Report errors during preparation of crates

collector/README.md

@@ -450,3 +450,17 @@ profilers whose results are not affected by system noise (e.g. `callgrind` or `e
 
 `RUST_LOG=debug` can be specified to enable verbose logging, which is useful
 for debugging `collector` itself.
+
+
+## How `rustc` wrapping works
+When a crate is benchmarked or profiled, the real `rustc` is replaced with the `rustc-fake` binary,
+which parses commands passed from the `collector` and invokes the actual profiling or benchmarking
+tool.
+
+Profiling/benchmarking a crate is performed in two steps:
+1) Preparation - here all dependencies are compiled and build scripts are executed.
+During this step, `cargo` is invoked with `... -- --skip-this-rustc`, which causes `rustc-fake` to skip
+compilation of the final/leaf crate. Cargo only passes arguments after `--` to the final crate,
+therefore this does not affect the compilation of dependencies.
+2) Profiling/benchmarking - `cargo` is invoked with `--wrap-rustc-with <TOOL>`, which executes the
+specified profiling tool by `rustc-fake`.

collector/src/benchmark/mod.rs

@@ -253,15 +253,18 @@ impl Benchmark {
             for (profile, prep_dir) in &profile_dirs {
                 let server = server.clone();
                 s.spawn::<_, anyhow::Result<()>>(move |_| {
+                    // Panic the thread if an error occurs to make sure that the whole scope will
+                    // also panic if there was some error.
                     self.mk_cargo_process(compiler, prep_dir.path(), *profile)
                         .jobserver(server)
-                        .run_rustc(false)?;
+                        .run_rustc(false)
+                        .unwrap();
                     Ok(())
                 });
             }
             Ok(())
         })
-        .unwrap()?;
+        .expect("Preparation has failed")?;
 
         for (profile, prep_dir) in profile_dirs {
             eprintln!("Running {}: {:?} + {:?}", self.name, profile, scenarios);