Rollup merge of rust-lang#104797 - weihanglo:stream-write-dwp, r=jackh726

rustc_codegen_ssa: write `.dwp` in a streaming fashion

When writing a `.dwp` file, rustc writes to a Vec first then to a BufWriter-wrapped file. It seems very likely that we can write in a streaming fashion to avoid double buffering in an intermediate Vec.

On my Linux machine, `.dwp` from the latest rust-lang/cargo is 113MiB. It may worth a stream writer, though I didn't do any benchmark 🙇🏾‍♂️.

Author: matthiaskrgr

compiler/rustc_codegen_ssa/src/back/link.rs

@@ -676,17 +676,18 @@ fn link_dwarf_object<'a>(
             thorin::MissingReferencedObjectBehaviour::Skip,
         )?;
 
-        let output = package.finish()?.write()?;
-        let mut output_stream = BufWriter::new(
+        let output_stream = BufWriter::new(
             OpenOptions::new()
                 .read(true)
                 .write(true)
                 .create(true)
                 .truncate(true)
                 .open(dwp_out_filename)?,
         );
-        output_stream.write_all(&output)?;
-        output_stream.flush()?;
+        let mut output_stream = object::write::StreamingBuffer::new(output_stream);
+        package.finish()?.emit(&mut output_stream)?;
+        output_stream.result()?;
+        output_stream.into_inner().flush()?;
 
         Ok(())
     }) {