ISLE: run rustfmt on generated code

cranelift/codegen/build.rs

@@ -218,9 +218,44 @@ fn rebuild_isle(crate_dir: &std::path::Path) -> Result<(), Box<dyn std::error::E
             impl std::error::Error for DebugReport {}
         })?;
 
+        let code = rustfmt(&code).unwrap_or_else(|e| {
+            println!(
+                "cargo:warning=Failed to run `rustfmt` on ISLE-generated code: {:?}",
+                e
+            );
+            code
+        });
+
         println!("Writing ISLE-generated Rust code to {}", out_file.display());
         std::fs::write(out_file, code)?;
     }
 
-    Ok(())
+    return Ok(());
+
+    fn rustfmt(code: &str) -> std::io::Result<String> {
+        use std::io::Write;
+
+        let mut rustfmt = std::process::Command::new("rustfmt")
+            .stdin(std::process::Stdio::piped())
+            .stdout(std::process::Stdio::piped())
+            .spawn()?;
+
+        let mut stdin = rustfmt.stdin.take().unwrap();
+        stdin.write_all(code.as_bytes())?;
+        drop(stdin);
+
+        let mut stdout = rustfmt.stdout.take().unwrap();
+        let mut data = vec![];
+        stdout.read_to_end(&mut data)?;
+
+        let status = rustfmt.wait()?;
+        if !status.success() {
+            return Err(std::io::Error::new(
+                std::io::ErrorKind::Other,
+                format!("`rustfmt` exited with status {}", status),
+            ));
+        }
+
+        Ok(String::from_utf8(data).expect("rustfmt always writs utf-8 to stdout"))
+    }
 }