Introduce a simpler function that hides the creation of the Error**.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 rust/util/src/error.rs | 52 ++++++++++++++++--------------------------
 1 file changed, 20 insertions(+), 32 deletions(-)

diff --git a/rust/util/src/error.rs b/rust/util/src/error.rs
index bdbf2634170..5a0dd7786b5 100644
--- a/rust/util/src/error.rs
+++ b/rust/util/src/error.rs
@@ -14,8 +14,7 @@
 //!   [`ptr_or_propagate`](crate::Error::ptr_or_propagate) can be used to build
 //!   a C return value while also propagating an error condition
 //!
-//! * [`err_or_else`](crate::Error::err_or_else) and
-//!   [`err_or_unit`](crate::Error::err_or_unit) can be used to build a `Result`
+//! * [`with_errp`](crate::Error::with_errp) can be used to build a `Result`
 //!
 //! This module is most commonly used at the boundary between C and Rust code;
 //! other code will usually access it through the
@@ -208,35 +207,21 @@ pub unsafe fn propagate(self, errp: *mut *mut bindings::Error) {
         }
     }
 
-    /// Convert a C `Error*` into a Rust `Result`, using
-    /// `Ok(())` if `c_error` is NULL.  Free the `Error*`.
+    /// Pass a C `Error*` to the closure, and convert the result
+    /// (either the return value of the closure, or the error)
+    /// into a Rust `Result`.
     ///
     /// # Safety
     ///
-    /// `c_error` must be `NULL` or valid; typically it was initialized
-    /// with `ptr::null_mut()` and passed by reference to a C function.
-    pub unsafe fn err_or_unit(c_error: *mut bindings::Error) -> Result<()> {
-        // SAFETY: caller guarantees c_error is valid
-        unsafe { Self::err_or_else(c_error, || ()) }
-    }
+    /// One exit from `f`, `c_error` must be unchanged or point to a
+    /// valid C [`struct Error`](bindings::Error).
+    pub unsafe fn with_errp<T, F: FnOnce(&mut *mut bindings::Error) -> T>(f: F) -> Result<T> {
+        let mut c_error: *mut bindings::Error = ptr::null_mut();
 
-    /// Convert a C `Error*` into a Rust `Result`, calling `f()` to
-    /// obtain an `Ok` value if `c_error` is NULL.  Free the `Error*`.
-    ///
-    /// # Safety
-    ///
-    /// `c_error` must be `NULL` or point to a valid C [`struct
-    /// Error`](bindings::Error); typically it was initialized with
-    /// `ptr::null_mut()` and passed by reference to a C function.
-    pub unsafe fn err_or_else<T, F: FnOnce() -> T>(
-        c_error: *mut bindings::Error,
-        f: F,
-    ) -> Result<T> {
-        // SAFETY: caller guarantees c_error is valid
-        let err = unsafe { Option::<Self>::from_foreign(c_error) };
-        match err {
-            None => Ok(f()),
-            Some(err) => Err(err),
+        // SAFETY: guaranteed by the postcondition of `f`
+        match (f(&mut c_error), unsafe { c_error.into_native() }) {
+            (result, None) => Ok(result),
+            (_, Some(err)) => Err(err),
         }
     }
 }
@@ -421,13 +406,16 @@ fn test_ptr_or_propagate() {
     }
 
     #[test]
-    fn test_err_or_unit() {
+    fn test_with_errp() {
         unsafe {
-            let result = Error::err_or_unit(ptr::null_mut());
-            assert_match!(result, Ok(()));
+            let result = Error::with_errp(|_errp| true);
+            assert_match!(result, Ok(true));
 
-            let err = error_for_test(c"msg");
-            let err = Error::err_or_unit(err.into_inner()).unwrap_err();
+            let err = Error::with_errp(|errp| {
+                *errp = error_for_test(c"msg").into_inner();
+                false
+            })
+            .unwrap_err();
             assert_eq!(&*format!("{err}"), "msg");
         }
     }
-- 
2.51.0