rustfmt: support const block items

Author: GrigorenkoPV

src/tools/rustfmt/src/visitor.rs

@@ -7,7 +7,9 @@ use rustc_span::{BytePos, Ident, Pos, Span, symbol};
 use tracing::debug;
 
 use crate::attr::*;
-use crate::comment::{CodeCharKind, CommentCodeSlices, contains_comment, rewrite_comment};
+use crate::comment::{
+    CodeCharKind, CommentCodeSlices, contains_comment, recover_comment_removed, rewrite_comment,
+};
 use crate::config::{BraceStyle, Config, MacroSelector, StyleEdition};
 use crate::coverage::transform_missing_snippet;
 use crate::items::{
@@ -533,6 +535,28 @@ impl<'b, 'a: 'b> FmtVisitor<'a> {
                 ast::ItemKind::Static(..) | ast::ItemKind::Const(..) => {
                     self.visit_static(&StaticParts::from_item(item));
                 }
+                ast::ItemKind::ConstBlock(ast::ConstBlockItem {
+                    id: _,
+                    span,
+                    ref block,
+                }) => {
+                    let context = &self.get_context();
+                    let offset = self.block_indent;
+                    self.push_rewrite(
+                        item.span,
+                        block
+                            .rewrite(
+                                context,
+                                Shape::legacy(
+                                    context.budget(offset.block_indent),
+                                    offset.block_only(),
+                                ),
+                            )
+                            .map(|rhs| {
+                                recover_comment_removed(format!("const {rhs}"), span, context)
+                            }),
+                    );
+                }
                 ast::ItemKind::Fn(ref fn_kind) => {
                     let ast::Fn {
                         defaultness,

src/tools/rustfmt/tests/source/const-block-items.rs

@@ -0,0 +1,24 @@
+#![feature(const_block_items)]
+
+ const {
+
+
+  assert!(true)
+                  }
+
+  #[cfg(false)] const { assert!(false) }
+
+
+  #[cfg(false)]
+// foo
+ const
+
+ {
+  // bar
+   assert!(false)
+  // baz
+   } // 123
+
+
+  #[expect(unused)]
+pub    const {  let a = 1;  assert!(true); }

src/tools/rustfmt/tests/target/const-block-items.rs

@@ -0,0 +1,20 @@
+#![feature(const_block_items)]
+
+const { assert!(true) }
+
+#[cfg(false)]
+const { assert!(false) }
+
+#[cfg(false)]
+// foo
+const {
+    // bar
+    assert!(false)
+    // baz
+} // 123
+
+#[expect(unused)]
+const {
+    let a = 1;
+    assert!(true);
+}

tests/ui/parser/const-block-items/inner-attr.rs

@@ -1,5 +1,8 @@
 #![feature(const_block_items)]
 
+// ATTENTION: if we ever start accepting inner attributes here, make sure `rustfmt` can handle them.
+//            see: https://github.com/rust-lang/rustfmt/issues/6158
+
 const {
     #![expect(unused)] //~ ERROR: an inner attribute is not permitted in this context
     let a = 1;

tests/ui/parser/const-block-items/inner-attr.stderr

@@ -1,5 +1,5 @@
 error: an inner attribute is not permitted in this context
-  --> $DIR/inner-attr.rs:4:5
+  --> $DIR/inner-attr.rs:7:5
    |
 LL |     #![expect(unused)]
    |     ^^^^^^^^^^^^^^^^^^