--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/async-google-apis-common/src/multipart.rs	Mon Oct 19 19:27:51 2020 +0200
@@ -0,0 +1,34 @@
+//! Format and stream a multipart/related request, for uploads.
+
+use hyper::{self, body::Bytes};
+use radix64::STD;
+use serde::Serialize;
+use std::io::Write;
+
+pub const MIME_BOUNDARY: &'static str = "PB0BHe6XN3O6Q4bpnWQgS1pKfMfglTZdifFvh8YIc2APj4Cz3C";
+
+pub fn format_multipart<Req: Serialize>(req: &Req, data: Bytes) -> anyhow::Result<Bytes> {
+    let meta = serde_json::to_string(req)?;
+    let mut buf = Vec::with_capacity(meta.len() + (1.5 * (data.len() as f64)) as usize);
+
+    // Write metadata.
+    buf.write(format!("--{}\n", MIME_BOUNDARY).as_bytes())
+        .unwrap();
+    buf.write("Content-Type: application/json; charset=UTF-8\n\n".as_bytes())
+        .unwrap();
+    buf.write(meta.as_bytes())?;
+
+    buf.write(format!("\n\n--{}\n", MIME_BOUNDARY).as_bytes())
+        .unwrap();
+    buf.write("Content-Transfer-Encoding: base64\n\n".as_bytes())
+        .unwrap();
+
+    // Write data contents.
+    let mut ew = radix64::io::EncodeWriter::new(STD, buf);
+    ew.write(data.as_ref())?;
+
+    let mut buf = ew.finish()?;
+    buf.write(format!("\n\n--{}--\n", MIME_BOUNDARY).as_bytes())
+        .unwrap();
+    Ok(Bytes::from(buf))
+}