[librsvg: 1/2] Return a meaningful error instead of using unit as an error type

[Federico Mena Quintero <federico src gnome org>]

commit 5f76e38afaf0762fe73ba07a035473a21bccf369
Author: Sven Neumann <sven svenfoo org>
Date:   Tue Feb 2 19:58:46 2021 +0100

    Return a meaningful error instead of using unit as an error type
    
    https://rust-lang.github.io/rust-clippy/master/index.html#result_unit_err

 src/aspect_ratio.rs | 18 +++++++++++++++---
 src/drawing_ctx.rs  |  2 +-
 2 files changed, 16 insertions(+), 4 deletions(-)
---
diff --git a/src/aspect_ratio.rs b/src/aspect_ratio.rs
index b5f5be68..b2e9fa64 100644
--- a/src/aspect_ratio.rs
+++ b/src/aspect_ratio.rs
@@ -16,6 +16,7 @@
 //! [spec]: https://www.w3.org/TR/SVG/coords.html#PreserveAspectRatioAttribute
 
 use cssparser::{BasicParseError, Parser};
+use std::fmt;
 use std::ops::Deref;
 
 use crate::error::*;
@@ -79,6 +80,17 @@ struct Align {
     fit: FitMode,
 }
 
+#[derive(Debug, PartialEq, Eq)]
+pub struct NonInvertibleTransform;
+
+impl fmt::Display for NonInvertibleTransform {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "Not invertible")
+    }
+}
+
+impl std::error::Error for NonInvertibleTransform {}
+
 #[derive(Debug, Copy, Clone, PartialEq, Eq)]
 pub struct AspectRatio {
     defer: bool,
@@ -148,7 +160,7 @@ impl AspectRatio {
         &self,
         vbox: Option<ViewBox>,
         viewport: &Rect,
-    ) -> Result<Option<Transform>, ()> {
+    ) -> Result<Option<Transform>, NonInvertibleTransform> {
         // width or height set to 0 disables rendering of the element
         // https://www.w3.org/TR/SVG/struct.html#SVGElementWidthAttribute
         // https://www.w3.org/TR/SVG/struct.html#UseElementWidthAttribute
@@ -179,7 +191,7 @@ impl AspectRatio {
         if transform.is_invertible() {
             Ok(Some(transform))
         } else {
-            Err(())
+            Err(NonInvertibleTransform)
         }
     }
 }
@@ -447,6 +459,6 @@ mod tests {
             &Rect::new(1.0, 1.0, 2.0, 2.0),
         );
 
-        assert_eq!(t, Err(()));
+        assert_eq!(t, Err(NonInvertibleTransform));
     }
 }
diff --git a/src/drawing_ctx.rs b/src/drawing_ctx.rs
index 5a2d7a3f..7d9c01e8 100644
--- a/src/drawing_ctx.rs
+++ b/src/drawing_ctx.rs
@@ -409,7 +409,7 @@ impl DrawingCtx {
 
         preserve_aspect_ratio
             .viewport_to_viewbox_transform(vbox, &viewport)
-            .unwrap_or_else(|_e: ()| {
+            .unwrap_or_else(|_e| {
                 match vbox {
                     None => unreachable!(
                         "viewport_to_viewbox_transform only returns errors when vbox != None"