refactor(tvix/eval): remove todo!() calls in compiler

It is impossible for tvixbolt to recover from panics, so the user
experience of typing an expression using an unsupported feature was
that it would get sad and stop responding to input.

Instead, raise a normal value-level error of a new variant and
continue where possible.

Change-Id: Ibe016c92cacb87b85095c0f83758eddc6468053e
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6528
Autosubmit: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler/attrs.rs

@@ -30,7 +30,11 @@ impl Compiler<'_, '_> {
     /// 3. Attribute sets can (optionally) be recursive.
     pub(super) fn compile_attr_set(&mut self, slot: LocalIdx, node: ast::AttrSet) {
         if node.rec_token().is_some() {
-            todo!("recursive attribute sets are not yet implemented")
+            let span = self.span_for(&node);
+            self.emit_warning(
+                span,
+                WarningKind::NotImplemented("recursive attribute sets are not yet implemented"),
+            );
         }
 
         // Open a scope to track the positions of the temporaries used

tvix/eval/src/compiler/mod.rs

@@ -187,7 +187,10 @@ impl Compiler<'_, '_> {
             // their value on the stack.
             ast::Expr::Paren(paren) => self.compile(slot, paren.expr().unwrap()),
 
-            ast::Expr::LegacyLet(_) => todo!("legacy let"),
+            ast::Expr::LegacyLet(_) => {
+                let span = self.span_for(&expr);
+                self.emit_error(span, ErrorKind::NotImplemented("legacy let syntax"));
+            }
 
             ast::Expr::Root(_) => unreachable!("there cannot be more than one root"),
             ast::Expr::Error(_) => unreachable!("compile is only called on validated trees"),
@@ -238,7 +241,14 @@ impl Compiler<'_, '_> {
             buf
         } else {
             // TODO: decide what to do with findFile
-            todo!("other path types (e.g. <...> lookups) not yet implemented")
+            let span = self.span_for(&node);
+            self.emit_error(
+                span,
+                ErrorKind::NotImplemented(
+                    "other path types (e.g. <...> lookups) not yet implemented",
+                ),
+            );
+            return;
         };
 
         // TODO: Use https://github.com/rust-lang/rfcs/issues/2208
@@ -622,7 +632,12 @@ impl Compiler<'_, '_> {
             };
 
             if path.len() != 1 {
-                todo!("nested bindings in let expressions :(")
+                let span = self.span_for(&entry);
+                self.emit_error(
+                    span,
+                    ErrorKind::NotImplemented("nested bindings in let expressions :("),
+                );
+                continue;
             }
 
             let idx = self.declare_local(&entry.attrpath().unwrap(), path.pop().unwrap());