3 files changed, 20 insertions, 0 deletions
diff --git a/src/libexpr/parser/parser.cc b/src/libexpr/parser/parser.cc
index 6e8ab2fe4..17463056c 100644
--- a/src/libexpr/parser/parser.cc
+++ b/src/libexpr/parser/parser.cc
@@ -668,6 +668,16 @@ template<> struct BuildAST<grammar::expr::uri> {
 
 template<> struct BuildAST<grammar::expr::ancient_let> : change_head<BindingsState> {
     static void success(const auto & in, BindingsState & b, ExprState & s, State & ps) {
+        // Added 2024-09-18. Turn into an error at some point in the future.
+        // See the documentation on deprecated features for more details.
+        if (!ps.featureSettings.isEnabled(Dep::AncientLet))
+            warn(
+                "%s found at %s. This feature is deprecated and will be removed in the future. Use %s to silence this warning.",
+                "let {",
+                ps.positions[ps.at(in)],
+                "--extra-deprecated-features ancient-let"
+            );
+
         b.attrs.pos = ps.at(in);
         b.attrs.recursive = true;
         s.pushExpr<ExprSelect>(b.attrs.pos, b.attrs.pos, std::make_unique<ExprAttrs>(std::move(b.attrs)), ps.s.body);
diff --git a/src/libutil/deprecated-features.cc b/src/libutil/deprecated-features.cc
index de4efd93b..4de4c8ec7 100644
--- a/src/libutil/deprecated-features.cc
+++ b/src/libutil/deprecated-features.cc
@@ -37,6 +37,15 @@ constexpr std::array<DeprecatedFeatureDetails, numDepFeatures> depFeatureDetails
         )",
     },
     {
+        .tag = Dep::AncientLet,
+        .name = "ancient-let",
+        .description = R"(
+            Allow the ancient `let { body = …; … }` syntax.
+
+            Use the `let … in` syntax instead.
+        )",
+    },
+    {
         .tag = Dep::UrlLiterals,
         .name = "url-literals",
         .description = R"(
diff --git a/src/libutil/deprecated-features.hh b/src/libutil/deprecated-features.hh
index d4d8a57a9..bdff1bcdb 100644
--- a/src/libutil/deprecated-features.hh
+++ b/src/libutil/deprecated-features.hh
@@ -19,6 +19,7 @@ namespace nix {
 enum struct DeprecatedFeature
 {
     RecSetOverrides,
+    AncientLet,
     UrlLiterals,
 };