Prepare for new riscv ecosystem

rtic-macros/src/syntax/backend/riscv_slic.rs

@@ -1,16 +1,27 @@
 use syn::{
     parse::{Parse, ParseStream},
-    Ident, Result,
+    Result,
 };
 
 #[derive(Debug)]
 pub struct BackendArgs {
-    pub hart_id: Ident,
+    #[cfg(feature = "riscv-clint")]
+    pub hart_id: syn::Ident,
 }
 
 impl Parse for BackendArgs {
     fn parse(input: ParseStream) -> Result<Self> {
-        let hart_id = input.parse()?;
-        Ok(BackendArgs { hart_id })
+        match () {
+            #[cfg(feature = "riscv-clint")]
+            () => {
+                let hart_id = input.parse()?;
+                Ok(BackendArgs { hart_id })
+            }
+            #[cfg(feature = "riscv-mecall")]
+            () => Err(syn::Error::new(
+                input.span(),
+                "riscv-mecall backend does not accept any arguments",
+            )),
+        }
     }
 }