Constify trait aliases #144291

oli-obk · 2025-07-22T07:29:27Z

Allow const trait Foo = Bar + [const] Baz; trait alias declarations. Their rules are the same as with super traits of const traits. So [const] Baz or const Baz is only required for [const] Foo or const Foo bounds respectively.

tracking issue #41517 (part of the general trait alias feature gate, but I can split it out into a separate const trait alias feature gate. I just assumed that const traits would stabilize before trait aliases, and we'd want to stabilize trait aliases together with const trait aliases at the same time)

r? @compiler-errors @fee1-dead

rustbot · 2025-07-22T07:29:34Z

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

HIR ty lowering was modified

cc @fmease

Changes to the size of AST and/or HIR nodes.

cc @nnethercote

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

nnethercote · 2025-07-22T10:19:12Z

compiler/rustc_ast/src/ast.rs

-    static_assert_size!(Item, 144);
-    static_assert_size!(ItemKind, 80);
+    static_assert_size!(Item, 136);
+    static_assert_size!(ItemKind, 72);


blyxyas · 2025-07-22T14:02:38Z

Clippy changes are approved.

src/tools/rustfmt/src/visitor.rs

bors · 2025-07-23T10:09:45Z

☔ The latest upstream changes (presumably #143897) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2025-07-23T18:10:31Z

☔ The latest upstream changes (presumably #143843) made this pull request unmergeable. Please resolve the merge conflicts.

bors · 2025-07-27T16:48:43Z

☔ The latest upstream changes (presumably #143884) made this pull request unmergeable. Please resolve the merge conflicts.

compiler-errors · 2025-09-14T18:31:50Z

r? fee1-dead or re-roll perhaps

fee1-dead

quick review because i didn't get enough time

Didn't look at the solver parts yet, will do later

View changes since this review

compiler/rustc_ast/src/visit.rs

compiler/rustc_ast_lowering/src/item.rs

compiler/rustc_ast_pretty/src/pprust/state/item.rs

compiler/rustc_parse/src/parser/item.rs

fee1-dead · 2025-09-16T03:12:41Z

The commits should probably be squashed.

oli-obk · 2025-09-16T07:23:48Z

The commits should probably be squashed.

Hmm... I split them on purpose to only make atomic testable changes

fee1-dead

r=me after nit

View changes since this review

fee1-dead · 2025-10-02T00:24:13Z

compiler/rustc_next_trait_solver/src/solve/effect_goals.rs

+            ecx.add_goals(GoalSource::Misc, where_clause_bounds);
+
+            let const_conditions = cx
+                .const_conditions(goal.predicate.def_id().into())
+                .iter_instantiated(cx, goal.predicate.trait_ref.args)
+                .map(|bound_trait_ref| {
+                    goal.with(
+                        cx,
+                        bound_trait_ref.to_host_effect_clause(cx, goal.predicate.constness),
+                    )
+                });
+            // While you could think of trait aliases to have a single builtin impl
+            // which uses its implied trait bounds as where-clauses, using
+            // `GoalSource::ImplWhereClause` here would be incorrect, as we also
+            // impl them, which means we're "stepping out of the impl constructor"
+            // again. To handle this, we treat these cycles as ambiguous for now.
+            ecx.add_goals(GoalSource::Misc, const_conditions);


The comment really applies to both of those lines, so

Suggested change

ecx.add_goals(GoalSource::Misc, where_clause_bounds);

let const_conditions = cx

.const_conditions(goal.predicate.def_id().into())

.iter_instantiated(cx, goal.predicate.trait_ref.args)

.map(|bound_trait_ref| {

goal.with(

cx,

bound_trait_ref.to_host_effect_clause(cx, goal.predicate.constness),

)

});

// While you could think of trait aliases to have a single builtin impl

// which uses its implied trait bounds as where-clauses, using

// `GoalSource::ImplWhereClause` here would be incorrect, as we also

// impl them, which means we're "stepping out of the impl constructor"

// again. To handle this, we treat these cycles as ambiguous for now.

ecx.add_goals(GoalSource::Misc, const_conditions);

let const_conditions = cx

.const_conditions(goal.predicate.def_id().into())

.iter_instantiated(cx, goal.predicate.trait_ref.args)

.map(|bound_trait_ref| {

goal.with(

cx,

bound_trait_ref.to_host_effect_clause(cx, goal.predicate.constness),

)

});

// While you could think of trait aliases to have a single builtin impl

// which uses its implied trait bounds as where-clauses, using

// `GoalSource::ImplWhereClause` here would be incorrect, as we also

// impl them, which means we're "stepping out of the impl constructor"

// again. To handle this, we treat these cycles as ambiguous for now.

ecx.add_goals(GoalSource::Misc, where_clause_bounds);

ecx.add_goals(GoalSource::Misc, const_conditions);

bors · 2025-10-22T10:27:02Z

☔ The latest upstream changes (presumably #147957) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2025-10-28T11:28:04Z

This PR was rebased onto a different master 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.

oli-obk · 2025-10-30T18:21:38Z

@bors r=fee1-dead

bors · 2025-10-30T18:21:41Z

📌 Commit ebb249d has been approved by fee1-dead

It is now in the queue for this repository.

…dead Constify trait aliases Allow `const trait Foo = Bar + [const] Baz;` trait alias declarations. Their rules are the same as with super traits of const traits. So `[const] Baz` or `const Baz` is only required for `[const] Foo` or `const Foo` bounds respectively. tracking issue rust-lang#41517 (part of the general trait alias feature gate, but I can split it out into a separate const trait alias feature gate. I just assumed that const traits would stabilize before trait aliases, and we'd want to stabilize trait aliases together with const trait aliases at the same time) r? `@compiler-errors` `@fee1-dead`

Rollup of 4 pull requests Successful merges: - #144291 (Constify trait aliases) - #147633 (Add new `--bypass-ignore-backends` option) - #148262 (Fix types being marked as dead when they are inferred generic arguments) - #148268 (rustdoc: fix `--emit=dep-info` on scraped examples) r? `@ghost` `@rustbot` modify labels: rollup

Rollup of 4 pull requests Successful merges: - #144291 (Constify trait aliases) - #147633 (Add new `--bypass-ignore-backends` option) - #148252 (Improve diagnose for unconditional panic when resource limit) - #148262 (Fix types being marked as dead when they are inferred generic arguments) r? `@ghost` `@rustbot` modify labels: rollup

Rollup merge of #144291 - oli-obk:const_trait_alias, r=fee1-dead Constify trait aliases Allow `const trait Foo = Bar + [const] Baz;` trait alias declarations. Their rules are the same as with super traits of const traits. So `[const] Baz` or `const Baz` is only required for `[const] Foo` or `const Foo` bounds respectively. tracking issue #41517 (part of the general trait alias feature gate, but I can split it out into a separate const trait alias feature gate. I just assumed that const traits would stabilize before trait aliases, and we'd want to stabilize trait aliases together with const trait aliases at the same time) r? ``@compiler-errors`` ``@fee1-dead``

rustbot assigned compiler-errors Jul 22, 2025

nnethercote reviewed Jul 22, 2025

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ytmimi reviewed Jul 22, 2025

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src/tools/rustfmt/src/visitor.rs Show resolved Hide resolved

oli-obk force-pushed the const_trait_alias branch 2 times, most recently from cb809b6 to 7508ac9 Compare July 23, 2025 14:30

oli-obk force-pushed the const_trait_alias branch from 7508ac9 to 5d3eb69 Compare July 24, 2025 07:25

rustbot assigned fee1-dead and unassigned compiler-errors Sep 14, 2025

fee1-dead requested changes Sep 16, 2025

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rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 16, 2025

oli-obk force-pushed the const_trait_alias branch from 5d3eb69 to 4591683 Compare September 19, 2025 11:03

rustbot added the T-rustfmt Relevant to the rustfmt team, which will review and decide on the PR/issue. label Sep 19, 2025

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oli-obk force-pushed the const_trait_alias branch from 4591683 to 156e480 Compare September 22, 2025 11:29

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oli-obk 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 Sep 23, 2025

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Trait aliases are rare large ast nodes, box them

3ca752f

oli-obk force-pushed the const_trait_alias branch from 82973ed to 906e528 Compare October 28, 2025 11:27

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oli-obk added 7 commits October 30, 2025 08:05

Constify trait aliases

5f6772c

Generate const predicates for const trait aliases

8b122f1

Make const trait aliases work in next solver

544a5a3

Prepare test for old solver

bc4d612

Make const trait aliases work in the old solver

b9e7cf6

Add more tests

a6ca44c

Fix formatting of const trait aliases

ebb249d

oli-obk force-pushed the const_trait_alias branch from 906e528 to ebb249d Compare October 30, 2025 08:06

bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 30, 2025

GuillaumeGomez mentioned this pull request Oct 30, 2025

Rollup of 4 pull requests #148308

Closed

matthiaskrgr mentioned this pull request Oct 31, 2025

Rollup of 4 pull requests #148314

Merged

bors merged commit 149ad71 into rust-lang:master Oct 31, 2025
11 checks passed

rustbot added this to the 1.93.0 milestone Oct 31, 2025

Constify trait aliases #144291

Constify trait aliases #144291

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