Disallow setting built-in cfgs via set the command-line

compiler/rustc_lint/src/context/diagnostics.rs

@@ -438,5 +438,8 @@ pub(super) fn decorate_lint(sess: &Session, diagnostic: BuiltinLintDiag, diag: &
         BuiltinLintDiag::OutOfScopeMacroCalls { path } => {
             lints::OutOfScopeMacroCalls { path }.decorate_lint(diag)
         }
+        BuiltinLintDiag::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by } => {
+            lints::UnexpectedBuiltinCfg { cfg, cfg_name, controlled_by }.decorate_lint(diag)
+        }
     }
 }

compiler/rustc_lint/src/lints.rs

@@ -2379,6 +2379,16 @@ pub mod unexpected_cfg_value {
     }
 }
 
+#[derive(LintDiagnostic)]
+#[diag(lint_unexpected_builtin_cfg)]
+#[note(lint_controlled_by)]
+#[note(lint_incoherent)]
+pub struct UnexpectedBuiltinCfg {
+    pub(crate) cfg: String,
+    pub(crate) cfg_name: Symbol,
+    pub(crate) controlled_by: &'static str,
+}
+
 #[derive(LintDiagnostic)]
 #[diag(lint_macro_use_deprecated)]
 #[help]