better type assertion ergonomic

fix-vm/src/instructions/with_scope.rs

@@ -2,7 +2,8 @@ use fix_common::Symbol;
 use fix_error::Error;
 use gc_arena::Gc;
 
-use crate::{BytecodeReader, CallFrame, StepResult, WithEnv};
+use crate::{BytecodeReader, CallFrame, Step, WithEnv};
+use crate::value::*;
 
 impl<'gc> crate::Vm<'gc> {
     #[inline(always)]
@@ -11,7 +12,7 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &gc_arena::Mutation<'gc>,
-    ) -> StepResult {
+    ) -> Step {
         let env = reader.read_operand_data(ctx).resolve(mc, self);
         let scope = Gc::new(
             mc,
@@ -21,20 +22,20 @@ impl<'gc> crate::Vm<'gc> {
             },
         );
         self.with_env = Some(scope);
-        StepResult::Continue
+        Step::Continue
     }
 
     #[inline(always)]
-    pub(crate) fn op_pop_with(&mut self) -> StepResult {
+    pub(crate) fn op_pop_with(&mut self) -> Step {
         let Some(scope) = self.with_env else {
             unreachable!("no with_scope to pop");
         };
         self.with_env = scope.prev;
-        StepResult::Continue
+        Step::Continue
     }
 
     #[inline(always)]
-    pub(crate) fn op_prepare_with(&mut self) -> StepResult {
+    pub(crate) fn op_prepare_with(&mut self) -> Step {
         self.call_stack.push(CallFrame {
             pc: usize::MAX,
             stack_depth: 0,
@@ -42,7 +43,7 @@ impl<'gc> crate::Vm<'gc> {
             env: self.env,
             with_env: self.with_env,
         });
-        StepResult::Continue
+        Step::Continue
     }
 
     #[inline(always)]
@@ -51,7 +52,7 @@ impl<'gc> crate::Vm<'gc> {
         ctx: &mut impl crate::VmContext,
         reader: &mut BytecodeReader<'_>,
         mc: &gc_arena::Mutation<'gc>,
-    ) -> StepResult {
+    ) -> Step {
         let name = reader.read_string_id();
 
         let Some(&WithEnv { env, prev }) = self.with_env.as_deref() else {
@@ -64,23 +65,26 @@ impl<'gc> crate::Vm<'gc> {
                 Symbol::from(ctx.resolve_string(name))
             )));
         };
-        self.push_stack(env);
+        self.push(env);
         if let Some(step) = self.try_force(0, reader, mc) {
             return step;
         }
 
-        let env = self.pop_stack().as_gc::<crate::AttrSet>().unwrap();
+        let env = match self.pop_forced_expect_gc::<AttrSet>() {
+            Ok(val) => val,
+            Err(got) => return self.finish_type_err(NixType::List, got)
+        };
         let Some(val) = env.lookup(name) else {
             reader.set_pc(reader.inst_start_pc());
             self.with_env = prev;
-            return StepResult::Continue;
+            return Step::Continue;
         };
 
-        self.push_stack(val);
+        self.push(val);
         let Some(CallFrame { with_env, .. }) = self.call_stack.pop() else {
             unreachable!()
         };
         self.with_env = with_env;
-        StepResult::Continue
+        Step::Continue
     }
 }