fix(ser): Error on too-large u64

This is better than going negative

Fixes #511

crates/toml/tests/testsuite/serde.rs

@@ -955,8 +955,8 @@ fn integer_too_big() {
     let native = Foo { a_b: u64::MAX };
     let err = Table::try_from(native.clone()).unwrap_err();
     snapbox::assert_eq("u64 value was too large", err.to_string());
-    let err = toml::to_string(&native).unwrap();
-    snapbox::assert_eq("a_b = -1\n", err.to_string());
+    let err = toml::to_string(&native).unwrap_err();
+    snapbox::assert_eq("out-of-range value for u64 type", err.to_string());
 }
 
 #[test]

crates/toml_edit/src/ser/mod.rs

@@ -20,6 +20,8 @@ use crate::visit_mut::VisitMut;
 pub enum Error {
     /// Type could not be serialized to TOML
     UnsupportedType(Option<&'static str>),
+    /// Value was out of range for the given type
+    OutOfRange(Option<&'static str>),
     /// `None` could not be serialized to TOML
     UnsupportedNone,
     /// Key was not convertable to `String` for serializing to TOML
@@ -53,6 +55,8 @@ impl std::fmt::Display for Error {
         match self {
             Self::UnsupportedType(Some(t)) => write!(formatter, "unsupported {t} type"),
             Self::UnsupportedType(None) => write!(formatter, "unsupported rust type"),
+            Self::OutOfRange(Some(t)) => write!(formatter, "out-of-range value for {t} type"),
+            Self::OutOfRange(None) => write!(formatter, "out-of-range value"),
             Self::UnsupportedNone => "unsupported None value".fmt(formatter),
             Self::KeyNotString => "map key was not a string".fmt(formatter),
             Self::DateInvalid => "a serialized date was invalid".fmt(formatter),

crates/toml_edit/src/ser/value.rs

@@ -98,7 +98,10 @@ impl serde::ser::Serializer for ValueSerializer {
     }
 
     fn serialize_u64(self, v: u64) -> Result<Self::Ok, Self::Error> {
-        self.serialize_i64(v as i64)
+        let v: i64 = v
+            .try_into()
+            .map_err(|_err| Error::OutOfRange(Some("u64")))?;
+        self.serialize_i64(v)
     }
 
     fn serialize_f32(self, v: f32) -> Result<Self::Ok, Self::Error> {