feat(tvix/eval): implement legacy let syntax

... and emit a warning if anyone decides to use.

Change-Id: Iaa6fe9fa932340e6d0fa9f357155e78823702576
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6611
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler/mod.rs

@@ -188,10 +188,7 @@ impl Compiler<'_, '_> {
             // their value on the stack.
             ast::Expr::Paren(paren) => self.compile(slot, paren.expr().unwrap()),
 
-            ast::Expr::LegacyLet(_) => {
-                let span = self.span_for(&expr);
-                self.emit_error(span, ErrorKind::NotImplemented("legacy let syntax"));
-            }
+            ast::Expr::LegacyLet(legacy_let) => self.compile_legacy_let(slot, legacy_let),
 
             ast::Expr::Root(_) => unreachable!("there cannot be more than one root"),
             ast::Expr::Error(_) => unreachable!("compile is only called on validated trees"),
@@ -997,6 +994,17 @@ impl Compiler<'_, '_> {
         self.push_op(OpCode::OpCall, &node);
     }
 
+    fn compile_legacy_let(&mut self, slot: LocalIdx, node: ast::LegacyLet) {
+        self.scope_mut().begin_scope();
+        self.compile_recursive_scope(slot, true, &node);
+        self.push_op(OpCode::OpAttrs(Count(node.entries().count())), &node);
+        self.emit_constant(Value::String(SmolStr::new_inline("body").into()), &node);
+        self.push_op(OpCode::OpAttrsSelect, &node);
+
+        let warning_span = self.span_for(&node);
+        self.emit_warning(warning_span, WarningKind::DeprecatedLegacyLet);
+    }
+
     /// Compile an expression into a runtime thunk which should be
     /// lazily evaluated when accessed.
     // TODO: almost the same as Compiler::compile_lambda; unify?

tvix/eval/src/warnings.rs

@@ -10,6 +10,7 @@ pub enum WarningKind {
     UselessInherit,
     UnusedBinding,
     ShadowedGlobal(&'static str),
+    DeprecatedLegacyLet,
 
     /// Tvix internal warning for features triggered by users that are
     /// not actually implemented yet, but do not cause runtime failures.
@@ -75,6 +76,10 @@ impl EvalWarning {
                 format!("declared variable '{}' shadows a built-in global!", name)
             }
 
+            WarningKind::DeprecatedLegacyLet => {
+                "legacy `let` syntax used, please rewrite this as `let .. in ...`".to_string()
+            }
+
             WarningKind::NotImplemented(what) => {
                 format!("feature not yet implemented in tvix: {}", what)
             }
@@ -89,6 +94,7 @@ impl EvalWarning {
             WarningKind::UselessInherit => "W002",
             WarningKind::UnusedBinding => "W003",
             WarningKind::ShadowedGlobal(_) => "W004",
+            WarningKind::DeprecatedLegacyLet => "W005",
             WarningKind::NotImplemented(_) => "W999",
         }
     }