Include constness in impl blocks (#4215)

Closes #4084

rustfmt-core/rustfmt-lib/src/formatting/items.rs

@@ -905,6 +905,7 @@ fn format_impl_ref_and_type(
         unsafety,
         polarity,
         defaultness,
+        constness,
         ref generics,
         of_trait: ref trait_ref,
         ref self_ty,
@@ -920,6 +921,7 @@ fn format_impl_ref_and_type(
         let shape = Shape::indented(offset + last_line_width(&result), context.config);
         let generics_str = rewrite_generics(context, "impl", generics, shape)?;
         result.push_str(&generics_str);
+        result.push_str(format_constness_right(constness));
 
         let polarity_str = match polarity {
             ast::ImplPolarity::Negative(_) => "!",

rustfmt-core/rustfmt-lib/src/formatting/utils.rs

@@ -102,6 +102,14 @@ 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 {

rustfmt-core/rustfmt-lib/tests/source/impls.rs

@@ -160,3 +160,11 @@ impl<'a, 'b, 'c> SomeThing<Something> for (&'a mut SomethingLong, &'b mut Someth
 
 // #2746
 impl<'seq1, 'seq2, 'body, 'scope, Channel> Adc12< Dual, MasterRunningDma<'seq1, 'body, 'scope, Channel>, SlaveRunningDma<'seq2, 'body, 'scope>, > where Channel: DmaChannel, {}
+
+// #4084
+impl     const       std::default::Default for Struct {
+    #[inline]
+    fn default() -> Self {
+        Self { f: 12.5 }
+    }
+}

rustfmt-core/rustfmt-lib/tests/target/impls.rs

@@ -234,3 +234,11 @@ where
     Channel: DmaChannel,
 {
 }
+
+// #4084
+impl const std::default::Default for Struct {
+    #[inline]
+    fn default() -> Self {
+        Self { f: 12.5 }
+    }
+}