feat(tvix/eval): emit warnings when globals are being shadowed

Change-Id: I7dae6978c2a4548382d7fa059b20ccdf35d2cf7f
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6257
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/compiler.rs

@@ -900,6 +900,7 @@ impl Compiler {
         };
 
         if let Some(global_ident) = key {
+            self.emit_warning(node.clone(), WarningKind::ShadowedGlobal(global_ident));
             self.scope_mut().poison(global_ident, depth);
         }
 

tvix/eval/src/warnings.rs

@@ -6,6 +6,7 @@ pub enum WarningKind {
     DeprecatedLiteralURL,
     UselessInherit,
     UnusedBinding,
+    ShadowedGlobal(&'static str),
 }
 
 #[derive(Debug)]