Fix formatting of applies in backpassing

compiler/fmt/src/expr.rs

@@ -861,6 +861,14 @@ fn fmt_backpassing<'a>(
         indent
     };
 
+    let pattern_needs_parens = loc_patterns
+        .iter()
+        .any(|p| pattern_needs_parens_when_backpassing(&p.value));
+
+    if pattern_needs_parens {
+        buf.push('(');
+    }
+
     let mut it = loc_patterns.iter().peekable();
 
     while let Some(loc_pattern) = it.next() {
@@ -876,6 +884,10 @@ fn fmt_backpassing<'a>(
         }
     }
 
+    if pattern_needs_parens {
+        buf.push(')');
+    }
+
     if arguments_are_multiline {
         newline(buf, indent);
     } else {
@@ -896,7 +908,7 @@ fn fmt_backpassing<'a>(
     // the body of the Backpass can be on the same line, or
     // on a new line. If it's on the same line, insert a space.
 
-    match &loc_ret.value {
+    match &loc_body.value {
         SpaceBefore(_, _) => {
             // the body starts with (first comment and then) a newline
             // do nothing
@@ -911,6 +923,16 @@ fn fmt_backpassing<'a>(
     loc_ret.format_with_options(buf, Parens::NotNeeded, Newlines::Yes, indent);
 }
 
+fn pattern_needs_parens_when_backpassing(pat: &Pattern) -> bool {
+    match pat {
+        Pattern::Apply(_, _) => true,
+        Pattern::SpaceBefore(a, _) | Pattern::SpaceAfter(a, _) => {
+            pattern_needs_parens_when_backpassing(a)
+        }
+        _ => false,
+    }
+}
+
 fn fmt_record<'a>(
     buf: &mut String<'a>,
     update: Option<&'a Located<Expr<'a>>>,

compiler/fmt/tests/test_fmt.rs

@@ -2714,6 +2714,46 @@ mod test_fmt {
         ));
     }
 
+    #[test]
+    fn backpassing_simple() {
+        expr_formats_same(indoc!(
+            r#"
+                getChar = \ctx ->
+                    x <- Task.await (getCharScope scope)
+                    42
+
+                42
+            "#
+        ));
+    }
+
+    #[test]
+    fn backpassing_apply_tag() {
+        expr_formats_same(indoc!(
+            r#"
+                getChar = \ctx ->
+                    (T val newScope) <- Task.await (getCharScope scope)
+                    42
+
+                42
+            "#
+        ));
+    }
+
+    #[test]
+    fn backpassing_body_on_newline() {
+        expr_formats_same(indoc!(
+            r#"
+                getChar = \ctx ->
+                    x <-
+                        Task.await (getCharScope scope)
+                    42
+
+                42
+            "#
+        ));
+    }
+
     // this is a parse error atm
     //    #[test]
     //    fn multiline_apply() {