From ebb249d69e84df66f183b45b45c51e21ad0d0988 Mon Sep 17 00:00:00 2001
From: Oli Scherer <github333195615777966@oli-obk.de>
Date: Wed, 23 Jul 2025 14:29:01 +0000
Subject: [PATCH] Fix formatting of const trait aliases

---
 src/tools/rustfmt/src/items.rs                | 15 +++++++--------
 src/tools/rustfmt/src/visitor.rs              |  9 +--------
 src/tools/rustfmt/tests/target/const_trait.rs |  5 +++++
 src/tools/rustfmt/tests/target/trait.rs       |  2 +-
 4 files changed, 14 insertions(+), 17 deletions(-)
 create mode 100644 src/tools/rustfmt/tests/target/const_trait.rs

diff --git a/src/tools/rustfmt/src/items.rs b/src/tools/rustfmt/src/items.rs
index a2e89c10a480..ecaa4670f61d 100644
--- a/src/tools/rustfmt/src/items.rs
+++ b/src/tools/rustfmt/src/items.rs
@@ -1375,22 +1375,21 @@ fn rewrite_result(&self, context: &RewriteContext<'_>, shape: Shape) -> RewriteR
 
 pub(crate) fn format_trait_alias(
     context: &RewriteContext<'_>,
-    ident: symbol::Ident,
+    ta: &ast::TraitAlias,
     vis: &ast::Visibility,
-    generics: &ast::Generics,
-    generic_bounds: &ast::GenericBounds,
     shape: Shape,
 ) -> Option<String> {
-    let alias = rewrite_ident(context, ident);
+    let alias = rewrite_ident(context, ta.ident);
     // 6 = "trait ", 2 = " ="
     let g_shape = shape.offset_left(6)?.sub_width(2)?;
-    let generics_str = rewrite_generics(context, alias, generics, g_shape).ok()?;
+    let generics_str = rewrite_generics(context, alias, &ta.generics, g_shape).ok()?;
     let vis_str = format_visibility(context, vis);
-    let lhs = format!("{vis_str}trait {generics_str} =");
+    let constness = format_constness(ta.constness);
+    let lhs = format!("{vis_str}{constness}trait {generics_str} =");
     // 1 = ";"
     let trait_alias_bounds = TraitAliasBounds {
-        generic_bounds,
-        generics,
+        generic_bounds: &ta.bounds,
+        generics: &ta.generics,
     };
     rewrite_assign_rhs(
         context,
diff --git a/src/tools/rustfmt/src/visitor.rs b/src/tools/rustfmt/src/visitor.rs
index 633a17a40108..c521b497a2d9 100644
--- a/src/tools/rustfmt/src/visitor.rs
+++ b/src/tools/rustfmt/src/visitor.rs
@@ -499,14 +499,7 @@ pub(crate) fn visit_item(&mut self, item: &ast::Item) {
                 }
                 ast::ItemKind::TraitAlias(ref ta) => {
                     let shape = Shape::indented(self.block_indent, self.config);
-                    let rw = format_trait_alias(
-                        &self.get_context(),
-                        ta.ident,
-                        &item.vis,
-                        &ta.generics,
-                        &ta.bounds,
-                        shape,
-                    );
+                    let rw = format_trait_alias(&self.get_context(), ta, &item.vis, shape);
                     self.push_rewrite(item.span, rw);
                 }
                 ast::ItemKind::ExternCrate(..) => {
diff --git a/src/tools/rustfmt/tests/target/const_trait.rs b/src/tools/rustfmt/tests/target/const_trait.rs
new file mode 100644
index 000000000000..c4c88b17fc50
--- /dev/null
+++ b/src/tools/rustfmt/tests/target/const_trait.rs
@@ -0,0 +1,5 @@
+#![feature(trait_alias, const_trait_impl)]
+
+const trait Bar {}
+
+const trait Foo = Bar;
diff --git a/src/tools/rustfmt/tests/target/trait.rs b/src/tools/rustfmt/tests/target/trait.rs
index cc2b59dc6162..7a65b13d629e 100644
--- a/src/tools/rustfmt/tests/target/trait.rs
+++ b/src/tools/rustfmt/tests/target/trait.rs
@@ -219,4 +219,4 @@ trait Visible {
     pub fn g() {}
 }
 
-trait Foomp = Hash;
+const trait Foomp = Hash;