syntax: tweak Debug impl for Hir

The default derive(Debug) impl for Hir is very noisy because it lists
out the properties for every Hir value. We change the default to just
print out the actual expressions and omit the properties. But one can
opt back into seeing the properties via the "alternate" impl. i.e.,
{:#?} instead of {:?}.

Author: BurntSushi

regex-syntax/src/hir/mod.rs

@@ -157,7 +157,14 @@ impl core::fmt::Display for ErrorKind {
 /// proportional to the size of the `Hir`. The regex it prints is guaranteed to
 /// be _semantically_ equivalent to the original concrete syntax, but it may
 /// look very different. (And potentially not practically readable by a human.)
-#[derive(Clone, Debug, Eq, PartialEq)]
+///
+/// An `Hir`'s `fmt::Debug` implementation currently does not use constant
+/// stack space. The default implementation will also suppress some details
+/// (such as the `Properties` inlined into every `Hir` value to make it less
+/// noisy), but using the "[alternate]" format option will show everything.
+///
+/// [alternate]: https://doc.rust-lang.org/std/fmt/struct.Formatter.html#method.alternate
+#[derive(Clone, Eq, PartialEq)]
 pub struct Hir {
     /// The underlying HIR kind.
     kind: HirKind,
@@ -413,6 +420,19 @@ impl HirKind {
     }
 }
 
+impl core::fmt::Debug for Hir {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        if f.alternate() {
+            f.debug_struct("Hir")
+                .field("kind", &self.kind)
+                .field("props", &self.props)
+                .finish()
+        } else {
+            self.kind.fmt(f)
+        }
+    }
+}
+
 /// Print a display representation of this Hir.
 ///
 /// The result of this is a valid regular expression pattern string.