Don't support async functions

It doesn't make sense to time async functions right now. Maybe something could
be devised for async functions in the future (ah. ah.), but right now this would
just be misleading.

libs/micro-timer-macros/src/lib.rs

@@ -19,6 +19,13 @@ use syn::spanned::Spanned;
 /// #[timed]  // Can only be used on functions
 /// struct Thing;
 /// ```
+///
+/// ```compile_fail
+/// use micro_timer::timed;
+///
+/// #[timed]  // Can only be used on sync functions
+/// async fn func() {}
+/// ```
 #[proc_macro_attribute]
 pub fn timed(
     attrs: proc_macro::TokenStream,
@@ -75,7 +82,16 @@ fn inner_timed(_attr_ts: TokenStream, fn_ts: TokenStream) -> TokenStream {
 
 fn parse_function(item: syn::Item) -> Result<syn::ItemFn, TokenStream> {
     match item {
-        syn::Item::Fn(func) => Ok(func),
+        syn::Item::Fn(func) => {
+            if func.sig.asyncness.is_some() {
+                return Err(quote_spanned! {
+                    func.span()=>
+                    compile_error!("Cannot use `#[timed]` on async functions")
+                    #func
+                });
+            }
+            Ok(func)
+        }
         i => Err(quote_spanned! {
             i.span()=>
             compile_error!("`#[timed]` can only be used on functions");