Support trait objects in type info reflection

[izagawd]

Tracking issue: #146922

Adds type_info support for trait object types by introducing a DynTrait variant

I can't seem to get it to work correctly with dyn for<'a> Foo<'a>, though it works fine for normal dyn Foo trait objects

r? @oli-obk

[rustbot]

The reflection data structures are tied exactly to the implementation
in the compiler. Make sure to also adjust rustc_const_eval/src/const_eval/type_info.rs

cc @oli-obk

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

[rustbot]

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

[SpriteOvO]

trait A {}
trait B: A {}
trait C: B {}

The quoted dump is the type info for dyn C + Send. How do we determine whether it has trait C?

[izagawd]

Sorry, I don't think I fully understand the question. Could you elaborate?

[SpriteOvO]

For clear, I replaced the type ID number with type name in the following examples.

First, TypeId(dyn C) != TypeId(dyn C + Send), then the type info of dyn C + Send is

Type {
    kind: DynTrait(
        DynTrait {
            super_traits: [
                TypeId(B),
                TypeId(A),
            ],
            is_auto: false,
            auto_traits: [
                TypeId(Send),
            ],
        },
    ),
    size: None,
}

For the current PR implementation, if given a dyn C + Send (or type ID of it), we can know there are A, B and Send but no way of knowing C.

[izagawd]

I did not include C in it since C is not a super trait of itself. I am assuming you want a field that represents the TypeId of the trait object itself?

[SpriteOvO]

Not exactly, dyn C and dyn C + Send are two different things, so it's not "the trait object itself".

Intuitively, I think DynTrait's layout should probably look something like this. Take dyn C + Send as an example again.

Type {
    kind: DynTrait(
        DynTrait {
            predicates: [ Predicate(Trait(C)), Predicate(Trait(Send)) ]
        },
    ),
    size: None,
}

Predicate {
    trait,
    negative: bool, // Not sure if possible, #144241
}

Trait(C) = Trait {
    supers: [ Trait(B), Trait(A) ],
    is_auto: false,
}

Trait(Send) = Trait {
    supers: [],
    is_auto: true,
}

Trait(B) = ...
Trait(A) = ...

[izagawd]

This makes a lot of sense! I will go with this

[SpriteOvO]

like this?

#[derive(Debug)]
// FIXME(type_info): Add supertraits
pub struct DynTraitPredicate {
    ...
}

Also, we might need to do that for generics as well no?

Yea. BTW, I'm adding generics info for ADT types in PR #151142. Once your PR is merged, I can also add generics info to traits later.

[SpriteOvO]

LGTM, waiting for further review by @oli-obk. (I don't have r+ permission)

View changes since this review

[BD103]

This is very much non-blocking, but I would prefer this be named TraitObject, as that is how they are presented in the book and the reference. Definitely something that should be done in a follow-up PR, as I don't want to hold up merging this due to bikeshedding :)

[oli-obk]

The nomenclature has moved away from trait object, looks like we have more places to update

[izagawd]

What else do I have to do to get this PR merged @oli-obk ?

[oli-obk]

🤦 forgot to submit the review

@bors delegate+

r=me after moving the code to a new module

View changes since this review

[rust-bors]

✌️ @izagawd, you can now approve this pull request!

If @oli-obk told you to "r=me" after making some further change, then please make that change and post @bors r=oli-obk.

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[rustbot]

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.

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

##[group]Runner Image Provisioner
Hosted Compute Agent
Version: 20260115.477
Commit: 4b342d620503cbe250a3154040964899ea7c9b00
Build Date: 2026-01-15T22:32:41Z
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Azure Region: westcentralus
##[endgroup]
##[group]Operating System
Ubuntu
24.04.3
LTS
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ghcr.io/dependabot/dependabot-updater-core   latest    354d02aa29ac   8 days ago   783MB
=> Removing docker images...
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---
tests/ui/drain_collect.fixed ... ok
tests/ui/drop_non_drop.rs ... ok
tests/ui/duplicate_underscore_argument.rs ... ok
tests/ui/duplicated_attributes.rs ... ok
tests/ui/duration_suboptimal_units.rs ... ok
tests/ui/duration_suboptimal_units_days_weeks.rs ... ok
tests/ui/double_parens.fixed ... ok
tests/ui/duration_subsec.rs ... ok
tests/ui/duration_suboptimal_units_days_weeks.fixed ... ok
tests/ui/duration_suboptimal_units.fixed ... ok
tests/ui/duration_subsec.fixed ... ok
tests/ui/else_if_without_else.rs ... ok
tests/ui/eager_transmute.rs ... ok
tests/ui/elidable_lifetime_names.rs ... ok
tests/ui/empty_docs.rs ... ok
---
...............................................    (147/147)

======== tests/rustdoc-gui/globals.goml ========

[ERROR] line 14: The following errors happened: [Property named `"searchIndex"` doesn't exist]: for command `assert-window-property-false: {"searchIndex": null}`
    at <file:///checkout/obj/build/x86_64-unknown-linux-gnu/test/rustdoc-gui/doc/test_docs/index.html?search=Foo>

======== tests/rustdoc-gui/search-result-display.goml ========

[WARNING] line 39: Delta is 0 for "x", maybe try to use `compare-elements-position` instead?

[izagawd]

@bors r=oli-obk

[rust-bors]

📌 Commit a1893d3 has been approved by oli-obk

It is now in the queue for this repository.