Sugar for const _: () =

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -537,6 +537,7 @@ macro_rules! gate_all {
     gate_all!(super_let, "`super let` is experimental");
     gate_all!(frontmatter, "frontmatters are experimental");
     gate_all!(coroutines, "coroutine syntax is experimental");
+    gate_all!(const_block_items, "const block items are experimental");
 
     if !visitor.features.never_patterns() {
         if let Some(spans) = spans.get(&sym::never_patterns) {

compiler/rustc_builtin_macros/src/cfg_eval.rs

@@ -9,7 +9,7 @@
 use rustc_expand::config::StripUnconfigured;
 use rustc_expand::configure;
 use rustc_feature::Features;
-use rustc_parse::parser::{ForceCollect, Parser};
+use rustc_parse::parser::{ConstBlockItemsAllowed, ForceCollect, Parser};
 use rustc_session::Session;
 use rustc_span::{Span, sym};
 use smallvec::SmallVec;
@@ -110,7 +110,8 @@ fn configure_annotatable(mut self, annotatable: Annotatable) -> Annotatable {
         let res: PResult<'_, Annotatable> = try {
             match annotatable {
                 Annotatable::Item(_) => {
-                    let item = parser.parse_item(ForceCollect::Yes)?.unwrap();
+                    let item =
+                        parser.parse_item(ForceCollect::Yes, ConstBlockItemsAllowed::No)?.unwrap();
                     Annotatable::Item(self.flat_map_item(item).pop().unwrap())
                 }
                 Annotatable::AssocItem(_, ctxt) => {

compiler/rustc_builtin_macros/src/source_util.rs

@@ -13,7 +13,7 @@
 };
 use rustc_expand::module::DirOwnership;
 use rustc_parse::lexer::StripTokens;
-use rustc_parse::parser::ForceCollect;
+use rustc_parse::parser::{ConstBlockItemsAllowed, ForceCollect};
 use rustc_parse::{new_parser_from_file, unwrap_or_emit_fatal, utf8_error};
 use rustc_session::lint::builtin::INCOMPLETE_INCLUDE;
 use rustc_session::parse::ParseSess;
@@ -168,7 +168,7 @@ fn make_expr(self: Box<ExpandInclude<'a>>) -> Option<Box<ast::Expr>> {
             ));
             let mut ret = SmallVec::new();
             loop {
-                match p.parse_item(ForceCollect::No) {
+                match p.parse_item(ForceCollect::No, ConstBlockItemsAllowed::No) {
                     Err(err) => {
                         err.emit();
                         break;

compiler/rustc_expand/src/base.rs

@@ -22,7 +22,7 @@
 use rustc_hir::{Stability, find_attr};
 use rustc_lint_defs::RegisteredTools;
 use rustc_parse::MACRO_ARGUMENTS;
-use rustc_parse::parser::{ForceCollect, Parser};
+use rustc_parse::parser::{ConstBlockItemsAllowed, ForceCollect, Parser};
 use rustc_session::Session;
 use rustc_session::parse::ParseSess;
 use rustc_span::def_id::{CrateNum, DefId, LocalDefId};
@@ -1472,7 +1472,7 @@ pub(crate) fn stream_pretty_printing_compatibility_hack(
             let mut parser = Parser::new(psess, stream.clone(), None);
             // No need to collect tokens for this simple check.
             parser
-                .parse_item(ForceCollect::No)
+                .parse_item(ForceCollect::No, ConstBlockItemsAllowed::No)
                 .expect("failed to reparse item")
                 .expect("an actual item")
         }

compiler/rustc_expand/src/expand.rs

@@ -25,8 +25,8 @@
 use rustc_hir::def::MacroKinds;
 use rustc_hir::limit::Limit;
 use rustc_parse::parser::{
-    AttemptLocalParseRecovery, CommaRecoveryMode, ForceCollect, Parser, RecoverColon, RecoverComma,
-    token_descr,
+    AttemptLocalParseRecovery, CommaRecoveryMode, ConstBlockItemsAllowed, ForceCollect, Parser,
+    RecoverColon, RecoverComma, token_descr,
 };
 use rustc_session::Session;
 use rustc_session::lint::builtin::{UNUSED_ATTRIBUTES, UNUSED_DOC_COMMENTS};
@@ -1099,7 +1099,7 @@ pub fn parse_ast_fragment<'a>(
     Ok(match kind {
         AstFragmentKind::Items => {
             let mut items = SmallVec::new();
-            while let Some(item) = this.parse_item(ForceCollect::No)? {
+            while let Some(item) = this.parse_item(ForceCollect::No, ConstBlockItemsAllowed::No)? {
                 items.push(item);
             }
             AstFragment::Items(items)

compiler/rustc_expand/src/proc_macro.rs

@@ -1,7 +1,7 @@
 use rustc_ast::tokenstream::TokenStream;
 use rustc_errors::ErrorGuaranteed;
 use rustc_middle::ty::{self, TyCtxt};
-use rustc_parse::parser::{ForceCollect, Parser};
+use rustc_parse::parser::{ConstBlockItemsAllowed, ForceCollect, Parser};
 use rustc_session::Session;
 use rustc_session::config::ProcMacroExecutionStrategy;
 use rustc_span::profiling::SpannedEventArgRecorder;
@@ -160,7 +160,7 @@ fn expand(
         let mut items = vec![];
 
         loop {
-            match parser.parse_item(ForceCollect::No) {
+            match parser.parse_item(ForceCollect::No, ConstBlockItemsAllowed::No) {
                 Ok(None) => break,
                 Ok(Some(item)) => {
                     if is_stmt {

compiler/rustc_feature/src/unstable.rs

@@ -414,6 +414,8 @@ pub fn internal(&self, feature: Symbol) -> bool {
     (unstable, cmse_nonsecure_entry, "1.48.0", Some(75835)),
     /// Allows `async {}` expressions in const contexts.
     (unstable, const_async_blocks, "1.53.0", Some(85368)),
+    /// Allows `const { ... }` as a shorthand for `const _: () = const { ... };` for module items.
+    (unstable, const_block_items, "CURRENT_RUSTC_VERSION", Some(149226)),
     /// Allows `const || {}` closures in const contexts.
     (incomplete, const_closures, "1.68.0", Some(106003)),
     /// Allows using `[const] Destruct` bounds and calling drop impls in const contexts.

compiler/rustc_parse/src/parser/attr.rs

@@ -9,7 +9,8 @@
 use tracing::debug;
 
 use super::{
-    AttrWrapper, Capturing, FnParseMode, ForceCollect, Parser, PathStyle, Trailing, UsePreAttrPos,
+    AttrWrapper, Capturing, ConstBlockItemsAllowed, FnParseMode, ForceCollect, Parser, PathStyle,
+    Trailing, UsePreAttrPos,
 };
 use crate::parser::FnContext;
 use crate::{errors, exp, fluent_generated as fluent};
@@ -203,6 +204,7 @@ fn annotate_following_item_if_applicable(
             false,
             FnParseMode { req_name: |_, _| true, context: FnContext::Free, req_body: true },
             ForceCollect::No,
+            ConstBlockItemsAllowed::No,
         ) {
             Ok(Some(item)) => {
                 // FIXME(#100717)

compiler/rustc_parse/src/parser/item.rs

@@ -22,8 +22,8 @@
 use super::diagnostics::{ConsumeClosingDelim, dummy_arg};
 use super::ty::{AllowPlus, RecoverQPath, RecoverReturnSign};
 use super::{
-    AttrWrapper, ExpKeywordPair, ExpTokenPair, FollowedByType, ForceCollect, Parser, PathStyle,
-    Recovered, Trailing, UsePreAttrPos,
+    AttrWrapper, ConstBlockItemsAllowed, ExpKeywordPair, ExpTokenPair, FollowedByType,
+    ForceCollect, Parser, PathStyle, Recovered, Trailing, UsePreAttrPos,
 };
 use crate::errors::{self, FnPointerCannotBeAsync, FnPointerCannotBeConst, MacroExpandsToAdtField};
 use crate::{exp, fluent_generated as fluent};
@@ -69,7 +69,7 @@ pub fn parse_mod(
         // `parse_item` consumes the appropriate semicolons so any leftover is an error.
         loop {
             while self.maybe_consume_incorrect_semicolon(items.last().map(|x| &**x)) {} // Eat all bad semicolons
-            let Some(item) = self.parse_item(ForceCollect::No)? else {
+            let Some(item) = self.parse_item(ForceCollect::No, ConstBlockItemsAllowed::Yes)? else {
                 break;
             };
             items.push(item);
@@ -123,21 +123,34 @@ enum ReuseKind {
 }
 
 impl<'a> Parser<'a> {
-    pub fn parse_item(&mut self, force_collect: ForceCollect) -> PResult<'a, Option<Box<Item>>> {
+    pub fn parse_item(
+        &mut self,
+        force_collect: ForceCollect,
+        const_block_items_allowed: ConstBlockItemsAllowed,
+    ) -> PResult<'a, Option<Box<Item>>> {
         let fn_parse_mode =
             FnParseMode { req_name: |_, _| true, context: FnContext::Free, req_body: true };
-        self.parse_item_(fn_parse_mode, force_collect).map(|i| i.map(Box::new))
+        self.parse_item_(fn_parse_mode, force_collect, const_block_items_allowed)
+            .map(|i| i.map(Box::new))
     }
 
     fn parse_item_(
         &mut self,
         fn_parse_mode: FnParseMode,
         force_collect: ForceCollect,
+        const_block_items_allowed: ConstBlockItemsAllowed,
     ) -> PResult<'a, Option<Item>> {
         self.recover_vcs_conflict_marker();
         let attrs = self.parse_outer_attributes()?;
         self.recover_vcs_conflict_marker();
-        self.parse_item_common(attrs, true, false, fn_parse_mode, force_collect)
+        self.parse_item_common(
+            attrs,
+            true,
+            false,
+            fn_parse_mode,
+            force_collect,
+            const_block_items_allowed,
+        )
     }
 
     pub(super) fn parse_item_common(
@@ -147,10 +160,11 @@ pub(super) fn parse_item_common(
         attrs_allowed: bool,
         fn_parse_mode: FnParseMode,
         force_collect: ForceCollect,
+        const_block_items_allowed: ConstBlockItemsAllowed,
     ) -> PResult<'a, Option<Item>> {
-        if let Some(item) =
-            self.eat_metavar_seq(MetaVarKind::Item, |this| this.parse_item(ForceCollect::Yes))
-        {
+        if let Some(item) = self.eat_metavar_seq(MetaVarKind::Item, |this| {
+            this.parse_item(ForceCollect::Yes, const_block_items_allowed)
+        }) {
             let mut item = item.expect("an actual item");
             attrs.prepend_to_nt_inner(&mut item.attrs);
             return Ok(Some(*item));
@@ -163,6 +177,7 @@ pub(super) fn parse_item_common(
             let kind = this.parse_item_kind(
                 &mut attrs,
                 mac_allowed,
+                const_block_items_allowed,
                 lo,
                 &vis,
                 &mut def,
@@ -209,6 +224,7 @@ fn parse_item_kind(
         &mut self,
         attrs: &mut AttrVec,
         macros_allowed: bool,
+        const_block_items_allowed: ConstBlockItemsAllowed,
         lo: Span,
         vis: &Visibility,
         def: &mut Defaultness,
@@ -257,6 +273,34 @@ fn parse_item_kind(
         } else if self.check_impl_frontmatter(0) {
             // IMPL ITEM
             self.parse_item_impl(attrs, def_(), false)?
+        } else if let ConstBlockItemsAllowed::Yes = const_block_items_allowed
+            && self.token.is_keyword(kw::Const)
+            && self.look_ahead(1, |t| *t == token::OpenBrace || t.is_metavar_block())
+        {
+            // CONST BLOCK ITEM
+            self.psess.gated_spans.gate(sym::const_block_items, self.token.span);
+            let block = self.parse_const_block(DUMMY_SP)?;
+            ItemKind::Const(Box::new(ConstItem {
+                defaultness: Defaultness::Final,
+                ident: Ident { name: kw::Underscore, span: DUMMY_SP },
+                generics: Generics {
+                    params: thin_vec![],
+                    where_clause: WhereClause {
+                        has_where_token: false,
+                        predicates: thin_vec![],
+                        span: DUMMY_SP,
+                    },
+                    span: DUMMY_SP,
+                },
+                ty: Box::new(Ty {
+                    id: DUMMY_NODE_ID,
+                    kind: TyKind::Tup(thin_vec![]),
+                    span: DUMMY_SP,
+                    tokens: None,
+                }),
+                rhs: Some(ConstItemRhs::Body(block)),
+                define_opaque: None,
+            }))
         } else if let Const::Yes(const_span) = self.parse_constness(case) {
             // CONST ITEM
             self.recover_const_mut(const_span);
@@ -316,6 +360,7 @@ fn parse_item_kind(
             return self.parse_item_kind(
                 attrs,
                 macros_allowed,
+                const_block_items_allowed,
                 lo,
                 vis,
                 def,
@@ -1068,7 +1113,7 @@ fn parse_assoc_item(
         fn_parse_mode: FnParseMode,
         force_collect: ForceCollect,
     ) -> PResult<'a, Option<Option<Box<AssocItem>>>> {
-        Ok(self.parse_item_(fn_parse_mode, force_collect)?.map(
+        Ok(self.parse_item_(fn_parse_mode, force_collect, ConstBlockItemsAllowed::No)?.map(
             |Item { attrs, id, span, vis, kind, tokens }| {
                 let kind = match AssocItemKind::try_from(kind) {
                     Ok(kind) => kind,
@@ -1320,7 +1365,7 @@ pub fn parse_foreign_item(
             context: FnContext::Free,
             req_body: false,
         };
-        Ok(self.parse_item_(fn_parse_mode, force_collect)?.map(
+        Ok(self.parse_item_(fn_parse_mode, force_collect, ConstBlockItemsAllowed::No)?.map(
             |Item { attrs, id, span, vis, kind, tokens }| {
                 let kind = match ForeignItemKind::try_from(kind) {
                     Ok(kind) => kind,
@@ -2394,7 +2439,7 @@ fn recover_nested_adt_item(&mut self, keyword: Symbol) -> PResult<'a, bool> {
         {
             let kw_token = self.token;
             let kw_str = pprust::token_to_string(&kw_token);
-            let item = self.parse_item(ForceCollect::No)?;
+            let item = self.parse_item(ForceCollect::No, ConstBlockItemsAllowed::No)?;
             let mut item = item.unwrap().span;
             if self.token == token::Comma {
                 item = item.to(self.token.span);

compiler/rustc_parse/src/parser/mod.rs

@@ -145,6 +145,13 @@ pub enum ForceCollect {
     No,
 }
 
+/// Whether to accept `const { ... }` as a shorthand for `const _: () = const { ... }`.
+#[derive(Clone, Copy, Debug, PartialEq)]
+pub enum ConstBlockItemsAllowed {
+    Yes,
+    No,
+}
+
 /// If the next tokens are ill-formed `$ty::` recover them as `<$ty>::`.
 #[macro_export]
 macro_rules! maybe_recover_from_interpolated_ty_qpath {

compiler/rustc_parse/src/parser/nonterminal.rs

@@ -6,7 +6,9 @@
 
 use crate::errors::UnexpectedNonterminal;
 use crate::parser::pat::{CommaRecoveryMode, RecoverColon, RecoverComma};
-use crate::parser::{FollowedByType, ForceCollect, ParseNtResult, Parser, PathStyle};
+use crate::parser::{
+    ConstBlockItemsAllowed, FollowedByType, ForceCollect, ParseNtResult, Parser, PathStyle,
+};
 
 impl<'a> Parser<'a> {
     /// Checks whether a non-terminal may begin with a particular token.
@@ -118,7 +120,9 @@ pub fn parse_nonterminal(&mut self, kind: NonterminalKind) -> PResult<'a, ParseN
         match kind {
             // Note that TT is treated differently to all the others.
             NonterminalKind::TT => Ok(ParseNtResult::Tt(self.parse_token_tree())),
-            NonterminalKind::Item => match self.parse_item(ForceCollect::Yes)? {
+            NonterminalKind::Item => match self
+                .parse_item(ForceCollect::Yes, ConstBlockItemsAllowed::Yes)?
+            {
                 Some(item) => Ok(ParseNtResult::Item(item)),
                 None => Err(self.dcx().create_err(UnexpectedNonterminal::Item(self.token.span))),
             },

compiler/rustc_parse/src/parser/stmt.rs

@@ -20,8 +20,8 @@
 use super::pat::{PatternLocation, RecoverComma};
 use super::path::PathStyle;
 use super::{
-    AttrWrapper, BlockMode, FnContext, FnParseMode, ForceCollect, Parser, Restrictions,
-    SemiColonMode, Trailing, UsePreAttrPos,
+    AttrWrapper, BlockMode, ConstBlockItemsAllowed, FnContext, FnParseMode, ForceCollect, Parser,
+    Restrictions, SemiColonMode, Trailing, UsePreAttrPos,
 };
 use crate::errors::{self, MalformedLoopLabel};
 use crate::exp;
@@ -156,6 +156,7 @@ pub fn parse_stmt_without_recovery(
             true,
             FnParseMode { req_name: |_, _| true, context: FnContext::Free, req_body: true },
             force_collect,
+            ConstBlockItemsAllowed::No,
         )? {
             self.mk_stmt(lo.to(item.span), StmtKind::Item(Box::new(item)))
         } else if self.eat(exp!(Semi)) {

compiler/rustc_span/src/symbol.rs

@@ -730,6 +730,7 @@
         console,
         const_allocate,
         const_async_blocks,
+        const_block_items,
         const_closures,
         const_compare_raw_pointers,
         const_constructor,

src/librustdoc/doctest/make.rs

@@ -539,7 +539,10 @@ fn check_item(item: &ast::Item, info: &mut ParseSourceInfo, crate_name: &Option<
 
     let mut prev_span_hi = 0;
     let not_crate_attrs = &[sym::forbid, sym::allow, sym::warn, sym::deny, sym::expect];
-    let parsed = parser.parse_item(rustc_parse::parser::ForceCollect::No);
+    let parsed = parser.parse_item(
+        rustc_parse::parser::ForceCollect::No,
+        rustc_parse::parser::ConstBlockItemsAllowed::No,
+    );
 
     let result = match parsed {
         Ok(Some(ref item))

tests/ui/feature-gates/feature-gate-const-block-items.rs

@@ -0,0 +1,7 @@
+//@ check-fail
+
+const { //~ ERROR: const block items are experimental
+    assert!(true)
+}
+
+fn main() {}

tests/ui/feature-gates/feature-gate-const-block-items.stderr

@@ -0,0 +1,13 @@
+error[E0658]: const block items are experimental
+  --> $DIR/feature-gate-const-block-items.rs:3:1
+   |
+LL | const {
+   | ^^^^^
+   |
+   = note: see issue #149226 <https://github.com/rust-lang/rust/issues/149226> for more information
+   = help: add `#![feature(const_block_items)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0658`.