Add compile error on const fn instrumentation

Author: andrewpollack

tracing-attributes/src/lib.rs

@@ -385,6 +385,15 @@ fn instrument_precise(
     let input = syn::parse::<ItemFn>(item)?;
     let instrumented_function_name = input.sig.ident.to_string();
 
+    macro_rules! error_on_const_fn {
+        ($e:expr) => {
+            if $e {
+                compile_error!("instrumenting const fn not supported");
+            }
+        };
+    }
+    error_on_const_fn!(input.sig.constness.is_some());
+
     // check for async_trait-like patterns in the block, and instrument
     // the future instead of the wrapper
     if let Some(async_like) = expand::AsyncInfo::from_fn(&input) {