Add some additional comments

Author: Aaron1011

src/bin/cargo-miri.rs

@@ -526,6 +526,11 @@ fn in_cargo_miri() {
             cmd.arg(arg);
         }
 
+        // Serialize our actual args into a special environemt variable.
+        // This will be read by `inside_cargo_rustc` when we go to invoke
+        // our actual target crate (the binary or the test we are running).
+        // Since we're using "cargo check", we have no other way of passing
+        // these arguments.
         let args_vec: Vec<String> = args.collect();
         cmd.env("MIRI_MAGIC_ARGS", serde_json::to_string(&args_vec).expect("failed to serialize args"));
 
@@ -559,6 +564,10 @@ fn inside_cargo_rustc() {
 
     let in_build_script = is_build_dep();
 
+    // Build scripts need to be compiled to actual runnable executables,
+    // and therefore completely bypass Miri. We make sure to only specify
+    // our custom Xargo sysroot for non-build-script crate - that is,
+    // crates which are ultimately going to get interpreted by Miri.
     let mut args = if in_build_script {
         rustc_args.collect()
     } else {