Ignore artifact messages on rustc stderr

These recently started getting emitted, but we don't want them polluting
eprintln files. This filters them out based on the expected prefix for these
lines, which should hopefully not happen by chance (e.g., if someone is
intentionally adding eprintlns).

Author: Mark-Simulacrum

collector/src/execute.rs

@@ -12,8 +12,7 @@ use std::env;
 use std::fmt;
 use std::fs::{self, File};
 use std::hash;
-use std::io::Read;
-use std::io::Write;
+use std::io::{BufRead, Read, Write};
 use std::mem::ManuallyDrop;
 use std::path::{Path, PathBuf};
 use std::process::{self, Command};
@@ -1144,7 +1143,21 @@ impl<'a> Processor for ProfileProcessor<'a> {
                 let tmp_eprintln_file = filepath(data.cwd.as_ref(), "eprintln");
                 let eprintln_file = filepath(self.output_dir, &out_file("eprintln"));
 
-                fs::copy(&tmp_eprintln_file, &eprintln_file)?;
+                let mut final_file =
+                    std::io::BufWriter::new(std::fs::File::create(&tmp_eprintln_file)?);
+
+                for line in std::io::BufReader::new(std::fs::File::open(&eprintln_file)?).lines() {
+                    let line = line?;
+                    // rustc under Cargo currently ~always emits artifact
+                    // messages -- which we don't want in final
+                    // eprintln output. These messages generally look like:
+                    // {"artifact":"/tmp/.tmpjIe45J/...","emit":"dep-info"}
+                    if line.starts_with(r#"{"artifact":"#) {
+                        continue;
+                    }
+
+                    writeln!(&mut final_file, "{}", line)?;
+                }
             }
 
             // mono item results are redirected (via rustc-fake) to a file