[librsvg] Add more error cases to ReferenceError

[Federico Mena Quintero <federico src gnome org>]

commit 42b445ee9d8ccb01ec36183ef5e5557dd843c792
Author: Federico Mena Quintero <federico gnome org>
Date:   Wed Nov 28 12:09:49 2018 -0600

    Add more error cases to ReferenceError

 rsvg_internals/src/defs.rs | 23 ++++++++++++++++++-----
 1 file changed, 18 insertions(+), 5 deletions(-)
---
diff --git a/rsvg_internals/src/defs.rs b/rsvg_internals/src/defs.rs
index aef34c0f..adecfde7 100644
--- a/rsvg_internals/src/defs.rs
+++ b/rsvg_internals/src/defs.rs
@@ -82,17 +82,30 @@ pub enum Reference {
     UriWithFragmentId(String, String),
 }
 
+/// Errors returned when creating a `Reference` out of an href
 #[derive(Debug, PartialEq)]
 pub enum ReferenceError {
+    /// The href is an invalid URI or has empty components.
     ParseError,
+
+    /// A fragment identifier ("`#foo`") is not allowed here
+    ///
+    /// For example, the SVG `<image>` element only allows referencing
+    /// resources without fragment identifiers like
+    /// `xlink:href="foo.png"`.
+    FragmentForbidden,
+
+    /// A fragment identifier ("`#foo`") was required but not found.  For example,
+    /// the SVG `<use>` element requires one, as in `<use xlink:href="foo.svg#bar">`.
+    FragmentRequired,
 }
 
 impl Reference {
-    pub fn parse(s: &str) -> Result<Reference, ReferenceError> {
-        let (uri, fragment) = match s.rfind('#') {
-            None => (Some(s), None),
-            Some(p) if p == 0 => (None, Some(&s[1..])),
-            Some(p) => (Some(&s[..p]), Some(&s[(p + 1)..])),
+    pub fn parse(href: &str) -> Result<Reference, ReferenceError> {
+        let (uri, fragment) = match href.rfind('#') {
+            None => (Some(href), None),
+            Some(p) if p == 0 => (None, Some(&href[1..])),
+            Some(p) => (Some(&href[..p]), Some(&href[(p + 1)..])),
         };
 
         match (uri, fragment) {