From f81f4066a237409a3d4774b6e193457613e9dc58 Mon Sep 17 00:00:00 2001
From: Louis Pilfold <louis@lpil.uk>
Date: Mon, 29 Apr 2024 13:14:51 +0100
Subject: [PATCH] Tweak and comment

---
 compiler-core/src/type_/expression.rs | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/compiler-core/src/type_/expression.rs b/compiler-core/src/type_/expression.rs
index ed9bfc74b33..17be88b5f9f 100644
--- a/compiler-core/src/type_/expression.rs
+++ b/compiler-core/src/type_/expression.rs
@@ -650,12 +650,17 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
     ) -> Result<TypedExpr, Error> {
         let (fun, args, typ) = self.do_infer_call(fun, args, location)?;
 
-        let _ = match fun {
+        // One common mistake is to think that the syntax for adding a message
+        // to a `todo` or a `panic` exception is to `todo("...")`, but really
+        // this does nothing as the `todo` or `panic` throws the exception
+        // before it gets to the function call `("...")`.
+        // If we find code doing this then emit a warning.
+        let todopanic = match fun {
             TypedExpr::Todo { .. } => Some((location, TodoOrPanic::Todo)),
             TypedExpr::Panic { .. } => Some((location, TodoOrPanic::Panic)),
             _ => None,
-        }
-        .map(|(location, kind)| {
+        };
+        if let Some((location, kind)) = todopanic {
             let args_location = match (args.first(), args.last()) {
                 (Some(first), Some(last)) => Some(SrcSpan {
                     start: first.location().start,
@@ -663,7 +668,6 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
                 }),
                 _ => None,
             };
-
             self.environment
                 .warnings
                 .emit(Warning::TodoOrPanicUsedAsFunction {
@@ -672,7 +676,7 @@ impl<'a, 'b> ExprTyper<'a, 'b> {
                     args_location,
                     args: args.len(),
                 });
-        });
+        }
 
         Ok(TypedExpr::Call {
             location,