implement string context

fix-abstract-vm/src/host.rs

@@ -4,8 +4,7 @@ use fix_error::Source;
 use hashbrown::HashSet;
 
 use crate::{
-    AttrSet, Closure, ExtraScope, List, NixString, NixType, Null, Path, PrimOp, PrimOpApp,
-    StaticValue, StrictValue, Thunk, ThunkState, Value,
+    AttrSet, Closure, ExtraScope, List, NixString, NixType, Null, Path, PrimOp, PrimOpApp, StaticValue, StrictValue, StringContext, Thunk, ThunkState, Value
 };
 
 pub trait VmContext {
@@ -36,6 +35,9 @@ pub trait VmRuntimeCtxExt: VmRuntimeCtx {
         &'a mut self,
         val: StrictValue<'gc>,
     ) -> std::result::Result<StringId, NixType>;
+    /// Returns the string context attached to `val`, or `&[]` if `val` is
+    /// either a non-string or a string without context.
+    fn get_string_context<'gc>(&self, val: StrictValue<'gc>) -> &'gc StringContext;
     fn convert_value(&self, val: Value) -> fix_common::Value;
 }
 
@@ -73,6 +75,14 @@ impl<T: VmRuntimeCtx> VmRuntimeCtxExt for T {
         }
     }
 
+    fn get_string_context<'gc>(&self, val: StrictValue<'gc>) -> &'gc StringContext {
+        if let Some(ns) = val.as_gc::<NixString>() {
+            ns.as_ref().context()
+        } else {
+            StringContext::empty()
+        }
+    }
+
     fn convert_value(&self, val: Value) -> fix_common::Value {
         self.convert_value_with_seen(val, &mut HashSet::new())
     }