gen-protos: special case path for cfg(buck_build)

This is needed to emit the `.rs` files into the right build directory in a
follow up diff to add `BUCK` files.

lib/gen-protos/src/main.rs

@@ -23,11 +23,29 @@ fn main() -> Result<()> {
         "working_copy.proto",
     ];
 
-    let root = Path::new(env!("CARGO_MANIFEST_DIR")).parent().unwrap();
-    let protos_dir = root.join("src").join("protos");
+    let root = Path::new(env!("CARGO_MANIFEST_DIR"));
+    let protos_dir = if cfg!(buck_build) {
+        // NOTE (aseipp, buck_build): the cargo build allows `../src/protos`
+        // like below, but this is really awkward in the buck2 design since
+        // directory names are autogenerated. use `src/protos` instead.
+        root.join("src").join("protos")
+    } else {
+        root.parent().unwrap().join("src").join("protos")
+    };
+
+    let out_dir = if cfg!(buck_build) {
+        // NOTE (aseipp, buck_build): under `cfg(buck_build)` the genrule() that
+        // invokes this program will automatically set $OUT, so always use that
+        // and fail otherwise.
+        std::env::var("OUT")
+            .map(|s| Path::new(&s).to_path_buf())
+            .expect("OUT environment variable not set during cfg(buck_build)")
+    } else {
+        protos_dir.clone()
+    };
 
     prost_build::Config::new()
-        .out_dir(&protos_dir)
+        .out_dir(&out_dir)
         .include_file("mod.rs")
         // For old protoc versions. 3.12.4 needs this, but 3.21.12 doesn't.
         .protoc_arg("--experimental_allow_proto3_optional")