layer: usize -> u8

fix/src/lib.rs

@@ -431,9 +431,10 @@ impl<'ctx: 'ir, 'id, 'ir, R: VmRuntimeCtx> DowngradeContext<'id, 'ir>
                     abs_layer,
                 } => {
                     if param_sym == sym {
-                        return Ok(MaybeThunk::Arg {
-                            layer: self.thunk_scopes.len() - abs_layer,
-                        });
+                        let layers: u8 =
+                            self.thunk_scopes.len().try_into().expect("scope too deep!");
+                        let layer = layers - abs_layer;
+                        return Ok(MaybeThunk::Arg { layer });
                     }
                 }
             }
@@ -484,7 +485,7 @@ impl<'ctx: 'ir, 'id, 'ir, R: VmRuntimeCtx> DowngradeContext<'id, 'ir>
     {
         self.scopes.push(Scope::Param {
             sym,
-            abs_layer: self.thunk_scopes.len(),
+            abs_layer: self.thunk_scopes.len().try_into().expect("scope too deep!"),
         });
         let mut guard = ScopeGuard { ctx: self };
         f(guard.as_ctx())
@@ -510,6 +511,9 @@ impl<'ctx: 'ir, 'id, 'ir, R: VmRuntimeCtx> DowngradeContext<'id, 'ir>
     where
         F: FnOnce(&mut Self) -> Ret,
     {
+        if self.thunk_scopes.len() == u8::MAX as usize {
+            panic!("scope too deep!");
+        }
         self.thunk_scopes.push(ThunkScope::new_in(self.bump));
         let ret = f(self);
         (
@@ -564,7 +568,7 @@ enum Scope<'ctx> {
     Repl(&'ctx HashSet<StringId>),
     ScopedImport(HashSet<StringId>),
     Let(HashMap<StringId, ThunkId>),
-    Param { sym: StringId, abs_layer: usize },
+    Param { sym: StringId, abs_layer: u8 },
 }
 
 struct ScopeGuard<'a, 'ctx, 'id, 'ir, R: VmRuntimeCtx> {