Passing first test

.gitignore

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+/target
+Cargo.lock

Cargo.toml

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+[workspace]
+members = ["instant-xml", "instant-xml-macros"]

instant-xml-macros/Cargo.toml

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+[package]
+name = "instant-xml-macros"
+version = "0.1.0"
+edition = "2021"
+workspace = ".."
+
+[lib]
+proc-macro = true
+
+[dependencies]
+heck = "0.4"
+quote = "1.0.18"
+syn = { version = "1.0.86", features = ["full"] }

instant-xml-macros/src/lib.rs

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+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::parse_macro_input;
+
+#[proc_macro_derive(ToXml)]
+pub fn to_xml(input: TokenStream) -> TokenStream {
+    let ast = parse_macro_input!(input as syn::ItemStruct);
+    let ident = &ast.ident;
+    let name = ident.to_string();
+    TokenStream::from(quote!(
+        impl ToXml for #ident {
+            fn write_xml<W: ::std::fmt::Write>(&self, write: &mut W) -> Result<(), instant_xml::Error> {
+                write.write_str("<")?;
+                write.write_fmt(format_args!("{}", #name))?;
+                write.write_str("/>")?;
+                Ok(())
+            }
+        }
+    ))
+}

instant-xml/Cargo.toml

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+[package]
+name = "instant-xml"
+version = "0.1.0"
+edition = "2021"
+workspace = ".."
+
+[dependencies]
+macros = { package = "instant-xml-macros", version = "0.1", path = "../instant-xml-macros" }
+thiserror = "1.0.29"
+xmlparser = "0.13.3"

instant-xml/src/lib.rs

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+use std::fmt;
+
+use thiserror::Error;
+
+pub use macros::ToXml;
+
+pub trait ToXml {
+    fn write_xml<W: fmt::Write>(&self, write: &mut W) -> Result<(), Error>;
+
+    fn to_xml(&self) -> Result<String, Error> {
+        let mut out = String::new();
+        self.write_xml(&mut out)?;
+        Ok(out)
+    }
+}
+
+pub trait FromXml<'xml>: Sized {
+    fn from_xml(input: &str) -> Result<Self, Error>;
+}
+
+pub trait FromXmlOwned: for<'xml> FromXml<'xml> {}
+
+#[derive(Debug, Error)]
+pub enum Error {
+    #[error("format: {0}")]
+    Format(#[from] fmt::Error),
+}

instant-xml/tests/all.rs

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+use instant_xml::ToXml;
+
+#[derive(ToXml)]
+struct Unit;
+
+#[test]
+fn unit() {
+    assert_eq!(Unit.to_xml().unwrap(), "<Unit/>");
+}