`const` blocks as a `mod` item #149174

GrigorenkoPV · 2025-11-21T12:02:00Z

Tracking issue: #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 Add warn-by-default lint for visibility on const _ declarations #147136). Reject it. Should this be rejected by the parser or smth?
Improve diagnostics (preferably they should not mention the fake _ ident).

rustbot · 2025-11-29T21:40:07Z

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

rustbot · 2025-11-29T21:40:09Z

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jdonszelmann · 2025-12-08T14:40:47Z

then I think it might be better not to.

jdonszelmann · 2025-12-08T14:41:04Z

the const block will get its own defid, so with it we effectively allocate two

jdonszelmann · 2025-12-09T05:21:41Z

@rustbot author

joshtriplett · 2025-12-10T04:50:42Z

I wonder about maybe doing this a different way, like how we allow omitting -> () on functions, so maybe we could instead allow const _ = …; that has to be unit-typed? That way it'd clearly be an item even in a place where inline const expressions are also valid.

That's not much of an improvement over const _: () = ...;; either way it's a kludge to hide a const block inside another kind of construct.

const { ... } makes perfect sense as an item. const { assert!(...) } is self-explanatory, for instance.

rustbot · 2025-12-10T22:26:58Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

GrigorenkoPV · 2025-12-10T22:37:07Z

Changed the expansion to const _: () = { ... };.

@rustbot ready

jdonszelmann · 2025-12-11T09:07:57Z

compiler/rustc_parse/src/parser/mod.rs

+pub enum AllowConstBlockItems {
+    Yes,
+    No,
+    DoesNotMatter,


could you just document what does not matter means? e.g. "in the parsing context it doesn't matter so we default to no"

jdonszelmann · 2025-12-11T09:10:15Z

r=me after that

jdonszelmann · 2025-12-11T09:10:32Z

@rustbot author

ytmimi · 2025-12-11T22:26:22Z

src/tools/rustfmt/src/expr.rs

        shape
    };
-    let rhs = rewrite_assign_rhs_expr(context, &lhs, ex, shape, rhs_kind, rhs_tactics)?;
+    let rhs: String = rewrite_assign_rhs_expr(context, &lhs, ex, shape, rhs_kind, rhs_tactics)?;


was this change necessary? If not please revert it.

ytmimi · 2025-12-11T22:29:03Z

src/tools/rustfmt/src/items.rs

Please revert all of these changes, they don't look like they're necessary for const block formatting. It's best not to make refactors like this to subtrees that are maintained outside of r-l/rust

ytmimi · 2025-12-11T22:49:54Z

src/tools/rustfmt/src/visitor.rs

+                    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)
+                            }),
+                    );


I think you're going to need to call rewrite_block() directly here instead of block.rewrite() so that you can pass along attributes from the Item. I wan to avoid inner attributes getting removed like they were in rust-lang/rustfmt#6158.

ytmimi · 2025-12-11T22:50:29Z

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

Can we add a test case similar to the one in rust-lang/rustfmt#6158 that has inner attributes.

ytmimi

Need to make some rustfmt changes

View changes since this review

GrigorenkoPV force-pushed the const_block_item branch from 7ec6ad3 to 0b65188 Compare November 21, 2025 12:08