Rollup merge of #149174 - GrigorenkoPV:const_block_item, r=me,ytmimi

`const` blocks as a `mod` item

Tracking issue: rust-lang/rust#149226

This adds support for writing `const { ... }` as an item in a module. In the current implementation, this is a unique AST item that gets lowered to `const _: () = const { ... };` in HIR.

rustfmt support included.

TODO:
- `pub const { ... }` does not make sense (see rust-lang/rust#147136). Reject it. Should this be rejected by the parser or smth?
- Improve diagnostics (preferably they should not mention the fake `_` ident).

Author: matthiaskrgr

src/parse/macros/cfg_if.rs

@@ -3,7 +3,7 @@ use std::panic::{AssertUnwindSafe, catch_unwind};
 use rustc_ast::ast;
 use rustc_ast::token::TokenKind;
 use rustc_parse::exp;
-use rustc_parse::parser::ForceCollect;
+use rustc_parse::parser::{AllowConstBlockItems, ForceCollect};
 use rustc_span::symbol::kw;
 
 use crate::parse::macros::build_stream_parser;
@@ -61,7 +61,7 @@ fn parse_cfg_if_inner<'a>(
         }
 
         while parser.token != TokenKind::CloseBrace && parser.token.kind != TokenKind::Eof {
-            let item = match parser.parse_item(ForceCollect::No) {
+            let item = match parser.parse_item(ForceCollect::No, AllowConstBlockItems::Yes) {
                 Ok(Some(item_ptr)) => *item_ptr,
                 Ok(None) => continue,
                 Err(err) => {

src/parse/macros/mod.rs

@@ -2,7 +2,7 @@ use rustc_ast::ast;
 use rustc_ast::token::{Delimiter, NonterminalKind, NtExprKind::*, NtPatKind::*, TokenKind};
 use rustc_ast::tokenstream::TokenStream;
 use rustc_parse::MACRO_ARGUMENTS;
-use rustc_parse::parser::{ForceCollect, Parser, Recovery};
+use rustc_parse::parser::{AllowConstBlockItems, ForceCollect, Parser, Recovery};
 use rustc_session::parse::ParseSess;
 use rustc_span::symbol;
 
@@ -67,7 +67,7 @@ fn parse_macro_arg<'a, 'b: 'a>(parser: &'a mut Parser<'b>) -> Option<MacroArg> {
     parse_macro_arg!(
         Item,
         NonterminalKind::Item,
-        |parser: &mut Parser<'b>| parser.parse_item(ForceCollect::No),
+        |parser: &mut Parser<'b>| parser.parse_item(ForceCollect::No, AllowConstBlockItems::Yes),
         |x: Option<Box<ast::Item>>| x
     );
 

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,

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); }

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);
+}