3 files changed, 36 insertions, 16 deletions

diff --git a/core/extensions.rs b/core/extensions.rs
index 94c4a2a79..9b4fb203a 100644
--- a/core/extensions.rs
+++ b/core/extensions.rs
@@ -23,23 +23,43 @@ pub enum ExtensionFileSourceCode {
   LoadedFromFsDuringSnapshot(PathBuf),
 }
 
-impl ExtensionFileSourceCode {
+#[derive(Clone, Debug)]
+pub struct ExtensionFileSource {
+  pub specifier: &'static str,
+  pub code: ExtensionFileSourceCode,
+}
+
+impl ExtensionFileSource {
+  fn find_non_ascii(s: &str) -> String {
+    s.chars().filter(|c| !c.is_ascii()).collect::<String>()
+  }
+
   pub fn load(&self) -> Result<ModuleCode, Error> {
-    match self {
-      ExtensionFileSourceCode::IncludedInBinary(code) => Ok((*code).into()),
+    match &self.code {
+      ExtensionFileSourceCode::IncludedInBinary(code) => {
+        debug_assert!(
+          code.is_ascii(),
+          "Extension code must be 7-bit ASCII: {} (found {})",
+          self.specifier,
+          Self::find_non_ascii(code)
+        );
+        Ok((*code).into())
+      }
       ExtensionFileSourceCode::LoadedFromFsDuringSnapshot(path) => {
         let msg = || format!("Failed to read \"{}\"", path.display());
-        Ok(std::fs::read_to_string(path).with_context(msg)?.into())
+        let s = std::fs::read_to_string(path).with_context(msg)?;
+        debug_assert!(
+          s.is_ascii(),
+          "Extension code must be 7-bit ASCII: {} (found {})",
+          self.specifier,
+          Self::find_non_ascii(&s)
+        );
+        Ok(s.into())
       }
     }
   }
 }
 
-#[derive(Clone, Debug)]
-pub struct ExtensionFileSource {
-  pub specifier: &'static str,
-  pub code: ExtensionFileSourceCode,
-}
 pub type OpFnRef = v8::FunctionCallback;
 pub type OpMiddlewareFn = dyn Fn(OpDecl) -> OpDecl;
 pub type OpStateFn = dyn FnOnce(&mut OpState);
diff --git a/core/modules.rs b/core/modules.rs
index 78efdedfd..cfd68d245 100644
--- a/core/modules.rs
+++ b/core/modules.rs
@@ -309,7 +309,7 @@ impl From<Cow<'static, [u8]>> for ModuleCode {
 impl From<&'static str> for ModuleCode {
   #[inline(always)]
   fn from(value: &'static str) -> Self {
-    assert!(value.is_ascii());
+    debug_assert!(value.is_ascii());
     ModuleCode::Static(value.as_bytes())
   }
 }
@@ -331,7 +331,7 @@ impl From<Vec<u8>> for ModuleCode {
 impl From<&'static [u8]> for ModuleCode {
   #[inline(always)]
   fn from(value: &'static [u8]) -> Self {
-    assert!(value.is_ascii());
+    debug_assert!(value.is_ascii());
     ModuleCode::Static(value)
   }
 }
@@ -339,7 +339,7 @@ impl From<&'static [u8]> for ModuleCode {
 impl<const N: usize> From<&'static [u8; N]> for ModuleCode {
   #[inline(always)]
   fn from(value: &'static [u8; N]) -> Self {
-    assert!(value.is_ascii());
+    debug_assert!(value.is_ascii());
     ModuleCode::Static(value)
   }
 }
@@ -583,7 +583,7 @@ impl ModuleLoader for ExtModuleLoader {
       let result = if let Some(load_callback) = &self.maybe_load_callback {
         load_callback(file_source)
       } else {
-        match file_source.code.load() {
+        match file_source.load() {
           Ok(code) => Ok(code),
           Err(err) => return futures::future::err(err).boxed_local(),
         }
@@ -1517,7 +1517,7 @@ impl ModuleMap {
   ) -> Option<v8::Local<'a, v8::String>> {
     match name {
       ModuleName::Static(s) => {
-        assert!(s.is_ascii());
+        debug_assert!(s.is_ascii());
         v8::String::new_external_onebyte_static(scope, s.as_bytes())
       }
       ModuleName::NotStatic(s) => v8::String::new(scope, s),
diff --git a/core/runtime.rs b/core/runtime.rs
index c5afbda53..787bac972 100644
--- a/core/runtime.rs
+++ b/core/runtime.rs
@@ -751,7 +751,7 @@ impl JsRuntime {
             realm.execute_script(
               self.v8_isolate(),
               file_source.specifier,
-              file_source.code.load()?,
+              file_source.load()?,
             )?;
           }
         }
@@ -2544,7 +2544,7 @@ impl JsRealm {
     let scope = &mut self.handle_scope(isolate);
 
     let source = Self::string_from_code(scope, &source_code).unwrap();
-    assert!(name.is_ascii());
+    debug_assert!(name.is_ascii());
     let name =
       v8::String::new_external_onebyte_static(scope, name.as_bytes()).unwrap();
     let origin = bindings::script_origin(scope, name);