Improve image loading error messages

crates/egui/src/context.rs

@@ -1921,15 +1921,9 @@ impl Context {
     pub fn is_loader_installed(&self, id: &str) -> bool {
         let loaders = self.loaders();
 
-        let in_bytes = loaders.bytes.lock().iter().any(|loader| loader.id() == id);
-        let in_image = loaders.image.lock().iter().any(|loader| loader.id() == id);
-        let in_texture = loaders
-            .texture
-            .lock()
-            .iter()
-            .any(|loader| loader.id() == id);
-
-        in_bytes || in_image || in_texture
+        loaders.bytes.lock().iter().any(|l| l.id() == id)
+            || loaders.image.lock().iter().any(|l| l.id() == id)
+            || loaders.texture.lock().iter().any(|l| l.id() == id)
     }
 
     /// Append an entry onto the chain of bytes loaders.
@@ -2028,7 +2022,7 @@ impl Context {
             }
         }
 
-        Err(load::LoadError::NotSupported)
+        Err(load::LoadError::NoMatchingBytesLoader)
     }
 
     /// Try loading the image from the given uri using any available image loaders.
@@ -2067,7 +2061,7 @@ impl Context {
             }
         }
 
-        Err(load::LoadError::NotSupported)
+        Err(load::LoadError::NoMatchingImageLoader)
     }
 
     /// Try loading the texture from the given uri using any available texture loaders.
@@ -2103,7 +2097,7 @@ impl Context {
             }
         }
 
-        Err(load::LoadError::NotSupported)
+        Err(load::LoadError::NoMatchingTextureLoader)
     }
 
     /// The loaders of bytes, images, and textures.

crates/egui/src/load.rs

@@ -72,24 +72,44 @@ use std::{error::Error as StdError, fmt::Display, sync::Arc};
 /// Represents a failed attempt at loading an image.
 #[derive(Clone, Debug)]
 pub enum LoadError {
-    /// There are no image loaders installed.
+    /// Programmer error: There are no image loaders installed.
     NoImageLoaders,
 
-    /// This loader does not support this schema, protocol or image format.
+    /// A specific loader does not support this schema, protocol or image format.
     NotSupported,
 
-    /// A custom error message (e.g. "File not found: foo.png").
-    Custom(String),
+    /// Programmer error: Failed to find the bytes for this image because
+    /// there was no [`BytesLoader`] supporting the schema.
+    NoMatchingBytesLoader,
+
+    /// Programmer error: Failed to parse the bytes as an image because
+    /// there was no [`ImageLoader`] supporting the schema.
+    NoMatchingImageLoader,
+
+    /// Programmer error: no matching [`TextureLoader`].
+    /// Because of the [`DefaultTextureLoader`], this error should never happen.
+    NoMatchingTextureLoader,
+
+    /// Runtime error: Loading was attempted, but failed (e.g. "File not found").
+    Loading(String),
 }
 
 impl Display for LoadError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
-            LoadError::NoImageLoaders => f.write_str(
+            Self::NoImageLoaders => f.write_str(
                 "No image loaders are installed. If you're trying to load some images \
                 for the first time, follow the steps outlined in https://docs.rs/egui/latest/egui/load/index.html"),
-            LoadError::NotSupported => f.write_str("not supported"),
-            LoadError::Custom(message) => f.write_str(message),
+
+            Self::NoMatchingBytesLoader => f.write_str("No matching BytesLoader. Either you need to call Context::include_bytes, or install some more bytes loaders, e.g. using egui_extras."),
+
+            Self::NoMatchingImageLoader => f.write_str("No matching ImageLoader. Either you need to call Context::include_bytes, or install some more bytes loaders, e.g. using egui_extras."),
+
+            Self::NoMatchingTextureLoader => f.write_str("No matching TextureLoader. Did you remove the default one?"),
+
+            Self::NotSupported => f.write_str("Iagge schema or URI not supported by this loader"),
+
+            Self::Loading(message) => f.write_str(message),
         }
     }
 }

crates/egui_extras/src/loaders/ehttp_loader.rs

@@ -72,7 +72,7 @@ impl BytesLoader for EhttpLoader {
                     bytes: Bytes::Shared(file.bytes),
                     mime: file.mime,
                 }),
-                Poll::Ready(Err(err)) => Err(LoadError::Custom(err)),
+                Poll::Ready(Err(err)) => Err(LoadError::Loading(err)),
                 Poll::Pending => Ok(BytesPoll::Pending { size: None }),
             }
         } else {

crates/egui_extras/src/loaders/file_loader.rs

@@ -45,7 +45,7 @@ impl BytesLoader for FileLoader {
                     bytes: Bytes::Shared(file.bytes),
                     mime: file.mime,
                 }),
-                Poll::Ready(Err(err)) => Err(LoadError::Custom(err)),
+                Poll::Ready(Err(err)) => Err(LoadError::Loading(err)),
                 Poll::Pending => Ok(BytesPoll::Pending { size: None }),
             }
         } else {

crates/egui_extras/src/loaders/image_loader.rs

@@ -50,7 +50,7 @@ impl ImageLoader for ImageCrateLoader {
         if let Some(entry) = cache.get(uri).cloned() {
             match entry {
                 Ok(image) => Ok(ImagePoll::Ready { image }),
-                Err(err) => Err(LoadError::Custom(err)),
+                Err(err) => Err(LoadError::Loading(err)),
             }
         } else {
             match ctx.try_load_bytes(uri) {
@@ -68,7 +68,7 @@ impl ImageLoader for ImageCrateLoader {
                     cache.insert(uri.into(), result.clone());
                     match result {
                         Ok(image) => Ok(ImagePoll::Ready { image }),
-                        Err(err) => Err(LoadError::Custom(err)),
+                        Err(err) => Err(LoadError::Loading(err)),
                     }
                 }
                 Ok(BytesPoll::Pending { size }) => Ok(ImagePoll::Pending { size }),

crates/egui_extras/src/loaders/svg_loader.rs

@@ -42,7 +42,7 @@ impl ImageLoader for SvgLoader {
         if let Some(entry) = cache.get(&(uri.clone(), size_hint)).cloned() {
             match entry {
                 Ok(image) => Ok(ImagePoll::Ready { image }),
-                Err(err) => Err(LoadError::Custom(err)),
+                Err(err) => Err(LoadError::Loading(err)),
             }
         } else {
             match ctx.try_load_bytes(&uri) {
@@ -60,7 +60,7 @@ impl ImageLoader for SvgLoader {
                     cache.insert((uri, size_hint), result.clone());
                     match result {
                         Ok(image) => Ok(ImagePoll::Ready { image }),
-                        Err(err) => Err(LoadError::Custom(err)),
+                        Err(err) => Err(LoadError::Loading(err)),
                     }
                 }
                 Ok(BytesPoll::Pending { size }) => Ok(ImagePoll::Pending { size }),