1 files changed, 10 insertions, 6 deletions
diff --git a/src/loading/mod.rs b/src/loading/mod.rs
index 3be74428..64a65580 100644
--- a/src/loading/mod.rs
+++ b/src/loading/mod.rs
@@ -6,6 +6,7 @@ mod fs;
 #[cfg(feature = "fs")]
 pub use fs::*;
 
+use std::io;
 use std::path::Path;
 
 use serde::{Deserialize, Serialize};
@@ -21,10 +22,13 @@ pub trait Loader {
     ///
     /// This should return the same id for all paths pointing to the same file
     /// and `None` if the file does not exist.
-    fn resolve_from(&self, base: FileId, path: &Path) -> Option<FileId>;
+    fn resolve_from(&self, base: FileId, path: &Path) -> io::Result<FileId>;
 
     /// Load a file by id.
-    fn load_file(&self, id: FileId) -> Option<Vec<u8>>;
+    ///
+    /// This must only be called with an `id` returned by a call to this
+    /// loader's `resolve_from` method.
+    fn load_file(&self, id: FileId) -> io::Result<Vec<u8>>;
 }
 
 /// A file id that can be [resolved](Loader::resolve_from) from a path.
@@ -53,11 +57,11 @@ impl Loader for BlankLoader {
         &[]
     }
 
-    fn resolve_from(&self, _: FileId, _: &Path) -> Option<FileId> {
-        None
+    fn resolve_from(&self, _: FileId, _: &Path) -> io::Result<FileId> {
+        Err(io::ErrorKind::NotFound.into())
     }
 
-    fn load_file(&self, _: FileId) -> Option<Vec<u8>> {
-        None
+    fn load_file(&self, _: FileId) -> io::Result<Vec<u8>> {
+        panic!("resolve_from never returns an id")
     }
 }