libexpr: Soft-deprecate ancient let syntax

Change-Id: I6802b26f038578870ea1fa1ed298f0c4b1f29c4a

2 files changed, 10 insertions, 0 deletions

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,
 };