Use -Zbuilt-std to build libstd on the fly from rust-src sources.

Author: eddyb

collector/src/bin/rustc-perf-collector/execute.rs

@@ -203,6 +203,9 @@ impl<'a> CargoProcess<'a> {
         loop {
             let mut cmd = self.base_command(self.cwd, "rustc");
             cmd.arg("-p").arg(self.get_pkgid(self.cwd));
+            cmd.arg("-Zbuild-std")
+                .arg("--target")
+                .arg(self.compiler.triple);
             match self.build_kind {
                 BuildKind::Check => {
                     cmd.arg("--profile").arg("check");
@@ -544,9 +547,7 @@ impl<'a> Processor for ProfileProcessor<'a> {
 
                 // Run `summarize`.
                 let mut summarize_cmd = Command::new("summarize");
-                summarize_cmd
-                    .arg("summarize")
-                    .arg(&zsp_files_prefix);
+                summarize_cmd.arg("summarize").arg(&zsp_files_prefix);
                 let output = summarize_cmd.output()?;
                 fs::write(&summarize_file, &output.stdout)?;