This allows QObject to be created from any serializable format, for
example JSON via serde_json.

This is not too useful, since QObjects are produced by
C code or by serializing structs, but it can be used for testing
and it is part of the full implementation of a serde format.

It is relatively simple and similar to the Serializer, just with
the extra indirection of a Visitor.

Co-authored-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/util/meson.build                |   1 +
 rust/util/src/qobject/deserialize.rs | 134 +++++++++++++++++++++++++++
 rust/util/src/qobject/mod.rs         |   1 +
 3 files changed, 136 insertions(+)
 create mode 100644 rust/util/src/qobject/deserialize.rs

diff --git a/rust/util/meson.build b/rust/util/meson.build
index 39f427b3456..9f8fbd49f00 100644
--- a/rust/util/meson.build
+++ b/rust/util/meson.build
@@ -39,6 +39,7 @@ _util_rs = static_library(
     {'.': _util_bindings_inc_rs,
     'qobject': [
       'src/qobject/mod.rs',
+      'src/qobject/deserialize.rs',
       'src/qobject/error.rs',
       'src/qobject/serializer.rs',
       'src/qobject/serialize.rs',
diff --git a/rust/util/src/qobject/deserialize.rs b/rust/util/src/qobject/deserialize.rs
new file mode 100644
index 00000000000..280a577b6be
--- /dev/null
+++ b/rust/util/src/qobject/deserialize.rs
@@ -0,0 +1,134 @@
+//! `QObject` deserialization
+//!
+//! This module implements the [`Deserialize`] trait for `QObject`,
+//! allowing it to be created from any serializable format, for
+//! example JSON.
+
+use core::fmt;
+use std::ffi::CString;
+
+use serde::de::{self, Deserialize, MapAccess, SeqAccess, Visitor};
+
+use super::{to_qobject, QObject};
+
+impl<'de> Deserialize<'de> for QObject {
+    #[inline]
+    fn deserialize<D>(deserializer: D) -> Result<QObject, D::Error>
+    where
+        D: serde::Deserializer<'de>,
+    {
+        struct ValueVisitor;
+
+        impl<'de> Visitor<'de> for ValueVisitor {
+            type Value = QObject;
+
+            fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
+                formatter.write_str("any valid JSON value")
+            }
+
+            #[inline]
+            fn visit_bool<E>(self, value: bool) -> Result<QObject, E> {
+                Ok(value.into())
+            }
+
+            #[inline]
+            fn visit_i64<E>(self, value: i64) -> Result<QObject, E> {
+                Ok(value.into())
+            }
+
+            fn visit_i128<E>(self, value: i128) -> Result<QObject, E>
+            where
+                E: serde::de::Error,
+            {
+                to_qobject(value).map_err(|_| de::Error::custom("number out of range"))
+            }
+
+            #[inline]
+            fn visit_u64<E>(self, value: u64) -> Result<QObject, E> {
+                Ok(value.into())
+            }
+
+            fn visit_u128<E>(self, value: u128) -> Result<QObject, E>
+            where
+                E: serde::de::Error,
+            {
+                to_qobject(value).map_err(|_| de::Error::custom("number out of range"))
+            }
+
+            #[inline]
+            fn visit_f64<E>(self, value: f64) -> Result<QObject, E> {
+                Ok(value.into())
+            }
+
+            #[inline]
+            fn visit_str<E>(self, value: &str) -> Result<QObject, E>
+            where
+                E: serde::de::Error,
+            {
+                CString::new(value)
+                    .map_err(|_| de::Error::custom("NUL character in string"))
+                    .map(QObject::from)
+            }
+
+            #[inline]
+            fn visit_string<E>(self, value: String) -> Result<QObject, E>
+            where
+                E: serde::de::Error,
+            {
+                CString::new(value)
+                    .map_err(|_| de::Error::custom("NUL character in string"))
+                    .map(QObject::from)
+            }
+
+            #[inline]
+            fn visit_none<E>(self) -> Result<QObject, E> {
+                Ok(().into())
+            }
+
+            #[inline]
+            fn visit_some<D>(self, deserializer: D) -> Result<QObject, D::Error>
+            where
+                D: serde::Deserializer<'de>,
+            {
+                Deserialize::deserialize(deserializer)
+            }
+
+            #[inline]
+            fn visit_unit<E>(self) -> Result<QObject, E> {
+                Ok(().into())
+            }
+
+            #[inline]
+            fn visit_seq<V>(self, mut visitor: V) -> Result<QObject, V::Error>
+            where
+                V: SeqAccess<'de>,
+            {
+                // TODO: insert elements one at a time
+                let mut vec = Vec::<QObject>::new();
+
+                while let Some(elem) = visitor.next_element()? {
+                    vec.push(elem);
+                }
+                Ok(QObject::from_iter(vec))
+            }
+
+            fn visit_map<V>(self, mut visitor: V) -> Result<QObject, V::Error>
+            where
+                V: MapAccess<'de>,
+            {
+                // TODO: insert elements one at a time
+                let mut vec = Vec::<(CString, QObject)>::new();
+
+                if let Some(first_key) = visitor.next_key()? {
+                    vec.push((first_key, visitor.next_value()?));
+                    while let Some((key, value)) = visitor.next_entry()? {
+                        vec.push((key, value));
+                    }
+                }
+                Ok(QObject::from_iter(vec))
+            }
+        }
+
+        deserializer.deserialize_any(ValueVisitor)
+    }
+}
diff --git a/rust/util/src/qobject/mod.rs b/rust/util/src/qobject/mod.rs
index f8bd195ca2b..80c496b8a63 100644
--- a/rust/util/src/qobject/mod.rs
+++ b/rust/util/src/qobject/mod.rs
@@ -6,6 +6,7 @@
 
 #![deny(clippy::unwrap_used)]
 
+mod deserialize;
 mod error;
 mod serialize;
 mod serializer;
-- 
2.51.0