Add better error message when using `timed` on something else than a function

src/lib.rs

@@ -5,14 +5,22 @@
 /// `extern crate` Required even for 2018 edition
 extern crate proc_macro;
 use proc_macro::TokenStream;
-use quote::quote;
-
-type StaticResult<T> = ::std::result::Result<T, &'static str>;
-
+use quote::{quote, quote_spanned};
+use syn::spanned::Spanned;
+
+/// ```compile_fail
+/// use micro_timer::timed;
+///
+/// #[timed]  // Can only be used on functions
+/// struct Thing;
+/// ```
 #[proc_macro_attribute]
 pub fn timed(_attr_ts: TokenStream, fn_ts: TokenStream) -> TokenStream {
     let ast = syn::parse(fn_ts.clone()).unwrap();
-    let func = parse_function(ast).unwrap();
+    let func = match parse_function(ast) {
+        Ok(f) => f,
+        Err(stream) => return stream,
+    };
 
     let mut outer = func.clone();
     outer.sig.ident = func.sig.ident.to_owned();
@@ -42,9 +50,13 @@ pub fn timed(_attr_ts: TokenStream, fn_ts: TokenStream) -> TokenStream {
     (quote! {#outer}).into()
 }
 
-fn parse_function(item: syn::Item) -> StaticResult<syn::ItemFn> {
+fn parse_function(item: syn::Item) -> Result<syn::ItemFn, TokenStream> {
     match item {
         syn::Item::Fn(func) => Ok(func),
-        _ => Err("`time_it` attribute can only be used on functions"),
+        i => Err(quote_spanned! {
+            i.span() =>
+            compile_error!("`#[timed]` can only be used on functions");
+        }
+        .into()),
     }
 }