Unrolled build for rust-lang#120096

Rollup merge of rust-lang#120096 - onur-ozkan:rustc_bootstrap, r=dtolnay

Set RUSTC_BOOTSTRAP=1 consistently

Fixes https://internals.rust-lang.org/t/rust-compiler-with-parallel-build/20099 which is a regression from rust-lang#120001

cc `@dtolnay` `@petrochenkov`

src/bootstrap/bootstrap.py

@@ -924,14 +924,17 @@ def build_bootstrap_cmd(self, env):
         # default toolchain is not nightly.
         #
         # But that setting has the collateral effect of rust-analyzer also
-        # passing RUSTC_BOOTSTRAP=1 to all x.py invocations too (the various
-        # overrideCommand). For compiling bootstrap, that is unwanted and can
-        # cause spurious rebuilding of bootstrap when rust-analyzer x.py
-        # invocations are interleaved with handwritten ones on the command line.
-        env.pop("RUSTC_BOOTSTRAP", None)
-
-        # preserve existing RUSTFLAGS
-        env.setdefault("RUSTFLAGS", "")
+        # passing RUSTC_BOOTSTRAP=1 to all x.py invocations too (the various overrideCommand).
+        # For compiling bootstrap that can cause spurious rebuilding of bootstrap when
+        # rust-analyzer x.py invocations are interleaved with handwritten ones on the
+        # command line.
+        #
+        # Set RUSTC_BOOTSTRAP=1 consistently.
+        env["RUSTC_BOOTSTRAP"] = "1"
+
+        default_rustflags = "" if env.get("RUSTFLAGS_BOOTSTRAP", "") else "-Zallow-features="
+
+        env.setdefault("RUSTFLAGS", default_rustflags)
 
         target_features = []
         if self.get_toml("crt-static", build_section) == "true":