proc-macros: add BUCK files

BUCK

@@ -3,5 +3,6 @@
     alias(name = name, actual = actual)
         for (name, actual) in [
             # (top-level name, fully-qualified target name)
+            ("proc-macros", "//lib/proc-macros:proc-macros"),
         ]
 ]

lib/proc-macros/BUCK

@@ -0,0 +1,21 @@
+
+load("//buck/shims/jj.bzl", "jj")
+
+alias(
+    # NOTE: default target for this package
+    name = 'proc-macros',
+    actual = ':jj-lib-proc-macros',
+    visibility = [ 'PUBLIC' ],
+)
+
+jj.rust_library(
+    name = 'jj-lib-proc-macros',
+    srcs = glob(["src/**/*.rs"]),
+    visibility = [ "PUBLIC" ],
+    proc_macro = True,
+    deps = [
+        'third-party//rust:proc-macro2',
+        'third-party//rust:quote',
+        'third-party//rust:syn',
+    ]
+)