Rollup merge of #118650 - RalfJung:flags-sync, r=clubby789

add comment about keeping flags in sync between bootstrap.py and bootstrap.rs

They got out of sync, probably because this comment was missing on the Python side (it only exists on the Rust side). #118642 brings the flags back in sync but does not fix the comment, so let's do that here.

r? clubby789

src/bootstrap/bootstrap.py

@@ -946,6 +946,8 @@ def build_bootstrap_cmd(self, env):
         target_linker = self.get_toml("linker", build_section)
         if target_linker is not None:
             env["RUSTFLAGS"] += " -C linker=" + target_linker
+        # When changing this list, also update the corresponding list in `Builder::cargo`
+        # in `src/bootstrap/src/core/builder.rs`.
         env["RUSTFLAGS"] += " -Wrust_2018_idioms -Wunused_lifetimes"
         if self.warnings == "default":
             deny_warnings = self.get_toml("deny-warnings", "rust") != "false"