compiletest: Add warning and comment about running tests without RUSTC

src/tools/compiletest/src/header/tests.rs

@@ -58,7 +58,22 @@ fn config() -> Config {
     ];
     let mut args: Vec<String> = args.iter().map(ToString::to_string).collect();
     args.push("--rustc-path".to_string());
-    args.push(std::env::var("RUSTC").unwrap_or_else(|_| "rustc".to_string()));
+    // This is a subtle/fragile thing. On rust-lang CI, there is no global
+    // `rustc`, and Cargo doesn't offer a convenient way to get the path to
+    // `rustc`. Fortunately bootstrap sets `RUSTC` for us, which is pointing
+    // to the stage0 compiler.
+    //
+    // Otherwise, if you are running compiletests's tests manually, you
+    // probably don't have `RUSTC` set, in which case this falls back to the
+    // global rustc. If your global rustc is too far out of sync with stage0,
+    // then this may cause confusing errors. Or if for some reason you don't
+    // have rustc in PATH, that would also fail.
+    args.push(std::env::var("RUSTC").unwrap_or_else(|_| {
+        eprintln!(
+            "warning: RUSTC not set, using global rustc (are you not running via bootstrap?)"
+        );
+        "rustc".to_string()
+    }));
     crate::parse_config(args)
 }