ZJIT: fmt::Debug for VALUE in hex. Shorthand for rb_obj_info()

The default `{:?}` still always prints the pointer address and never
dereferences it, but now in hex.

The "alternate" flag lets you do `println!("{my_ruby_object:#?}")` and
get a rich printout like `VALUE(0x000000010232fd00 T_CLASS/Object)`.

Author: XrXr

zjit/src/cruby.rs

@@ -248,7 +248,7 @@ pub struct rb_iseq_constant_body {
 /// that this is a handle. Sometimes the C code briefly uses VALUE as
 /// an unsigned integer type and don't necessarily store valid handles but
 /// thankfully those cases are rare and don't cross the FFI boundary.
-#[derive(Copy, Clone, PartialEq, Eq, Debug, Hash)]
+#[derive(Copy, Clone, PartialEq, Eq, Hash)]
 #[repr(transparent)] // same size and alignment as simply `usize`
 pub struct VALUE(pub usize);
 
@@ -757,6 +757,17 @@ impl IseqParameters {
     }
 }
 
+impl Debug for VALUE {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        // Only use rb_obj_info() when {:#?} since it dereferences the pointer so carries some risk.
+        if f.alternate() {
+            write!(f, "VALUE({})", self.obj_info())
+        } else {
+            write!(f, "VALUE(0x{:x})", self.0)
+        }
+    }
+}
+
 impl From<IseqPtr> for VALUE {
     /// For `.into()` convenience
     fn from(iseq: IseqPtr) -> Self {
@@ -1418,6 +1429,13 @@ pub mod test_utils {
     fn value_from_fixnum_too_small_isize() {
         assert_eq!(VALUE::fixnum_from_isize(RUBY_FIXNUM_MIN-1), VALUE(1));
     }
+
+    #[test]
+    fn value_fmt_debug() {
+        assert_eq!("VALUE(0xcafe)", format!("{:?}", VALUE(0xcafe)));
+        let alternate = format!("{:#?}", eval("::Hash"));
+        assert!(alternate.contains("Hash"), "'Hash' not substring of '{alternate}'");
+    }
 }
 #[cfg(test)]
 pub use test_utils::*;