syntax: improve Debug impl for Class

Previously, classes would show up in the debug representation as very
deeply nested things, making them more difficult to read than they need
to be. This removes at least a few pretty redundant layers and uses a
more compact range notation.

regex-syntax/src/hir/mod.rs

@@ -787,7 +787,7 @@ impl core::fmt::Debug for Literal {
 /// the author of the regular expression to disable Unicode mode, which in turn
 /// impacts the semantics of case insensitive matching. For example, `(?i)k`
 /// and `(?i-u)k` will not match the same set of strings.
-#[derive(Clone, Debug, Eq, PartialEq)]
+#[derive(Clone, Eq, PartialEq)]
 pub enum Class {
     /// A set of characters represented by Unicode scalar values.
     Unicode(ClassUnicode),
@@ -986,6 +986,27 @@ impl Class {
     }
 }
 
+impl core::fmt::Debug for Class {
+    fn fmt(&self, f: &mut core::fmt::Formatter) -> core::fmt::Result {
+        use crate::debug::Byte;
+
+        let mut fmter = f.debug_set();
+        match *self {
+            Class::Unicode(ref cls) => {
+                for r in cls.ranges().iter() {
+                    fmter.entry(&(r.start..=r.end));
+                }
+            }
+            Class::Bytes(ref cls) => {
+                for r in cls.ranges().iter() {
+                    fmter.entry(&(Byte(r.start)..=Byte(r.end)));
+                }
+            }
+        }
+        fmter.finish()
+    }
+}
+
 /// A set of characters represented by Unicode scalar values.
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub struct ClassUnicode {