libexpr: Soft-deprecate ancient let syntax

Change-Id: I6802b26f038578870ea1fa1ed298f0c4b1f29c4a

doc/manual/rl-next/deprecated-features.md

@@ -14,3 +14,4 @@ and can be disabled via the flags for backwards compatibility (opt-out with `--e
   This means that all URLs must be properly put within quotes like all other strings.
 - `rec-set-overrides`: **__overrides** is an old arcane syntax which has not been in use for more than a decade.
   It is soft-deprecated with a warning only, with the plan to turn that into an error in a future release.
+- `ancient-let`: **The old `let` syntax** (`let { body = …; … }`) is soft-deprecated with a warning as well. Use the regular `let … in` instead.

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);

src/libutil/deprecated-features.cc

@@ -36,6 +36,15 @@ constexpr std::array<DeprecatedFeatureDetails, numDepFeatures> depFeatureDetails
             Use fix point functions (e.g. `lib.fix` in Nixpkgs) instead.
         )",
     },
+    {
+        .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",

src/libutil/deprecated-features.hh

@@ -19,6 +19,7 @@ namespace nix {
 enum struct DeprecatedFeature
 {
     RecSetOverrides,
+    AncientLet,
     UrlLiterals,
 };
 

tests/functional/lang/eval-okay-let.flags

@@ -0,0 +1 @@
+--extra-deprecated-features ancient-let