Require expectation in AssertError

src/error.rs

@@ -15,8 +15,10 @@ pub struct AssertError {
 impl AssertError {
     /// Creates a new assertion error. Attach fields using the
     /// [`Self::with_field`] method.
-    pub const fn new() -> Self {
-        Self { fields: Vec::new() }
+    pub fn new(expectation: impl Into<Cow<'static, str>>) -> Self {
+        Self {
+            fields: vec![("expected", expectation.into())],
+        }
     }
 
     /// Attaches a custom field to the error. This will appear in the error when

src/expect.rs

@@ -126,8 +126,7 @@ impl<T> Assertable for TryExpectationRoot<T> {
         if satisfied {
             Ok(())
         } else {
-            let error = AssertError::new()
-                .with_field("expected", expectation.to_string())
+            let error = AssertError::new(expectation.to_string())
                 .with_field("at", self.source_info.to_string())
                 .with_field("original target", self.target_source);
             Err(error)