rules_rust: enable pipelined compilation.

Pipelined compilation allows better parallelism during builds as it
allows libraries to generate lightweight metadata files to unlock other
depencies. These metadata files (.rmeta) can only be used to unlock
library -> library dependencies and do not affect builds in any other
way. This is currently the default in cargo:
https://internals.rust-lang.org/t/evaluating-pipelined-rustc-compilation/10199

Author: gigaroby

docs/flatten.md

@@ -1298,8 +1298,8 @@ A test rule for performing `rustfmt --check` on a set of targets
 ## CrateInfo
 
 <pre>
-CrateInfo(<a href="#CrateInfo-aliases">aliases</a>, <a href="#CrateInfo-compile_data">compile_data</a>, <a href="#CrateInfo-deps">deps</a>, <a href="#CrateInfo-edition">edition</a>, <a href="#CrateInfo-is_test">is_test</a>, <a href="#CrateInfo-name">name</a>, <a href="#CrateInfo-output">output</a>, <a href="#CrateInfo-owner">owner</a>, <a href="#CrateInfo-proc_macro_deps">proc_macro_deps</a>, <a href="#CrateInfo-root">root</a>,
-          <a href="#CrateInfo-rustc_env">rustc_env</a>, <a href="#CrateInfo-srcs">srcs</a>, <a href="#CrateInfo-type">type</a>, <a href="#CrateInfo-wrapped_crate_type">wrapped_crate_type</a>)
+CrateInfo(<a href="#CrateInfo-aliases">aliases</a>, <a href="#CrateInfo-compile_data">compile_data</a>, <a href="#CrateInfo-deps">deps</a>, <a href="#CrateInfo-edition">edition</a>, <a href="#CrateInfo-is_test">is_test</a>, <a href="#CrateInfo-metadata">metadata</a>, <a href="#CrateInfo-name">name</a>, <a href="#CrateInfo-output">output</a>, <a href="#CrateInfo-owner">owner</a>,
+          <a href="#CrateInfo-proc_macro_deps">proc_macro_deps</a>, <a href="#CrateInfo-root">root</a>, <a href="#CrateInfo-rustc_env">rustc_env</a>, <a href="#CrateInfo-srcs">srcs</a>, <a href="#CrateInfo-type">type</a>, <a href="#CrateInfo-wrapped_crate_type">wrapped_crate_type</a>)
 </pre>
 
 A provider containing general Crate information.
@@ -1314,6 +1314,7 @@ A provider containing general Crate information.
 | <a id="CrateInfo-deps"></a>deps |  depset[DepVariantInfo]: This crate's (rust or cc) dependencies' providers.    |
 | <a id="CrateInfo-edition"></a>edition |  str: The edition of this crate.    |
 | <a id="CrateInfo-is_test"></a>is_test |  bool: If the crate is being compiled in a test context    |
+| <a id="CrateInfo-metadata"></a>metadata |  File: The rmeta file produced for this crate. It is optional.    |
 | <a id="CrateInfo-name"></a>name |  str: The name of this crate.    |
 | <a id="CrateInfo-output"></a>output |  File: The output File that will be produced, depends on crate type.    |
 | <a id="CrateInfo-owner"></a>owner |  Label: The label of the target that produced this CrateInfo    |
@@ -1331,7 +1332,8 @@ A provider containing general Crate information.
 
 <pre>
 DepInfo(<a href="#DepInfo-dep_env">dep_env</a>, <a href="#DepInfo-direct_crates">direct_crates</a>, <a href="#DepInfo-link_search_path_files">link_search_path_files</a>, <a href="#DepInfo-transitive_build_infos">transitive_build_infos</a>,
-        <a href="#DepInfo-transitive_crate_outputs">transitive_crate_outputs</a>, <a href="#DepInfo-transitive_crates">transitive_crates</a>, <a href="#DepInfo-transitive_noncrates">transitive_noncrates</a>)
+        <a href="#DepInfo-transitive_crate_outputs">transitive_crate_outputs</a>, <a href="#DepInfo-transitive_crates">transitive_crates</a>, <a href="#DepInfo-transitive_metadata_outputs">transitive_metadata_outputs</a>,
+        <a href="#DepInfo-transitive_noncrates">transitive_noncrates</a>)
 </pre>
 
 A provider containing information about a Crate's dependencies.
@@ -1347,6 +1349,7 @@ A provider containing information about a Crate's dependencies.
 | <a id="DepInfo-transitive_build_infos"></a>transitive_build_infos |  depset[BuildInfo]    |
 | <a id="DepInfo-transitive_crate_outputs"></a>transitive_crate_outputs |  depset[File]: All transitive crate outputs.    |
 | <a id="DepInfo-transitive_crates"></a>transitive_crates |  depset[CrateInfo]    |
+| <a id="DepInfo-transitive_metadata_outputs"></a>transitive_metadata_outputs |  depset[File]: All transitive crate metadata outputs.    |
 | <a id="DepInfo-transitive_noncrates"></a>transitive_noncrates |  depset[LinkerInput]: All transitive dependencies that aren't crates.    |
 
 

docs/providers.md

@@ -10,8 +10,8 @@
 ## CrateInfo
 
 <pre>
-CrateInfo(<a href="#CrateInfo-aliases">aliases</a>, <a href="#CrateInfo-compile_data">compile_data</a>, <a href="#CrateInfo-deps">deps</a>, <a href="#CrateInfo-edition">edition</a>, <a href="#CrateInfo-is_test">is_test</a>, <a href="#CrateInfo-name">name</a>, <a href="#CrateInfo-output">output</a>, <a href="#CrateInfo-owner">owner</a>, <a href="#CrateInfo-proc_macro_deps">proc_macro_deps</a>, <a href="#CrateInfo-root">root</a>,
-          <a href="#CrateInfo-rustc_env">rustc_env</a>, <a href="#CrateInfo-srcs">srcs</a>, <a href="#CrateInfo-type">type</a>, <a href="#CrateInfo-wrapped_crate_type">wrapped_crate_type</a>)
+CrateInfo(<a href="#CrateInfo-aliases">aliases</a>, <a href="#CrateInfo-compile_data">compile_data</a>, <a href="#CrateInfo-deps">deps</a>, <a href="#CrateInfo-edition">edition</a>, <a href="#CrateInfo-is_test">is_test</a>, <a href="#CrateInfo-metadata">metadata</a>, <a href="#CrateInfo-name">name</a>, <a href="#CrateInfo-output">output</a>, <a href="#CrateInfo-owner">owner</a>,
+          <a href="#CrateInfo-proc_macro_deps">proc_macro_deps</a>, <a href="#CrateInfo-root">root</a>, <a href="#CrateInfo-rustc_env">rustc_env</a>, <a href="#CrateInfo-srcs">srcs</a>, <a href="#CrateInfo-type">type</a>, <a href="#CrateInfo-wrapped_crate_type">wrapped_crate_type</a>)
 </pre>
 
 A provider containing general Crate information.
@@ -26,6 +26,7 @@ A provider containing general Crate information.
 | <a id="CrateInfo-deps"></a>deps |  depset[DepVariantInfo]: This crate's (rust or cc) dependencies' providers.    |
 | <a id="CrateInfo-edition"></a>edition |  str: The edition of this crate.    |
 | <a id="CrateInfo-is_test"></a>is_test |  bool: If the crate is being compiled in a test context    |
+| <a id="CrateInfo-metadata"></a>metadata |  File: The rmeta file produced for this crate. It is optional.    |
 | <a id="CrateInfo-name"></a>name |  str: The name of this crate.    |
 | <a id="CrateInfo-output"></a>output |  File: The output File that will be produced, depends on crate type.    |
 | <a id="CrateInfo-owner"></a>owner |  Label: The label of the target that produced this CrateInfo    |
@@ -43,7 +44,8 @@ A provider containing general Crate information.
 
 <pre>
 DepInfo(<a href="#DepInfo-dep_env">dep_env</a>, <a href="#DepInfo-direct_crates">direct_crates</a>, <a href="#DepInfo-link_search_path_files">link_search_path_files</a>, <a href="#DepInfo-transitive_build_infos">transitive_build_infos</a>,
-        <a href="#DepInfo-transitive_crate_outputs">transitive_crate_outputs</a>, <a href="#DepInfo-transitive_crates">transitive_crates</a>, <a href="#DepInfo-transitive_noncrates">transitive_noncrates</a>)
+        <a href="#DepInfo-transitive_crate_outputs">transitive_crate_outputs</a>, <a href="#DepInfo-transitive_crates">transitive_crates</a>, <a href="#DepInfo-transitive_metadata_outputs">transitive_metadata_outputs</a>,
+        <a href="#DepInfo-transitive_noncrates">transitive_noncrates</a>)
 </pre>
 
 A provider containing information about a Crate's dependencies.
@@ -59,6 +61,7 @@ A provider containing information about a Crate's dependencies.
 | <a id="DepInfo-transitive_build_infos"></a>transitive_build_infos |  depset[BuildInfo]    |
 | <a id="DepInfo-transitive_crate_outputs"></a>transitive_crate_outputs |  depset[File]: All transitive crate outputs.    |
 | <a id="DepInfo-transitive_crates"></a>transitive_crates |  depset[CrateInfo]    |
+| <a id="DepInfo-transitive_metadata_outputs"></a>transitive_metadata_outputs |  depset[File]: All transitive crate metadata outputs.    |
 | <a id="DepInfo-transitive_noncrates"></a>transitive_noncrates |  depset[LinkerInput]: All transitive dependencies that aren't crates.    |
 
 

proto/proto.bzl

@@ -212,6 +212,11 @@ def _rust_proto_compile(protos, descriptor_sets, imports, crate_name, ctx, is_gr
         crate_name,
         output_hash,
     ))
+    rust_metadata = ctx.actions.declare_file("%s/lib%s-%s.rmeta" % (
+        output_dir,
+        crate_name,
+        output_hash,
+    ))
 
     # Gather all dependencies for compilation
     compile_action_deps = depset(
@@ -234,6 +239,7 @@ def _rust_proto_compile(protos, descriptor_sets, imports, crate_name, ctx, is_gr
             proc_macro_deps = depset([]),
             aliases = {},
             output = rust_lib,
+            metadata = rust_metadata,
             edition = proto_toolchain.edition,
             rustc_env = {},
             is_test = False,

rust/private/common.bzl

@@ -47,6 +47,8 @@ def _create_crate_info(**kwargs):
     """
     if not "wrapped_crate_type" in kwargs:
         kwargs.update({"wrapped_crate_type": None})
+    if not "metadata" in kwargs:
+        kwargs.update({"metadata": None})
     return CrateInfo(**kwargs)
 
 rust_common = struct(

rust/private/providers.bzl

@@ -22,6 +22,7 @@ CrateInfo = provider(
         "deps": "depset[DepVariantInfo]: This crate's (rust or cc) dependencies' providers.",
         "edition": "str: The edition of this crate.",
         "is_test": "bool: If the crate is being compiled in a test context",
+        "metadata": "File: The rmeta file produced for this crate. It is optional.",
         "name": "str: The name of this crate.",
         "output": "File: The output File that will be produced, depends on crate type.",
         "owner": "Label: The label of the target that produced this CrateInfo",
@@ -49,6 +50,7 @@ DepInfo = provider(
         "transitive_build_infos": "depset[BuildInfo]",
         "transitive_crate_outputs": "depset[File]: All transitive crate outputs.",
         "transitive_crates": "depset[CrateInfo]",
+        "transitive_metadata_outputs": "depset[File]: All transitive crate metadata outputs.",
         "transitive_noncrates": "depset[LinkerInput]: All transitive dependencies that aren't crates.",
     },
 )

rust/private/rust.bzl

@@ -13,6 +13,7 @@
 # limitations under the License.
 
 # buildifier: disable=module-docstring
+load("@bazel_skylib//lib:paths.bzl", "paths")
 load("//rust/private:common.bzl", "rust_common")
 load("//rust/private:rustc.bzl", "rustc_compile_action")
 load(
@@ -26,7 +27,6 @@ load(
     "name_to_crate_name",
     "transform_deps",
 )
-
 # TODO(marco): Separate each rule into its own file.
 
 def _assert_no_deprecated_attributes(_ctx):
@@ -266,6 +266,20 @@ def _rust_library_common(ctx, crate_type):
         output_hash,
     )
     rust_lib = ctx.actions.declare_file(rust_lib_name)
+    rust_metadata = None
+
+    # We can only build metadata iif
+    # - Pipelined compilation is enabled,
+    # - we are building with process wrapper, and
+    # - the current crate is a lib or rlib
+    can_build_metadata = toolchain._pipelined_compilation and ctx.attr._process_wrapper and crate_type in ("rlib", "lib")
+
+    # can_build_metadata = ctx.attr._process_wrapper and crate_type in ("rlib", "lib")
+    if can_build_metadata:
+        rust_metadata = ctx.actions.declare_file(
+            paths.replace_extension(rust_lib_name, ".rmeta"),
+            sibling = rust_lib,
+        )
 
     deps = transform_deps(ctx.attr.deps)
     proc_macro_deps = transform_deps(ctx.attr.proc_macro_deps + get_import_macro_deps(ctx))
@@ -283,6 +297,7 @@ def _rust_library_common(ctx, crate_type):
             proc_macro_deps = depset(proc_macro_deps),
             aliases = ctx.attr.aliases,
             output = rust_lib,
+            metadata = rust_metadata,
             edition = get_edition(ctx.attr, toolchain, ctx.label),
             rustc_env = ctx.attr.rustc_env,
             is_test = False,