Add the generated_crate_root attribute to cargo_build_script

This change allows us to declare that a build script generates a crate root.
Dependent targets created with `rust_library` and `rust_binary` will respect
the setting.

Author: mrkkrp

cargo/private/cargo_build_script.bzl

@@ -610,6 +610,7 @@ def _cargo_build_script_impl(ctx):
         DefaultInfo(files = depset([out_dir])),
         BuildInfo(
             out_dir = out_dir,
+            build_script_crate_root = ctx.attr.generated_crate_root,
             rustc_env = env_out,
             dep_env = dep_env_out,
             flags = flags_out,
@@ -659,6 +660,9 @@ cargo_build_script = rule(
             providers = [[DepInfo], [CrateGroupInfo]],
             cfg = "exec",
         ),
+        "generated_crate_root": attr.string(
+            doc = "The location of the crate root when it is generated by the build script",
+        ),
         "link_deps": attr.label_list(
             doc = dedent("""\
                 The subset of the Rust (normal) dependencies of the crate that

rust/private/providers.bzl

@@ -85,6 +85,7 @@ BuildInfo = provider(
         "link_search_paths": "Optional[File]: file containing search paths to pass to rustc and linker",
         "linker_flags": "Optional[File]: file containing flags to pass to the linker invoked by rustc or cc_common.link",
         "out_dir": "Optional[File]: directory containing the result of a build script",
+        "build_script_crate_root": "Optional[String]: the crate root that was generated by a build script",
         "rustc_env": "Optional[File]: file containing additional environment variables to set for rustc.",
     },
 )

rust/private/rust.bzl

@@ -33,6 +33,7 @@ load(
     "can_build_metadata",
     "can_use_metadata_for_pipelining",
     "compute_crate_name",
+    "crate_root_from_build_scripts",
     "crate_root_src",
     "dedent",
     "deduplicate",
@@ -169,7 +170,7 @@ def _rust_library_common(ctx, crate_type):
 
     crate_root = getattr(ctx.file, "crate_root", None)
     if not crate_root:
-        crate_root = crate_root_src(ctx.attr.name, ctx.attr.crate_name, ctx.files.srcs, crate_type)
+        crate_root = crate_root_from_build_scripts(ctx) or crate_root_src(ctx.attr.name, ctx.attr.crate_name, ctx.files.srcs, crate_type)
     srcs, compile_data, crate_root = transform_sources(ctx, ctx.files.srcs, ctx.files.compile_data, crate_root)
 
     # Determine unique hash for this rlib.
@@ -265,7 +266,7 @@ def _rust_binary_impl(ctx):
 
     crate_root = getattr(ctx.file, "crate_root", None)
     if not crate_root:
-        crate_root = crate_root_src(ctx.attr.name, ctx.attr.crate_name, ctx.files.srcs, ctx.attr.crate_type)
+        crate_root = crate_root_from_build_scripts(ctx) or crate_root_src(ctx.attr.name, ctx.attr.crate_name, ctx.files.srcs, ctx.attr.crate_type)
     srcs, compile_data, crate_root = transform_sources(ctx, ctx.files.srcs, ctx.files.compile_data, crate_root)
 
     rust_metadata = None

rust/private/utils.bzl

@@ -756,6 +756,28 @@ def can_use_metadata_for_pipelining(toolchain, crate_type):
     return toolchain._pipelined_compilation and \
            crate_type in ("rlib", "lib")
 
+def crate_root_from_build_scripts(ctx):
+    """
+    Identify the generated crate root that comes from a build script (if any)
+    and return a symlink to it (File).
+
+    Args:
+        ctx: (ctx): The current rule's context object
+    """
+    candidate_filename = None
+    candidate_path = None
+    for dep in ctx.attr.deps:
+        if BuildInfo in dep:
+            if dep[BuildInfo].build_script_crate_root:
+                candidate_filename = dep[BuildInfo].build_script_crate_root
+                candidate_path = dep[BuildInfo].out_dir.path + "/" + candidate_filename
+                break
+    if candidate_path:
+        crate_root = ctx.actions.declare_symlink(candidate_filename)
+        symlink_target = relativize(candidate_path, crate_root.dirname)
+        ctx.actions.symlink(output = crate_root, target_path = symlink_target)
+        return crate_root
+
 def crate_root_src(name, crate_name, srcs, crate_type):
     """Determines the source file for the crate root, should it not be specified in `attr.crate_root`.