Add a new error variant for pre-serialization formatting errors (#8)

Implementing XmlSerialize on some types might require handling errors when formatting them prior to serializing to XML. This change adds a new error variant to handle such errors, using anyhow to be flexible enough to handle errors from third-party crates without having to special-case each one we want to support going forwards.

xml_struct/Cargo.toml

@@ -6,6 +6,7 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
+anyhow = "1.0.69"
 quick-xml = "0.31.0"
 thiserror = "1.0.56"
 xml_struct_derive = { version = "0.1.0", path = "../xml_struct_derive" }

xml_struct/src/lib.rs

@@ -134,4 +134,10 @@ pub trait XmlSerializeAttr {
 pub enum Error {
     #[error("failed to process XML document")]
     Xml(#[from] quick_xml::Error),
+
+    /// An error representing a failure in formatting a data structure prior to
+    /// serializing it into XML. Its inner type is generic on purpose, as the
+    /// specific error type might be defined by a third-party crate.
+    #[error("failed to serialize value as text")]
+    Value(#[from] anyhow::Error),
 }