diff --git a/compiler/rustc_span/src/lib.rs b/compiler/rustc_span/src/lib.rs
index b8cf472309ba5..f6ae748560750 100644
--- a/compiler/rustc_span/src/lib.rs
+++ b/compiler/rustc_span/src/lib.rs
@@ -19,6 +19,7 @@
 #![allow(internal_features)]
 #![cfg_attr(target_arch = "loongarch64", feature(stdarch_loongarch))]
 #![feature(core_io_borrowed_buf)]
+#![feature(diagnostic_on_unknown)]
 #![feature(map_try_insert)]
 #![feature(negative_impls)]
 #![feature(read_buf)]
diff --git a/compiler/rustc_span/src/symbol.rs b/compiler/rustc_span/src/symbol.rs
index 07d6eb03ae8da..801f16c0a308b 100644
--- a/compiler/rustc_span/src/symbol.rs
+++ b/compiler/rustc_span/src/symbol.rs
@@ -2927,9 +2927,13 @@ pub mod kw {
 ///
 /// Given that `sym` is imported, use them like `sym::symbol_name`.
 /// For example `sym::rustfmt` or `sym::u8`.
+#[diagnostic::on_unknown(
+    label = "`{Unresolved}` is not a pre-interned symbol",
+    note = "consider adding `{Unresolved}` to the `symbols!` invocation in compiler/rustc_span/src/symbol.rs"
+)]
 pub mod sym {
-    // Used from a macro in `librustc_feature/accepted.rs`
     use super::Symbol;
+    // Used from a macro in `librustc_feature/accepted.rs`
     pub use super::kw::MacroRules as macro_rules;
     #[doc(inline)]
     pub use super::sym_generated::*;