Improve error on const fn

src/lib.rs

@@ -168,6 +168,12 @@ pub fn no_panic(args: TokenStream, input: TokenStream) -> TokenStream {
 fn parse(args: TokenStream2, input: TokenStream2) -> Result<ItemFn> {
     let function: ItemFn = syn::parse2(input)?;
     let _: Nothing = syn::parse2::<Nothing>(args)?;
+    if function.sig.constness.is_some() {
+        return Err(Error::new(
+            Span::call_site(),
+            "no_panic attribute on const fn is not supported",
+        ));
+    }
     if function.sig.asyncness.is_some() {
         return Err(Error::new(
             Span::call_site(),

tests/ui/const-fn.stderr

@@ -1,9 +1,7 @@
-error[E0015]: cannot call non-const closure in constant functions
+error: no_panic attribute on const fn is not supported
  --> tests/ui/const-fn.rs:3:1
   |
 3 | #[no_panic]
   | ^^^^^^^^^^^
   |
-  = note: closures need an RFC before allowed to be called in constant functions
-  = note: calls in constant functions are limited to constant functions, tuple structs and tuple variants
   = note: this error originates in the attribute macro `no_panic` (in Nightly builds, run with -Z macro-backtrace for more info)