rustfmt: format const trait impls to const impl for syntax transition

src/tools/rustfmt/src/items.rs

@@ -960,6 +960,7 @@ fn format_impl_ref_and_type(
 
     if let Some(of_trait) = of_trait.as_deref() {
         result.push_str(format_defaultness(of_trait.defaultness));
+        result.push_str(format_constness(*constness));
         result.push_str(format_safety(of_trait.safety));
     } else {
         result.push_str(format_constness(*constness));
@@ -980,7 +981,6 @@ fn format_impl_ref_and_type(
 
     let trait_ref_overhead;
     if let Some(of_trait) = of_trait.as_deref() {
-        result.push_str(format_constness_right(*constness));
         let polarity_str = match of_trait.polarity {
             ast::ImplPolarity::Negative(_) => "!",
             ast::ImplPolarity::Positive => "",

src/tools/rustfmt/src/utils.rs

@@ -125,14 +125,6 @@ pub(crate) fn format_constness(constness: ast::Const) -> &'static str {
     }
 }
 
-#[inline]
-pub(crate) fn format_constness_right(constness: ast::Const) -> &'static str {
-    match constness {
-        ast::Const::Yes(..) => " const",
-        ast::Const::No => "",
-    }
-}
-
 #[inline]
 pub(crate) fn format_defaultness(defaultness: ast::Defaultness) -> &'static str {
     match defaultness {

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

@@ -4,10 +4,7 @@ const trait Bar {}
 
 const trait Foo = Bar;
 
-impl const Bar for () {}
-
-// const impl gets reformatted to impl const.. for now
-impl const Bar for u8 {}
+const impl Bar for () {}
 
 struct X;
 

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

@@ -244,7 +244,7 @@ where
 }
 
 // #4084
-impl const std::default::Default for Struct {
+const impl std::default::Default for Struct {
     #[inline]
     fn default() -> Self {
         Self { f: 12.5 }

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

@@ -153,7 +153,7 @@ const fn not_quite_const<S: [const] T>() -> i32 {
     <S as T>::CONST
 }
 
-impl const T for U {}
+const impl T for U {}
 
 fn apit(_: impl [const] T) {}