Draft: fix: Fn supertraits indentation

Author: Flowrey

src/expr.rs

@@ -2110,7 +2110,7 @@ fn choose_rhs<R: Rewrite>(
     expr: &R,
     shape: Shape,
     orig_rhs: Option<String>,
-    _rhs_kind: &RhsAssignKind<'_>,
+    rhs_kind: &RhsAssignKind<'_>,
     rhs_tactics: RhsTactics,
     has_rhs_comment: bool,
 ) -> Option<String> {
@@ -2141,7 +2141,11 @@ fn choose_rhs<R: Rewrite>(
                 (Some(ref orig_rhs), Some(ref new_rhs))
                     if prefer_next_line(orig_rhs, new_rhs, rhs_tactics) =>
                 {
-                    Some(format!("{new_indent_str}{new_rhs}"))
+                    if let RhsAssignKind::Bounds = rhs_kind {
+                        Some(new_rhs.lines().map(|l| format!("{new_indent_str}{l}")).join(""))
+                    } else {
+                        Some(format!("{new_indent_str}{new_rhs}"))
+                    }
                 }
                 (None, Some(ref new_rhs)) => Some(format!("{new_indent_str}{new_rhs}")),
                 (None, None) if rhs_tactics == RhsTactics::AllowOverflow => {

tests/source/issue-6127.rs

@@ -0,0 +1,9 @@
+trait Foo:
+    Fn(
+    ReallyLongTypeName,
+    ReallyLongTypeName,
+    ReallyLongTypeName,
+    ReallyLongTypeName,
+) -> ReallyLongTypeName
+{
+}

tests/target/issue-6127.rs

@@ -0,0 +1,9 @@
+trait Foo:
+    Fn(
+        ReallyLongTypeName,
+        ReallyLongTypeName,
+        ReallyLongTypeName,
+        ReallyLongTypeName,
+    ) -> ReallyLongTypeName
+{
+}