Fix unused field warning in cursor parsing

Rust nightly started to warn about an unused field while building
examples:

    warning: field `0` is never read
      --> x11rb/src/cursor/parse_cursor.rs:18:8
       |
    18 |     Io(std::io::Error),
       |     -- ^^^^^^^^^^^^^^
       |     |
       |     field in this variant
       |
       = note: `#[warn(dead_code)]` on by default

The code from x11rb::cursor::parse_cursor is only called from
x11rb::cursor::load_cursor(). This code completely ignores any errors
and turns them into ParseError::InvalidValue. Thus, this warning is
right.

Signed-off-by: Uli Schlachter <[email protected]>

x11rb/src/cursor/parse_cursor.rs

@@ -15,7 +15,7 @@ const IMAGE_MAX_SIZE: u16 = 0x7fff;
 #[derive(Debug)]
 pub(crate) enum Error {
     /// An I/O error occurred
-    Io(std::io::Error),
+    Io,
 
     /// The file did not begin with the expected magic
     InvalidMagic,
@@ -34,8 +34,8 @@ pub(crate) enum Error {
 }
 
 impl From<std::io::Error> for Error {
-    fn from(value: std::io::Error) -> Self {
-        Error::Io(value)
+    fn from(_: std::io::Error) -> Self {
+        Error::Io
     }
 }
 
@@ -320,7 +320,7 @@ mod test {
     fn read_image_too_short() {
         let data = [];
         match Image::read(&mut Cursor::new(&data[..]), IMAGE_TYPE, 4) {
-            Err(Error::Io(ref e)) if e.kind() == ErrorKind::UnexpectedEof => {}
+            Err(Error::Io) => {}
             r => panic!("Unexpected result {:?}", r),
         }
     }
@@ -380,7 +380,7 @@ mod test {
     fn parse_cursor_too_short() {
         let data = [];
         match parse_cursor(&mut Cursor::new(&data[..]), 10) {
-            Err(Error::Io(ref e)) if e.kind() == ErrorKind::UnexpectedEof => {}
+            Err(Error::Io) => {}
             r => panic!("Unexpected result {:?}", r),
         }
     }