Move struct placeholder pt2 by Jamesbarford · Pull Request #150271 · rust-lang/rust

Jamesbarford · 2025-12-22T15:02:51Z

r? ghost

lcnr · 2026-01-05T15:26:13Z

The reason HashStable isn't implemented is kinda stupid.

HashStable_NoContext is a "perfect derive", i.e. we simply add where-clauses that each field implements the trait. This means the derive for BoundRegionKind adds I::DefId: HasHStable<CTX> and I::Symbol: HashStable<CTX>. These are then considered to not hold in the manual impl. Though tbf, I don't get why the manual impl exists at all.

Try to replace the manual impl with a derive and see if anything breaks. If it does break, add I::DefId: HashStable<CTX> and I::Symbol: HashStable<CTX> to the manual impl

lcnr · 2026-01-26T09:06:49Z

+ fn lift_to_interner(self, cx: U) -> Option<Self::Lifted> {
+ Some(BoundTy { var: self.var, kind: self.kind.lift_to_interner(cx)? })
+ }
+}


why can this one not be derived?

I get this error without the implementation;
error[E0277]: the trait bound `BoundTy<TyCtxt<'_>>: Lift<TyCtxt<'tcx>>` is not satisfied --> compiler/rustc_infer/src/infer/region_constraints/mod.rs:672:59 | 672 | GenericKind::Placeholder(ref p) => write!(f, "{p}"), | -----------^^^-- | | | | | unsatisfied trait bound | in this macro invocation (#1) | --> /rustc/9b1f8ff42d110b0ca138116745be921df5dc97e7/library/core/src/macros/mod.rs:610:0 | = note: in this expansion of `write!` (#1) ::: /rustc/9b1f8ff42d110b0ca138116745be921df5dc97e7/library/core/src/macros/mod.rs:612:23 | = note: in this macro invocation (#2) ::: /rustc/9b1f8ff42d110b0ca138116745be921df5dc97e7/library/core/src/macros/mod.rs:997:4 | = note: in this expansion of `$crate::format_args!` (#2) | = help: the nightly-only, unstable trait `for<'tcx> Lift<rustc_middle::ty::TyCtxt<'tcx>>` is not implemented for `BoundTy<rustc_middle::ty::TyCtxt<'_>>` = help: the following other types implement trait `Lift<I>`: `&RawList<(), BoundVariableKind<TyCtxt<'a>>>` implements `Lift<rustc_middle::ty::TyCtxt<'tcx>>` `&RawList<(), GenericArg<'a>>` implements `Lift<rustc_middle::ty::TyCtxt<'tcx>>` `&RawList<(), Ty<'a>>` implements `Lift<rustc_middle::ty::TyCtxt<'tcx>>` `&RawList<(), Binder<TyCtxt<'a>, ...>>` implements `Lift<rustc_middle::ty::TyCtxt<'tcx>>` `()` implements `Lift<rustc_middle::ty::TyCtxt<'tcx>>` `AliasTerm<I>` implements `Lift<J>` `AliasTy<I>` implements `Lift<J>` `AllocId` implements `Lift<rustc_middle::ty::TyCtxt<'tcx>>` and 68 others = note: required for `Placeholder<TyCtxt<'_>, BoundTy<TyCtxt<'_>>>` to implement `for<'tcx> Lift<rustc_middle::ty::TyCtxt<'tcx>>` = note: required for `rustc_middle::ty::TyCtxt<'_>` to implement `IrPrint<Placeholder<TyCtxt<'_>, BoundTy<...>>>` = note: required for `Placeholder<TyCtxt<'_>, BoundTy<TyCtxt<'_>>>` to implement `std::fmt::Display` = note: 1 redundant requirement hidden = note: required for `&Placeholder<TyCtxt<'_>, BoundTy<TyCtxt<'_>>>` to implement `std::fmt::Display` = note: the full name for the type has been written to '/home/jambar02/Documents/arm/rust/build/x86_64-unknown-linux-gnu/stage1-rustc/x86_64-unknown-linux-gnu/release/deps/rustc_infer-29c3d638b97358db.long-type-10995706924995725161.txt' = note: consider using `--verbose` to print the full type name to the console

without the manual implementation but with a derive?

as in, using Lift_Generic

I get the same error when doing;
#[derive_where(Clone,Copy,PartialEq,Eq,Hash;I:Interner)]#[derive(Lift_Generic)]#[cfg_attr( feature = "nightly", derive(Encodable_NoContext,Decodable_NoContext,HashStable_NoContext))]pubstructBoundTy<I:Interner>{pubvar: ty::BoundVar,pubkind:BoundTyKind<I>,}

that might be as BoundVar doesn't implement Lift.
Does only deriving it for BoundRegionKind also error? The same with its HashStable impl

Doing it for BoundRegionKind allowed me to then also do it for BoundVariableKind and BoundTyKind.

lcnr · 2026-01-26T09:07:50Z

- type BoundTy: BoundVarLike<Self>;
- type PlaceholderTy: PlaceholderLike<Self, Bound = Self::BoundTy>;
- type Symbol: Copy + Hash + PartialEq + Eq + Debug;
+ type Symbol: Copy + Hash + PartialEq + Eq + Debug + Symbol<Self>;


please move all these item bounds to be super traits of Symbol instead

not actually resolved 😅

I didn't push my changes 🤦

lcnr

nits, then r=me

also, for future PRs. It would have been good to keep the BoundRegionKind::NamedForPrinting rename in a separate commit as that one isn't strictly related to the other changes

View changes since this review

lcnr

r=me after nit

View changes since this review

lcnr · 2026-01-29T10:24:21Z

@bors delegate+

rust-bors · 2026-01-29T10:24:24Z

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

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

Jamesbarford · 2026-01-29T13:21:51Z

@bors r=lcnr

rust-bors · 2026-01-29T13:21:54Z

📌 Commit 25c1365 has been approved by lcnr

It is now in the queue for this repository.

…uwer Rollup of 8 pull requests Successful merges: - #147387 (hir_owner_parent optimized to inlined call for non-incremental build) - #150271 (Move struct placeholder pt2) - #151283 (Suggest ignore returning value inside macro for unused_must_use lint) - #151565 (Rename, clarify, and document code for "erasing" query values) - #149482 (thread::scope: document how join interacts with TLS destructors) - #151827 (Use `Rustc` prefix for `rustc` attrs in `AttributeKind`) - #151833 (Treat unions as 'data types' in attr parsing diagnostics) - #151834 (Update `askama` version to `0.15.4`)

Rollup merge of #150271 - Jamesbarford:chore/refactor-struct-placeholder-pt2, r=lcnr Move struct placeholder pt2 r? ghost

…-placeholder-pt2, r=lcnr Move struct placeholder pt2 r? ghost

…uwer Rollup of 8 pull requests Successful merges: - rust-lang/rust#147387 (hir_owner_parent optimized to inlined call for non-incremental build) - rust-lang/rust#150271 (Move struct placeholder pt2) - rust-lang/rust#151283 (Suggest ignore returning value inside macro for unused_must_use lint) - rust-lang/rust#151565 (Rename, clarify, and document code for "erasing" query values) - rust-lang/rust#149482 (thread::scope: document how join interacts with TLS destructors) - rust-lang/rust#151827 (Use `Rustc` prefix for `rustc` attrs in `AttributeKind`) - rust-lang/rust#151833 (Treat unions as 'data types' in attr parsing diagnostics) - rust-lang/rust#151834 (Update `askama` version to `0.15.4`)

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Part 2 refactoring of moving placeholder types to rustc_type_ir
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rust-borsBot 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-review Status: Awaiting review from the assignee but also interested parties. labels Jan 29, 2026

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Rollup of 8 pull requests #151835
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Rollup merge of #150271 - Jamesbarford:chore/refactor-struct-placeholder-pt2, r=lcnr Move struct placeholder pt2 r? ghost

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Toolchain upgrade to nightly-2026-02-09 flux-rs/flux#1345
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Update BoundTyKind and BoundRegionKindrust-lang/rustc-dev-guide#2758
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Rollup merge ofrust-lang#150271- Jamesbarford:chore/refactor-struct…
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Internal Tracking Issue for type system librarification #159654
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