Support underscore as associated const name by makai410 · Pull Request #158970 · rust-lang/rust

makai410 · 2026-07-08T19:00:34Z

This PR doesn't change the current behavior of the dead_code lint for unused associated consts with underscore-prefixed names and treats associated consts named _ consistently.

There might still be some places that should've been changed to use named_items() instead of in_definition_order(), though I'm not quite sure.

Unresolved questions:

Whether to lint impl Private { pub const _: Private = Private; }, where the visibility qualifier here has no effect. It seems like there wasn't any disagreement on linting this, though what's proposed in the RFC currently diverges from that: RFC: Associated const underscore.

Tracking:

Tracking issue for RFC 3527: Associated const underscore #158944

rustbot · 2026-07-30T03:06:38Z

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a

HIR ty lowering was modified

cc @fmease

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

These commits modify tests/rustdoc-json.
rustdoc-json is a public (but unstable) interface.

Please ensure that if you've changed the output:

It's intentional.
The FORMAT_VERSION in src/librustdoc-json-types is bumped if necessary.

cc @obi1kenobi

rustbot · 2026-07-30T03:06:41Z

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- Adds `data: AssocConstData` to `AssocKind` to properly handle anonymous associated consts in ty. - Adds `named_items()` to `AssocItems` and changes some uses of `in_definition_order()` to use it instead.

rustbot · 2026-08-14T14:18:59Z

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.

jackh726 · 2026-08-15T17:59:37Z

+    /// Returns named associated items in definition order.
+    pub fn named_items(&self) -> impl '_ + Iterator<Item = &ty::AssocItem> {
+        self.items.iter().filter_map(|(name, v)| name.is_some().then(|| v))
+    }


I have a slight issue with adding this function.

The comment on in_definition_order has a comment about new code; this function doesn't have that comment. It's each to accidentally rely on definition order with realizing. So - at the very least that comment needs to be added here too.

Second, it makes me slightly worried that someone might use this when they should have been iterating over all items.

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jackh726 · 2026-08-15T18:00:32Z

                    }
-                    (ty::AssocKind::Const { name, .. }, ty::AssocContainer::Trait) => {
+                    (ty::AssocKind::Const { .. }, ty::AssocContainer::Trait) => {
+                        let name = this.item.name();


I would rather use name.expect(...) than this (here and elsewhere).

View changes since the review

jackh726 · 2026-08-15T18:03:41Z

    explicit_implied_const_bounds: Table<DefIndex, LazyArray<(ty::PolyTraitRef<'static>, Span)>>,
    inherent_impls: Table<DefIndex, LazyArray<DefIndex>>,
    opt_rpitit_info: Table<DefIndex, Option<LazyValue<ty::ImplTraitInTraitData>>>,
+    is_anon_assoc_const: Table<DefIndex, bool>,


:/ is this really necessary? Why not use opt_item_name in decoder?

View changes since the review

jackh726 · 2026-08-15T18:04:33Z

            }
        }
        DefKind::AssocFn | DefKind::AssocConst { .. } | DefKind::AssocTy => {
+            // As with free constants named `_`, associated constants named `_` are always live.


"live" -> "dead"?

View changes since the review

jackh726 · 2026-08-15T18:08:51Z

    let kind = match impl_item.kind {
        hir::ImplItemKind::Const(_, rhs) => {
-            ty::AssocKind::Const { name, is_type_const: matches!(rhs, ConstItemRhs::TypeConst(_)) }
+            let data = if impl_item.is_anon_const() {


This can just look at name directly? Is hir::ImplItem::is_anon_const used elsewhere? There are extra checks there that aren't meaningful because we already know that is a Const impl item, right?

View changes since the review

I suppose there is the impl_kind check?

jackh726 · 2026-08-15T18:18:14Z

+impl Public {
+    const _: Private = Private;
+}
+
+impl Private {
+    pub const _: Private = Private;
+    pub const _: () = {};
+}


Curious why these don't trigger the dead code lint?

View changes since the review

jackh726 · 2026-08-15T18:19:14Z

+    const _: () = panic!();
+    const _: () = assert!(false);
+    const _: [(); {
+        {
+            assert!(std::mem::size_of::<Self>() == 0)
+        };
+        0
+    }] = [];
+    const _: [(); {
+        {
+            assert!(true)
+        };
+        0
+    }] = [];


I would expect these all to trigger dead code lints?

View changes since the review

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 8, 2026

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makai410 force-pushed the us-assoc-cnst branch from f17f19f to b10ada5 Compare July 27, 2026 17:56

rustbot added A-rustdoc-json Area: Rustdoc JSON backend T-rustdoc-frontend Relevant to the rustdoc-frontend team, which will review and decide on the web UI/UX output. labels Jul 27, 2026

makai410 changed the title ~~[WIP] Support underscore as associated const name~~ Support underscore as associated const name Jul 27, 2026

makai410 force-pushed the us-assoc-cnst branch from b10ada5 to 17b62f7 Compare July 29, 2026 08:22

rustbot added the T-clippy Relevant to the Clippy team. label Jul 29, 2026

makai410 force-pushed the us-assoc-cnst branch from 17b62f7 to e91ca1c Compare July 29, 2026 14:29

makai410 marked this pull request as ready for review July 30, 2026 03:06

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 30, 2026

rustbot assigned jackh726 Jul 30, 2026

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makai410 force-pushed the us-assoc-cnst branch from e91ca1c to e803c41 Compare August 6, 2026 22:00

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Support underscore as associated const name

c664e8b

- Adds `data: AssocConstData` to `AssocKind` to properly handle anonymous associated consts in ty. - Adds `named_items()` to `AssocItems` and changes some uses of `in_definition_order()` to use it instead.

makai410 force-pushed the us-assoc-cnst branch from e803c41 to c664e8b Compare August 14, 2026 14:18

jackh726 reviewed Aug 15, 2026

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