TypeId implements_trait #151236

coolcatcoder · 2026-01-17T01:54:32Z

This is for #146922.

It allows checking if a TypeId implements a trait.

(I've never used git from the terminal before, so that was a fun challenge. I hope I did everything correctly.)
(I haven't added any tests yet, I'll do that soon.)
(Zulip.)

rustbot · 2026-01-17T01:54:35Z

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

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 · 2026-01-17T01:54:37Z

r? @chenyukang

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coolcatcoder · 2026-01-17T02:01:33Z

Also, because this pull request is pretty much just copying and pasting from vtable_for's implementation, should we de-duplicate the code by putting the implementation in one location that both intrinsics can call?

compiler/rustc_const_eval/src/const_eval/machine.rs

xonx4l · 2026-01-17T04:27:31Z

Also, because this pull request is pretty much just copying and pasting from vtable_for's implementation, should we de-duplicate the code by putting the implementation in one location that both intrinsics can call?

Sure , This way it's easier to maintain.

rustbot · 2026-01-17T05:37:01Z

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

compiler/rustc_const_eval/src/interpret/util.rs

SpriteOvO · 2026-01-17T09:55:35Z

library/core/src/mem/type_info.rs

+    /// Checks if the type represented by the `TypeId` implements the trait.
+    /// It can only be called at compile time.
+    pub const fn implements_trait<
+        T: ptr::Pointee<Metadata = ptr::DynMetadata<T>> + ?Sized + 'static,
+    >(
+        self,
+    ) -> bool {
+        type_id_implements_trait(self, TypeId::of::<T>())
+    }
+
+    /// Checks if the type represented by the `TypeId` implements the trait represented by the secondary `TypeId`.
+    /// Returns `None` if the `trait_represented_by_type_id` is not a trait represented by type id.
+    /// It can only be called at compile time.
+    pub const fn implements_trait_represented_by_type_id(
+        self,
+        trait_represented_by_type_id: Self,
+    ) -> Option<bool> {
+        if type_id_is_trait(trait_represented_by_type_id) {
+            Some(type_id_implements_trait(self, trait_represented_by_type_id))
+        } else {
+            None
+        }
+    }


Basically, type_info feature gate mainly introduces a struct Type, which contains all the type information reflected from a TypeId.

Regarding whether a struct implements a trait, I think this also relates to "reflection". Based on this, I would suggest that associating them with struct Type (becoming methods of it) might be more appropriate.

For naming, personally, has_trait feels more intuitive than implements_trait to me, though it's still too early to do name decisions.

Good points. I agree that has_trait does make more sense, especially when considering that the trait might not be implemented by the type specifically, and rather have come from a blanket.

SpriteOvO · 2026-01-17T10:18:41Z

library/core/src/intrinsics/mod.rs

+/// Check if a type represented by a `TypeId` is a dyn Trait.
+/// It can only be called at compile time, the backends do
+/// not implement it.
+#[rustc_intrinsic]
+#[unstable(feature = "core_intrinsics", issue = "none")]
+pub const fn type_id_is_trait(_trait: crate::any::TypeId) -> bool {
+    panic!("`TypeId::implements_trait_represented_by_type_id` can only be called at compile-time")
+}


This likely doesn't require a dedicated intrinsic. we can determine it through variants of enum TypeKind. Someone happens to be working on this, see #151239. Maybe you can wait for it to land.

SpriteOvO · 2026-01-17T11:31:32Z

compiler/rustc_const_eval/src/interpret/mod.rs

 mod step;
 mod traits;
-mod util;
+pub mod util;


I haven't looked closely at how CompileTimeMachine interacts with InterpCx, but I believe there's a better way to share code than putting it into the util mod.

SpriteOvO · 2026-01-17T11:46:00Z

@rustbot author

rustbot · 2026-01-17T11:46:04Z

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

rust-bors · 2026-01-19T16:52:53Z

☔ The latest upstream changes (presumably #151363) made this pull request unmergeable. Please resolve the merge conflicts.

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SpriteOvO force-pushed the type_id_implements_trait branch from f603826 to 2f97f63 Compare January 17, 2026 11:13

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coolcatcoder added 4 commits January 17, 2026 12:19

Implements funtionality of type_id_implements_trait and its ilk

31b74d2

De-duplicate!

65f28a7

Add test.

98f244d

Fmease fixes.

da054c7

SpriteOvO force-pushed the type_id_implements_trait branch from 2f97f63 to da054c7 Compare January 17, 2026 11:19

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ZXY595 mentioned this pull request Jan 17, 2026

Rewriting using Reflection ZXY595/odometries#1

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