Fix formatting of const trait aliases

Author: oli-obk

src/tools/rustfmt/src/items.rs

@@ -1387,22 +1387,21 @@ impl<'a> Rewrite for TraitAliasBounds<'a> {
 
 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,

src/tools/rustfmt/src/visitor.rs

@@ -499,14 +499,7 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
                 }
                 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(..) => {

src/tools/rustfmt/tests/target/trait.rs

@@ -219,4 +219,4 @@ trait Visible {
     pub fn g() {}
 }
 
-trait Foomp = Hash;
+const trait Foomp = Hash;