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

rust/private/common.bzl

@@ -48,6 +48,8 @@ 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

@@ -24,6 +24,7 @@ CrateInfo = provider(
         "is_test": "bool: If the crate is being compiled in a test context",
         "name": "str: The name of this crate.",
         "output": "File: The output File that will be produced, depends on crate type.",
+        "metadata": "File: The rmeta file produced for this crate. It is optional.",
         "owner": "Label: The label of the target that produced this CrateInfo",
         "proc_macro_deps": "depset[DepVariantInfo]: This crate's rust proc_macro dependencies' providers.",
         "root": "File: The source File entrypoint to this crate, eg. lib.rs",
@@ -48,6 +49,7 @@ DepInfo = provider(
         "link_search_path_files": "depset[File]: All transitive files containing search paths to pass to the linker",
         "transitive_build_infos": "depset[BuildInfo]",
         "transitive_crate_outputs": "depset[File]: All transitive crate outputs.",
+        "transitive_metadata_outputs": "depset[File]: All transitive crate metadata outputs.",
         "transitive_crates": "depset[CrateInfo]",
         "transitive_noncrates": "depset[LinkerInput]: All transitive dependencies that aren't crates.",
     },

rust/private/rust.bzl

@@ -26,7 +26,7 @@ load(
     "name_to_crate_name",
     "transform_deps",
 )
-
+load("@bazel_skylib//lib:paths.bzl", "paths")
 # TODO(marco): Separate each rule into its own file.
 
 def _assert_no_deprecated_attributes(_ctx):
@@ -263,6 +263,19 @@ 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))
@@ -280,6 +293,7 @@ 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),
             rustc_env = ctx.attr.rustc_env,
             is_test = False,