Support defaults for static EIIs by AsakuraMizu · Pull Request #156583 · rust-lang/rust

AsakuraMizu · 2026-05-14T18:44:38Z

Tracking issue: #125418

#154193 added EII support for statics, but left default implementations for "a followup PR". This PR implements it.

Maybe I should remove no-prefer-dynamic if #156577 is accepted.

rustbot · 2026-05-14T18:44:44Z

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mu001999

LGTM, but I’ll defer to @JonathanBrouwer in case I’ve missed anything.

r? JonathanBrouwer

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rustbot · 2026-05-15T07:01:51Z

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JonathanBrouwer · 2026-05-15T07:03:57Z

I'm currently on holidays but would like to take a look, might take me a few days to get around to it

JonathanBrouwer

@bors r+ rollup
Looks good, thanks!
Wanted to implement this immediately after my PR that implemented static EIIs but forgot :3

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rust-bors · 2026-05-17T18:03:09Z

📌 Commit d7f16d7 has been approved by JonathanBrouwer

It is now in the queue for this repository.

JonathanBrouwer · 2026-05-17T18:03:31Z

@bors r=JonathanBrouwer,mu001999

rust-bors · 2026-05-17T18:03:34Z

📌 Commit d7f16d7 has been approved by JonathanBrouwer,mu001999

It is now in the queue for this repository.

…JonathanBrouwer,mu001999 Support defaults for static EIIs Tracking issue: rust-lang#125418 rust-lang#154193 added EII support for statics, but left default implementations for "a followup PR". This PR implements it. Maybe I should remove `no-prefer-dynamic` if rust-lang#156577 is accepted.

JonathanBrouwer · 2026-05-18T01:13:35Z

@bors r-
#156679 (comment)

rust-bors · 2026-05-18T01:13:41Z

This pull request was unapproved.

This PR was contained in a rollup (#156679), which was unapproved.

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AsakuraMizu · 2026-05-20T09:32:17Z

After some investigation, I think this is hitting an LLVM issue: llvm/llvm-project#111321

For tests/ui/eii/static/auxiliary/decl_with_default.rs, rustc currently emits the following LLVM IR on aarch64-apple-darwin:

@_RNvNvCshLcOpHkWMCm_17decl_with_default1__5DECL1 =
  constant [8 x i8] c"\05\00\00\00\00\00\00\00", align 8

@_RNvCshLcOpHkWMCm_17decl_with_default5DECL1 =
  linkonce alias i64, ptr @_RNvNvCshLcOpHkWMCm_17decl_with_default1__5DECL1

The second symbol is the EII foreign item symbol. In the generated Mach-O object it becomes non-external:

0000000000000000 (__TEXT,__const) non-external __RNvCshLcOpHkWMCm_17decl_with_default5DECL1
0000000000000000 (__TEXT,__const) external     __RNvNvCshLcOpHkWMCm_17decl_with_default1__5DECL1

Then, when the aux crate is built as a dylib, ld64 fails because the linker’s initial-undefines list requires the EII symbol to be externally resolvable:

Undefined symbols for architecture arm64:
  "__RNvCshLcOpHkWMCm_17decl_with_default5DECL1", referenced from:
      <initial-undefines>

I also checked whether //@ no-prefer-dynamic would be enough. It is not: the aux crate can then be built as an rlib, but the EII alias symbol inside the object is still non-external, so the final executable link fails instead:

Undefined symbols for architecture arm64:
  "__RNv...decl_with_default...DECL1", referenced from:
      default_cross_crate::main

What should I do here?

Temporarily skip the new cross-crate static-default tests on Apple targets and leave a FIXME.
Refactor to avoid weak aliases for static EII default impls: emit the default value directly under the foreign item symbol with weak/linkonce linkage.
Fix the LLVM issue.

JonathanBrouwer · 2026-05-20T12:19:16Z

(I'm at Rustweek this week, I'll take a look at this next week)

JonathanBrouwer · 2026-05-29T13:40:40Z

If you're a wizard enough to even consider fixing the LLVM issue please take that route, but I don't really know anything about LLVM so I can't help you there
If not, which is perfectly fine as well, disable static EII defaults on MacOS (and then also disable the test, leaving a FIXME), emitting a diagnostic explaining that they're currently broken. Imo only disabling the test is just risking that we forget about this in the future

I'd avoid refactoring to avoid weak aliases, as that would require a lot of code changes and might work less well dynamically linked libraries.

(sorry for the late reply, have been very busy and then a bit ill)
@rustbot author

rustbot · 2026-05-29T13:41:01Z

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

AsakuraMizu · 2026-05-29T17:49:56Z

If you're a wizard enough to even consider fixing the LLVM issue please take that route, but I don't really know anything about LLVM so I can't help you there

Some update from my side: I actually tried patching LLVM locally a few days ago, and with that patch the Rust tests here passed on macOS. When I was about to open an LLVM PR, I found that this collided with llvm/llvm-project#198148. The approach there is basically the same as mine, so I did not open a competing PR. I have not tested that exact patch yet, though.

I am not sure how long the full path usually takes: LLVM review/merge, possible backport, updating https://github.com/rust-lang/llvm-project, then updating the LLVM submodule here. If that goes smoothly, waiting for the upstream LLVM fix would be fine, but I would guess it will not be immediate.

If not, which is perfectly fine as well, disable static EII defaults on MacOS (and then also disable the test, leaving a FIXME), emitting a diagnostic explaining that they're currently broken. Imo only disabling the test is just risking that we forget about this in the future

So I agree with this approach to work around this on the Rust side for now. One question about the diagnostic: should it point users somewhere? Maybe the tracking issue?

JonathanBrouwer · 2026-05-29T17:56:39Z

I actually tried patching LLVM locally a few days ago

Very impressive, good job! Indeed waiting on that to land sounds like the best option then.

One question about the diagnostic: should it point users somewhere? Maybe the tracking issue?

I'd say create a new issue (linked to the tracking issue) for this. Give this issue the F-external-item_impls label, so we don't forget about it in the future if we try to stabilize this or whatever, being optimistic here

rustbot · 2026-06-09T12:04:10Z

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.

AsakuraMizu · 2026-06-09T12:27:34Z

@rustbot ready

mu001999 · 2026-06-10T15:47:16Z

@bors r=JonathanBrouwer,mu001999

rust-bors · 2026-06-10T15:47:19Z

📌 Commit cdb6acc has been approved by JonathanBrouwer,mu001999

It is now in the queue for this repository.

…JonathanBrouwer,mu001999 Support defaults for static EIIs Tracking issue: rust-lang#125418 rust-lang#154193 added EII support for statics, but left default implementations for "a followup PR". This PR implements it. Maybe I should remove `no-prefer-dynamic` if rust-lang#156577 is accepted.

Rollup of 23 pull requests Successful merges: - #157280 (traits: Allow escaping self types in ExistentialTraitRef::with_self_ty) - #157282 (Fix post-monomorphization error note race in the parallel frontend) - #157352 (Make the retained dep graph deterministic under the parallel frontend) - #157601 (Emit error for unused target expression in glob and list delegations) - #157611 (Update `browser-ui-test` version to `0.24.0`) - #157620 (Add a strategy FnMut to inject behavior into the flush cycle) - #157645 (Windows TLS - Only register the `atexit` hook when `cleanup` can be unloaded) - #157647 (Start using comptime for reflection intrinsics and their wrapper functions) - #157719 (resolve: Partially revert "Remove a special case for dummy imports") - #156497 (fix-155516: Don't suggest wrong unwrap expect) - #156583 (Support defaults for static EIIs) - #157013 (Ensure inferred let pattern types are well-formed) - #157230 (borrowck: avoid ICE describing fields on generic params) - #157288 (platform support: add SNaN erratum to MIPS targets) - #157350 (compiletest: ignore SVG `y` offset in by-lines comparison) - #157355 (Add `or_try_*` variants for `HashMap` and `BTreeMap` Entry APIs) - #157384 (Add `#[rustc_dump_generics]` attribute) - #157577 (Fix parser error recovery treating 'dyn' as a strict keyword in Rust 2015 when used in `dyn + dyn`) - #157670 (Rename `errors.rs` file to `diagnostics.rs` (4/N)) - #157691 (Move symbol hiding code to a new file) - #157697 (Add more tests that exercise the well-formedness checking of lazy type aliases) - #157700 (Rename `errors.rs` file to `diagnostics.rs` (5/N)) - #157703 (Fix doc link to Instant sub in saturating caveat) Failed merges: - #157699 (Arg splat experiment - hir FnDecl impl)

…JonathanBrouwer,mu001999 Support defaults for static EIIs Tracking issue: rust-lang#125418 rust-lang#154193 added EII support for statics, but left default implementations for "a followup PR". This PR implements it. Maybe I should remove `no-prefer-dynamic` if rust-lang#156577 is accepted.

Rollup of 31 pull requests Successful merges: - #141030 (Expand free alias types during variance computation) - #154853 (mgca: Register `ConstArgHasType` when normalizing projection consts) - #155527 (Replace printables table with `unicode_data.rs` tables) - #156629 (Stabilize `core::range::{legacy, RangeFull, RangeTo}`) - #157280 (traits: Allow escaping self types in ExistentialTraitRef::with_self_ty) - #157282 (Fix post-monomorphization error note race in the parallel frontend) - #157352 (Make the retained dep graph deterministic under the parallel frontend) - #157601 (Emit error for unused target expression in glob and list delegations) - #157611 (Update `browser-ui-test` version to `0.24.0`) - #157620 (Add a strategy FnMut to inject behavior into the flush cycle) - #157645 (Windows TLS - Only register the `atexit` hook when `cleanup` can be unloaded) - #157646 (Keep rename-imported main alive in dead-code analysis under `--test`) - #157647 (Start using comptime for reflection intrinsics and their wrapper functions) - #157719 (resolve: Partially revert "Remove a special case for dummy imports") - #155153 (Ensure Send/Sync is not implemented for std::env::Vars{,Os}) - #155198 (fix(mgca): Allow specifying generic args (of enum) on enum itself in unit & tuple variant constructions in (direct) const args) - #155323 (refactor `TypeRelativePath::AssocItem` to use `AliasTerm`) - #156497 (fix-155516: Don't suggest wrong unwrap expect) - #156583 (Support defaults for static EIIs) - #157013 (Ensure inferred let pattern types are well-formed) - #157196 (Only suggest reborrowing a moved value where the reborrow is valid) - #157230 (borrowck: avoid ICE describing fields on generic params) - #157288 (platform support: add SNaN erratum to MIPS targets) - #157330 (remove `IsTypeConst` from `hir::TraitItemKind`) - #157350 (compiletest: ignore SVG `y` offset in by-lines comparison) - #157355 (Add `or_try_*` variants for `HashMap` and `BTreeMap` Entry APIs) - #157577 (Fix parser error recovery treating 'dyn' as a strict keyword in Rust 2015 when used in `dyn + dyn`) - #157670 (Rename `errors.rs` file to `diagnostics.rs` (4/N)) - #157691 (Move symbol hiding code to a new file) - #157700 (Rename `errors.rs` file to `diagnostics.rs` (5/N)) - #157703 (Fix doc link to Instant sub in saturating caveat) Failed merges: - #157699 (Arg splat experiment - hir FnDecl impl)

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AsakuraMizu force-pushed the eii-static-default branch from 81c9617 to d7f16d7 Compare May 15, 2026 19:27

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Rollup of 6 pull requests #156678

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Static EII defaults are currently unsupported on macOS #157649

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Support defaults for static EIIs

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AsakuraMizu force-pushed the eii-static-default branch from d7f16d7 to 26766eb Compare June 9, 2026 12:04

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AsakuraMizu force-pushed the eii-static-default branch from 26766eb to 80d5676 Compare June 9, 2026 12:24

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Comment thread compiler/rustc_builtin_macros/src/eii.rs

Reject static EII defaults on Apple targets

cdb6acc

AsakuraMizu force-pushed the eii-static-default branch from 80d5676 to cdb6acc Compare June 10, 2026 15:15

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Rollup of 17 pull requests #157720

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Rollup of 23 pull requests #157726

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Rollup of 28 pull requests #157735

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Rollup of 31 pull requests #157739

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