rust: vmstate: convert to use builder pattern

Similar to MemoryRegionOps, the builder pattern has two advantages:
1) it makes it possible to build a VMStateDescription that knows which
types it will be invoked on; 2) it provides a way to wrap the callbacks
and let devices avoid "unsafe".

Unfortunately, building a static VMStateDescription requires the
builder methods to be "const", and because the VMStateFields are
*also* static, this requires const_refs_static.  So this requires
Rust 1.83.0.

Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250908105005.2119297-8-pbonzini@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 10 insertions, 1 deletions

diff --git a/rust/qemu-api/src/errno.rs b/rust/qemu-api/src/errno.rs
index 18d101448b..507850fe33 100644
--- a/rust/qemu-api/src/errno.rs
+++ b/rust/qemu-api/src/errno.rs
@@ -7,7 +7,10 @@
 //! convention.  This module provides functions to portably convert an integer
 //! into an [`io::Result`] and back.
 
-use std::{convert::TryFrom, io, io::ErrorKind};
+use std::{
+    convert::{self, TryFrom},
+    io::{self, ErrorKind},
+};
 
 /// An `errno` value that can be converted into an [`io::Error`]
 pub struct Errno(pub u16);
@@ -99,6 +102,12 @@ impl From<io::Error> for Errno {
     }
 }
 
+impl From<convert::Infallible> for Errno {
+    fn from(_value: convert::Infallible) -> Errno {
+        panic!("unreachable")
+    }
+}
+
 /// Internal traits; used to enable [`into_io_result`] and [`into_neg_errno`]
 /// for the "right" set of types.
 mod traits {