Buck: build and execute proc-macro2 build script

third-party/BUCK

@@ -1,5 +1,7 @@
 # To be generated by Facebook's `reindeer` tool once that is open source.
 
+load("//tools/buck:rust_library.bzl", "rust_library")
+
 rust_library(
     name = "bitflags",
     srcs = glob(["vendor/bitflags-1.3.1/src/**"]),
@@ -42,15 +44,11 @@ rust_library(
 rust_library(
     name = "proc-macro2",
     srcs = glob(["vendor/proc-macro2-1.0.28/src/**"]),
+    build_script = "vendor/proc-macro2-1.0.28/build.rs",
     features = [
         "proc-macro",
         "span-locations",
     ],
-    rustc_flags = [
-        "--cfg=span_locations",
-        "--cfg=use_proc_macro",
-        "--cfg=wrap_proc_macro",
-    ],
     visibility = ["PUBLIC"],
     deps = [":unicode-xid"],
 )

tools/buck/rust_library.bzl

@@ -0,0 +1,34 @@
+load("//tools/buck:genrule.bzl", "genrule")
+
+def rust_library(
+        name,
+        srcs,
+        features = [],
+        rustc_flags = [],
+        build_script = None,
+        **kwargs):
+    if build_script:
+        rust_binary(
+            name = "%s@build" % name,
+            srcs = srcs + [build_script],
+            crate = "build",
+            crate_root = build_script,
+            features = features,
+            rustc_flags = rustc_flags,
+        )
+
+        genrule(
+            name = "%s@cfg" % name,
+            out = "output",
+            cmd = "env RUSTC=rustc TARGET= $(exe :%s@build) | sed -n s/^cargo:rustc-cfg=/--cfg=/p > ${OUT}" % name,
+        )
+
+        rustc_flags = rustc_flags + ["@$(location :%s@cfg)" % name]
+
+    native.rust_library(
+        name = name,
+        srcs = srcs,
+        features = features,
+        rustc_flags = rustc_flags,
+        **kwargs
+    )