tests: test that .dwp targets can be built

This has some things we'll eventually want to remove; see bazel-contrib#109 for some context.

tests/BUILD.bazel

@@ -14,6 +14,7 @@
 
 load("@bazel_skylib//rules:build_test.bzl", "build_test")
 load("@rules_cc//cc:defs.bzl", "cc_library", "cc_test")
+load(":transitions.bzl", "dwp_file")
 
 cc_library(
     name = "stdlib",
@@ -46,6 +47,32 @@ build_test(
     ]
 )
 
+# We want to test that `llvm-dwp` (used when assembling a `.dwp` file from
+# `.dwo` files) can be called.
+#
+# `--fission=yes` enables this for all compilation modes but we also need to
+# enable the `per_object_debug_info` feature manually because
+# `unix_cc_toolchain_config.bzl`'s` `cc_toolchain_config` does not (see #109).
+#
+# This is what `dwp_file` does using a transition.
+#
+# Unfortunately `per_object_debug_info` breaks on macOS which is why this
+# target is marked as only being compatible with Linux (see #109).
+dwp_file(
+    name = "stdlib.dwp",
+    src = ":stdlib_bin",
+    target_compatible_with = [
+        "@platforms//os:linux",
+    ],
+)
+
+build_test(
+    name = "dwp_test",
+    targets = [
+        ":stdlib.dwp",
+    ],
+)
+
 cc_test(
     name = "pthread_link_test",
     srcs = ["pthread_link_test.cc"],

tests/transitions.bzl

@@ -0,0 +1,58 @@
+"""Helper transitions for tests."""
+
+def _fission_transition_impl(settings, attr):
+    features = settings["//command_line_option:features"]
+    if "per_object_debug_info" in features:
+        features.remove("per_object_debug_info")
+
+    enable_per_object_debug_info = attr.per_object_debug_info
+    if enable_per_object_debug_info:
+        features.append("per_object_debug_info")
+
+    return {
+        "//command_line_option:fission": attr.fission,
+        "//command_line_option:features": features,
+    }
+
+fission_transition = transition(
+    implementation = _fission_transition_impl,
+    inputs = ["//command_line_option:features"],
+    outputs = [
+        "//command_line_option:features",
+        "//command_line_option:fission",
+    ],
+)
+
+def _dwp_file_impl(ctx):
+    file = ctx.attr.name
+    file = ctx.actions.declare_file(file)
+    ctx.actions.symlink(
+        output = file,
+        target_file = ctx.attr.src[0][DebugPackageInfo].dwp_file,
+    )
+
+    return [DefaultInfo(files = depset([file]))]
+
+dwp_file = rule(
+    implementation = _dwp_file_impl,
+    attrs = {
+        "src": attr.label(
+            cfg = fission_transition,
+            mandatory = True,
+            doc = "The actual target to build and grab the .dwp file from.",
+            providers = [DebugPackageInfo],
+        ),
+        # NOTE: we should eventually be able to remove this (see #109).
+        "per_object_debug_info": attr.bool(
+            default = True,
+        ),
+        "fission": attr.string(
+            default = "yes",
+            values = ["yes", "no", "dbg", "fastbuild", "opt"],
+        ),
+        "_allowlist_function_transition": attr.label(
+            default = "@bazel_tools//tools/allowlists/function_transition_allowlist"
+        ),
+    },
+    incompatible_use_toolchain_transition = True,
+)