feat(tvix/eval): emit warnings on useless inherit

Change-Id: Ifb9993cf8b85393eb43e1b204c7ab2f889b7113b
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6194
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler.rs

@@ -680,7 +680,14 @@ impl Compiler {
             match inherit.from() {
                 // Within a `let` binding, inheriting from the outer
                 // scope is practically a no-op.
-                None => continue,
+                None => {
+                    self.warnings.push(EvalWarning {
+                        node: inherit.node().clone(),
+                        kind: WarningKind::UselessInherit,
+                    });
+
+                    continue;
+                }
                 Some(_) => todo!("let inherit from attrs"),
             }
         }

tvix/eval/src/warnings.rs

@@ -4,6 +4,7 @@
 #[derive(Debug)]
 pub enum WarningKind {
     DeprecatedLiteralURL,
+    UselessInherit,
 }
 
 #[derive(Debug)]