Improve fidelity of rustc-crates-on-stable test

tests/run-make/rustc-crates-on-stable/rmake.rs

@@ -4,13 +4,19 @@
 use run_make_support::{cargo, rustc_path, source_root};
 
 fn main() {
-    // Use the stage0 beta cargo for the compilation (it shouldn't really matter which cargo we use)
+    // NOTE: in the following cargo invocation, make sure that no unstable cargo flags are used! We
+    // want to check that these compiler crates can compile on the stable channel, which includes
+    // treating this cargo as if it was a stable cargo! However, we can't really just "ask" a stage1
+    // cargo to pretend that it is a stable cargo, because these compiler crates are part of the
+    // same workspace, which necessarily requires that they can use unstable features and
+    // experimental editions (like edition 2024).
     cargo()
-        // Ensure `proc-macro2`'s nightly detection is disabled
+        // Ensure `proc-macro2`'s nightly detection is disabled: its build script avoids using
+        // nightly features when `RUSTC_STAGE` is set.
         .env("RUSTC_STAGE", "0")
         .env("RUSTC", rustc_path())
-        // We want to disallow all nightly features to simulate a stable build
-        .env("RUSTFLAGS", "-Zallow-features=")
+        // This forces the underlying rustc to think it is a stable rustc.
+        .env("RUSTC_BOOTSTRAP", "-1")
         .arg("build")
         .arg("--manifest-path")
         .arg(source_root().join("Cargo.toml"))