
+            // crates) don't cause unnecessary rebuilds.
+            skip_per_crate_rustc_flags: true,
             srcs: target.srcs.clone(),
             tags: {
                 let mut tags = BTreeSet::from_iter(krate.common_attrs.tags.iter().cloned());

crate_universe/src/utils/starlark.rs

@@ -210,6 +210,10 @@ pub(crate) struct CommonAttrs {
     pub(crate) rustc_env_files: SelectSet<String>,
     #[serde(skip_serializing_if = "SelectList::is_empty")]
     pub(crate) rustc_flags: SelectList<String>,
+    /// Trim per_crate_rustc_flag from configuration to improve cache hit rates.
+    /// This is set to true for third-party crates generated by crate_universe.
+    #[serde(skip_serializing_if = "std::ops::Not::not")]
+    pub(crate) skip_per_crate_rustc_flags: bool,
     pub(crate) srcs: Glob,
     #[serde(skip_serializing_if = "Set::is_empty")]
     pub(crate) tags: Set<String>,

rust/private/per_crate_flag_trim.bzl

@@ -0,0 +1,60 @@
+# Copyright 2024 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+"""Transition to trim per_crate_rustc_flag for non-matching targets.
+
+This module provides a configuration trimming mechanism for the
+`experimental_per_crate_rustc_flag` setting. When a target has
+`skip_per_crate_rustc_flags = True`, this transition clears the setting,
+putting the target back into a canonical configuration.
+
+This is useful for third-party crates (e.g., from crate_universe) that will
+never match any per-crate flag filter. Without trimming, these crates would
+be rebuilt unnecessarily when any per-crate flag is set, even though the
+filter doesn't match them.
+
+Usage:
+    Third-party crate generators (like crate_universe) should set
+    `skip_per_crate_rustc_flags = True` on generated rust_library targets.
+"""
+
+_PER_CRATE_FLAG_SETTING = "@rules_rust//rust/settings:experimental_per_crate_rustc_flag"
+
+def _per_crate_flag_trim_transition_impl(settings, attr):
+    """Clear per_crate_rustc_flag for targets marked to skip it.
+
+    Args:
+        settings: A dict of current build settings.
+        attr: The attributes of the target being configured.
+
+    Returns:
+        A dict with the per_crate_rustc_flag setting (cleared or preserved).
+    """
+    # If this target is marked to skip per-crate flags, clear the setting
+    # to return it to a canonical configuration
+    if getattr(attr, "skip_per_crate_rustc_flags", False):
+        return {
+            _PER_CRATE_FLAG_SETTING: [],
+        }
+
+    # Otherwise, keep the current value
+    return {
+        _PER_CRATE_FLAG_SETTING: settings[_PER_CRATE_FLAG_SETTING],
+    }
+
+per_crate_flag_trim_transition = transition(
+    implementation = _per_crate_flag_trim_transition_impl,
+    inputs = [_PER_CRATE_FLAG_SETTING],
+    outputs = [_PER_CRATE_FLAG_SETTING],
+)

rust/private/rust.bzl

@@ -25,6 +25,10 @@ load(
     "CrateInfo",
     "LintsInfo",
 )
+load(
+    ":per_crate_flag_trim.bzl",
+    "per_crate_flag_trim_transition",
+)
 load(
     ":rust_allocator_libraries.bzl",
     "RUSTC_ALLOCATOR_LIBRARIES_ATTRS",
@@ -56,6 +60,9 @@ load(
 
 # TODO(marco): Separate each rule into its own file.
 
+# Setting path for per_crate_rustc_flag, used in transition definitions
+_PER_CRATE_FLAG_SETTING = "@rules_rust//rust/settings:experimental_per_crate_rustc_flag"
+
 def _assert_no_deprecated_attributes(_ctx):
     """Forces a failure if any deprecated attributes were specified
 
@@ -808,6 +815,17 @@ _COMMON_ATTRS = {
         doc = "Enable collection of cfg flags with results stored in CrateInfo.cfgs.",
         default = Label("//rust/settings:collect_cfgs"),
     ),
+    "skip_per_crate_rustc_flags": attr.bool(
+        doc = dedent("""\
+            If True, the `experimental_per_crate_rustc_flag` setting is trimmed from this
+            target's configuration. This puts the target back into a canonical configuration,
+            improving cache hit rates for targets that would never match any per-crate filter.
+
+            This attribute is primarily used by crate_universe for generated third-party crates.
+            First-party crates should leave this as False (the default).
+        """),
+        default = False,
+    ),
 } | RUSTC_ATTRS | RUSTC_ALLOCATOR_LIBRARIES_ATTRS
 
 _PLATFORM_ATTRS = {
@@ -912,6 +930,7 @@ _RUST_TEST_ATTRS = {
 rust_library = rule(
     implementation = _rust_library_impl,
     provides = COMMON_PROVIDERS,
+    cfg = per_crate_flag_trim_transition,
     attrs = _COMMON_ATTRS | {
         "disable_pipelining": attr.bool(
             default = False,
@@ -921,6 +940,9 @@ rust_library = rule(
                 crates will instead use the `.rlib` file.
             """),
         ),
+        "_allowlist_function_transition": attr.label(
+            default = "@bazel_tools//tools/allowlists/function_transition_allowlist",
+        ),
     },
     fragments = ["cpp"],
     toolchains = [
@@ -994,17 +1016,22 @@ rust_library = rule(
 )
 
 def _rust_static_library_transition_impl(settings, attr):
+    # Trim per_crate_rustc_flag for third-party crates to improve cache hit rates
+    per_crate_flags = [] if getattr(attr, "skip_per_crate_rustc_flags", False) else settings[_PER_CRATE_FLAG_SETTING]
     return {
         "//command_line_option:platforms": str(attr.platform) if attr.platform else settings["//command_line_option:platforms"],
+        _PER_CRATE_FLAG_SETTING: per_crate_flags,
     }
 
 _rust_static_library_transition = transition(
     implementation = _rust_static_library_transition_impl,
     inputs = [
         "//command_line_option:platforms",
+        _PER_CRATE_FLAG_SETTING,
     ],
     outputs = [
         "//command_line_option:platforms",
+        _PER_CRATE_FLAG_SETTING,
     ],
 )
 
@@ -1035,17 +1062,22 @@ rust_static_library = rule(
 )
 
 def _rust_shared_library_transition_impl(settings, attr):
+    # Trim per_crate_rustc_flag for third-party crates to improve cache hit rates
+    per_crate_flags = [] if getattr(attr, "skip_per_crate_rustc_flags", False) else settings[_PER_CRATE_FLAG_SETTING]
     return {
         "//command_line_option:platforms": str(attr.platform) if attr.platform else settings["//command_line_option:platforms"],
+        _PER_CRATE_FLAG_SETTING: per_crate_flags,
     }
 
 _rust_shared_library_transition = transition(
     implementation = _rust_shared_library_transition_impl,
     inputs = [
         "//command_line_option:platforms",
+        _PER_CRATE_FLAG_SETTING,
     ],
     outputs = [
         "//command_line_option:platforms",
+        _PER_CRATE_FLAG_SETTING,
     ],
 )
 
@@ -1090,6 +1122,7 @@ _proc_macro_dep_transition = transition(
 rust_proc_macro = rule(
     implementation = _rust_proc_macro_impl,
     provides = COMMON_PROVIDERS,
+    cfg = per_crate_flag_trim_transition,
     # Start by copying the common attributes, then override the `deps` attribute
     # to apply `_proc_macro_dep_transition`. To add this transition we additionally
     # need to declare `_allowlist_function_transition`, see
@@ -1157,17 +1190,22 @@ _RUST_BINARY_ATTRS = {
 } | _EXPERIMENTAL_USE_CC_COMMON_LINK_ATTRS
 
 def _rust_binary_transition_impl(settings, attr):
+    # Trim per_crate_rustc_flag for third-party crates to improve cache hit rates
+    per_crate_flags = [] if getattr(attr, "skip_per_crate_rustc_flags", False) else settings[_PER_CRATE_FLAG_SETTING]
     return {
         "//command_line_option:platforms": str(attr.platform) if attr.platform else settings["//command_line_option:platforms"],
+        _PER_CRATE_FLAG_SETTING: per_crate_flags,
     }
 
 _rust_binary_transition = transition(
     implementation = _rust_binary_transition_impl,
     inputs = [
         "//command_line_option:platforms",
+        _PER_CRATE_FLAG_SETTING,
     ],
     outputs = [
         "//command_line_option:platforms",
+        _PER_CRATE_FLAG_SETTING,
     ],
 )
 
@@ -1404,17 +1442,22 @@ rust_test_without_process_wrapper_test = rule(
 )
 
 def _rust_test_transition_impl(settings, attr):
+    # Trim per_crate_rustc_flag for third-party crates to improve cache hit rates
+    per_crate_flags = [] if getattr(attr, "skip_per_crate_rustc_flags", False) else settings[_PER_CRATE_FLAG_SETTING]
     return {
         "//command_line_option:platforms": str(attr.platform) if attr.platform else settings["//command_line_option:platforms"],
+        _PER_CRATE_FLAG_SETTING: per_crate_flags,
     }
 
 _rust_test_transition = transition(
     implementation = _rust_test_transition_impl,
     inputs = [
         "//command_line_option:platforms",
+        _PER_CRATE_FLAG_SETTING,
     ],
     outputs = [
         "//command_line_option:platforms",
+        _PER_CRATE_FLAG_SETTING,
     ],
 )