crate_universe: Make module extension restarts less expensive.

crate_universe/extension.bzl

@@ -8,7 +8,7 @@ load("//crate_universe:defs.bzl", _crate_universe_crate = "crate")
 load("//crate_universe/private:crates_vendor.bzl", "CRATES_VENDOR_ATTRS", "generate_config_file", "generate_splicing_manifest")
 load("//crate_universe/private:generate_utils.bzl", "CARGO_BAZEL_GENERATOR_SHA256", "CARGO_BAZEL_GENERATOR_URL", "GENERATOR_ENV_VARS", "render_config")
 load("//crate_universe/private:urls.bzl", "CARGO_BAZEL_SHA256S", "CARGO_BAZEL_URLS")
-load("//crate_universe/private/module_extensions:cargo_bazel_bootstrap.bzl", "get_cargo_bazel_runner")
+load("//crate_universe/private/module_extensions:cargo_bazel_bootstrap.bzl", "get_cargo_bazel_runner", "get_host_cargo_rustc")
 load("//rust/platform:triple.bzl", "get_host_triple")
 
 # A list of labels which may be relative (and if so, is within the repo the rule is generated in).
@@ -228,7 +228,7 @@ def _package_to_json(p):
     })
 
 def _get_generator(module_ctx):
-    """Query Network Resources to local a `cargo-bazel` binary.  
+    """Query Network Resources to local a `cargo-bazel` binary.
 
     Based off get_generator in crates_universe/private/generate_utils.bzl
 
@@ -276,6 +276,16 @@ def _get_generator(module_ctx):
     return output
 
 def _crate_impl(module_ctx):
+    # Preload all external repositories. Calling `module_ctx.path` will cause restarts of the implementation
+    # function of the module extension, so we want to trigger all restarts before we start the actual work.
+    # Once https://github.com/bazelbuild/bazel/issues/22729 has been fixed, this code can be removed.
+    get_host_cargo_rustc(module_ctx)
+    for mod in module_ctx.modules:
+        for cfg in mod.tags.from_cargo:
+            module_ctx.path(cfg.cargo_lockfile)
+            for m in cfg.manifests:
+                module_ctx.path(m)
+
     cargo_bazel_output = _get_generator(module_ctx)
     cargo_bazel = get_cargo_bazel_runner(module_ctx, cargo_bazel_output)
 

crate_universe/private/module_extensions/cargo_bazel_bootstrap.bzl

@@ -15,21 +15,32 @@ cargo_bazel_bootstrap = module_extension(
     doc = """Module extension to generate the cargo_bazel binary.""",
 )
 
-def get_cargo_bazel_runner(module_ctx, cargo_bazel):
-    """A helper function to allow executing cargo_bazel in module extensions.
+def get_host_cargo_rustc(module_ctx):
+    """A helper function to get the path to the host cargo and rustc binaries.
 
     Args:
         module_ctx: The module extension's context.
-        cargo_bazel: Path The path to a `cargo-bazel` binary
     Returns:
-        A function that can be called to execute cargo_bazel.
-    """
+        A tuple of path to cargo, path to rustc.
 
+    """
     host_triple = get_host_triple(module_ctx)
     binary_ext = system_to_binary_ext(host_triple.system)
 
     cargo_path = str(module_ctx.path(Label("@rust_host_tools//:bin/cargo{}".format(binary_ext))))
     rustc_path = str(module_ctx.path(Label("@rust_host_tools//:bin/rustc{}".format(binary_ext))))
+    return cargo_path, rustc_path
+
+def get_cargo_bazel_runner(module_ctx, cargo_bazel):
+    """A helper function to allow executing cargo_bazel in module extensions.
+
+    Args:
+        module_ctx: The module extension's context.
+        cargo_bazel: Path The path to a `cargo-bazel` binary
+    Returns:
+        A function that can be called to execute cargo_bazel.
+    """
+    cargo_path, rustc_path = get_host_cargo_rustc(module_ctx)
 
     # Placing this as a nested function allows users to call this right at the
     # start of a module extension, thus triggering any restarts as early as