fix formatting of panic/todo block messages

Author: giacomocavalieri

CHANGELOG.md

@@ -451,6 +451,10 @@
   in a different function.
   ([Giacomo Cavalieri](https://github.com/giacomocavalieri))
 
+- Fixed a bug where the formatter would format a panic message adding more
+  nesting than necessary.
+  ([Giacomo Cavalieri](https://github.com/giacomocavalieri))
+
 - Fixed a bug where the compiler allowed to write a guard with an empty clause.
   ([Tristan-Mihai Radulescu](https://github.com/Courtcircuits))
 

compiler-core/src/format.rs

@@ -2912,12 +2912,27 @@ impl<'comments> Formatter<'comments> {
                 docvec![line(), comments, line(), self.expr(message).group()].nest(INDENT)
             ],
 
-            None => docvec![
-                doc.group(),
-                as_,
-                " ",
-                self.expr(message).group().nest(INDENT),
-            ],
+            None => {
+                let message = match (preceding_as, message) {
+                    // If we have `as` preceded by a keyword (like with `panic` and `todo`)
+                    // and the message is a block, we don't want to nest it any further. That is,
+                    // we want it to look like this:
+                    // ```gleam
+                    // panic as {
+                    //   wibble wobble
+                    // }
+                    // ```
+                    // instead of this:
+                    // ```gleam
+                    // panic as {
+                    //     wibble wobble
+                    //   }
+                    // ```
+                    (PrecedingAs::Keyword, UntypedExpr::Block { .. }) => self.expr(message).group(),
+                    _ => self.expr(message).group().nest(INDENT),
+                };
+                docvec![doc.group(), as_, " ", message]
+            }
         };
 
         doc.group()

compiler-core/src/format/tests.rs

@@ -6719,3 +6719,17 @@ fn pattern_unused_discard() {
 "#
     );
 }
+
+// https://github.com/gleam-lang/gleam/issues/4929
+#[test]
+fn format_panic_as_with_block_message() {
+    assert_format!(
+        r#"pub fn main() {
+  panic as {
+    // b
+    a
+  }
+}
+"#
+    );
+}