Proof of Concept: enforced homogeneity for Try (& Poll) by MusicalNinjaDad · Pull Request #163637 · rust-lang/rust

MusicalNinjaDad · 2026-10-02T08:06:17Z

This is a draft PR targeted at a crater run to validate the viability of enforcing homogeneity for Try. See #155368, #84277 & rust-lang/goals#654.

If successful, this will feed into a near-future design meeting on the key open points for stabilising Try. I will organise a design meeting once the crater results are available.

Requesting a crater run to validate the following points

homogeneous `Poll`

This is not expected to produce any regressions in stable code.

? desugaring has been gated to use non-homogeneous Poll unless feature(try_trait_v2) is enabled. See zulip: #t-lang/try > impl Try for std::task::Poll for more details & discussion.

`Iterator::try_fold`

Potential source of regression, low risk.

try_fold is currently stable and names the unstable trait Try in its signature, this means that gated ? desugaring cannot be leveraged. There is a potential for crater regressions in the case that try_fold is used on an iterator of Poll<Result<_,_> or Poll<Option<Result<_,_>>; I cannot conceive a viable use-case for doing this as it would require silently ignoring & throwing away any Pending/Ready(None) cases during folding.

enforced homogeneity

This is not expected to produce any regressions in stable code.

Only stdlib can name Try etc in stable code. x test passes locally, any regressions should be caught by the full CI test run & bors build before crater.

TODOs - this is a draft PR for the following reasons

decision (design meeting TBC): if taking this approach stabilisation of Try is fixed to an edition boundary. All unstable try_... features should not stabilise before try_trait_v2
decision (design meeting TBC): is enforced homogeneity desirable?
decision (design meeting TBC): Is Poll homogeneity desirable (feedback from tokio is "yes, but only on an edition boundary")?
fix compiler output for legacy desugaring. This is temporarily adjusted to the (unsuitable) output produced by this change to allow for validation of the underlying approach.
fix try_trait_v2 flagged as unused if only needed for desugaring
consistent naming (old vs legacy vs not-intending-to-open-for-bikeshedding)
update rust-analyzer (symbols)

LLM statement

Confirmed no LLM usage.

traits with associated lang items

tmandry · 2026-10-05T22:46:16Z

@bors try

Try poll

rust-bors · 2026-10-05T23:52:08Z

☀️ Try build successful (CI)
Build commit: 35f304e (35f304e0e45f840db2524fae65670886a86bf6ba)
Base parent: ea13733 (ea137335b78829b4514bf1b4c16302f74fab8581)

FIXME this needs to be reverted and done properly if the overall approach is sound

try_fold is stable but relies on unstable trait Try. add a dedicated qmark! macro which performs future desugaring in a stable context. This is sound because only stdlib can name and use Try in a stable context. All third-party implementations of try_fold must be unstable and make use of feature(try_trait_v2) in order to name Try. This does not introduce any new risks, any adjustments to unstable `Try` already need to consider the impact on try_fold. All other try_xxx methods in stdlib are unstable.

to explicitly use modern desugaring

to use qmark! get_or_try_insert_with is currently unstable and is the only such method which relies on `?`. All other unstable `try_xxx` methods manually implement a match on Try::branch.

This reverts commit 0f9e216.

MusicalNinjaDad · 2026-10-06T09:19:39Z

r? @tmandry can you please set up a crater run for this.

Thanks!

tmandry · 2026-10-06T21:12:49Z

@bors try

Proof of Concept: enforced homogeneity for Try (& Poll)

rust-bors · 2026-10-06T22:13:00Z

☀️ Try build successful (CI)
Build commit: 8773d2c (8773d2c75a8e90130dcce745a7ec4e66bb9963db)
Base parent: 8d1a764 (8d1a76430406c877b35d0b627e7f796dcf0dfeca)

tmandry · 2026-10-07T14:22:06Z

@craterbot run mode=check-only start=8d1a76430406c877b35d0b627e7f796dcf0dfeca+rustflags=-Aunfulfilled-lint-expectations end=8773d2c75a8e90130dcce745a7ec4e66bb9963db+rustflags=-Aunfulfilled-lint-expectations

-Aunfulfilled-lint-expectations requested by the author to filter out lint related noise due to the current state of the PR.

craterbot · 2026-10-07T14:22:25Z

👌 Experiment pr-163637 created and queued.
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

rust-bors · 2026-10-08T23:33:56Z

☔ The latest upstream changes (presumably #164005) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

MusicalNinjaDad force-pushed the try_poll branch from aa05e45 to a82da97 Compare October 3, 2026 06:23

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provide legacy desugaring via dedicated internal

7656e12

traits with associated lang items

MusicalNinjaDad force-pushed the try_poll branch from 9bce5a8 to e9d0fe5 Compare October 5, 2026 11:03

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MusicalNinjaDad linked an issue Oct 5, 2026 that may be closed by this pull request

Crater run for homogenous Poll MusicalNinjaDad/rust-lang#1

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MusicalNinjaDad force-pushed the try_poll branch from bd3608a to 9fa6156 Compare October 5, 2026 15:01

rustbot added A-CI Area: Our Github Actions CI T-clippy Relevant to the Clippy team. T-infra Relevant to the infrastructure team, which will review and decide on the PR/issue. labels Oct 5, 2026

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rust-bors Bot pushed a commit that referenced this pull request Oct 5, 2026

Auto merge of #163637 - MusicalNinjaDad:try_poll, r=<try>

35f304e

Try poll

MusicalNinjaDad added 8 commits October 6, 2026 07:19

adjust ui tests so we can get a crater run

f6e3493

FIXME this needs to be reverted and done properly if the overall approach is sound

update all implementations of Iterator::try_fold

f3c4576

to explicitly use modern desugaring

update Option::get_or_try_insert_with

0a84e4e

to use qmark! get_or_try_insert_with is currently unstable and is the only such method which relies on `?`. All other unstable `try_xxx` methods manually implement a match on Try::branch.

homogeneous Poll

0ecb4c8

require full homogeneity

3765446

run as much CI as possible each time

0f9e216

Revert "run as much CI as possible each time"

9d3e7b5

This reverts commit 0f9e216.

MusicalNinjaDad force-pushed the try_poll branch from 9fa6156 to 9d3e7b5 Compare October 6, 2026 07:20

MusicalNinjaDad changed the title ~~Try poll~~ Proof of Concept: enforced homogeneity for Try (& Poll) Oct 6, 2026

MusicalNinjaDad mentioned this pull request Oct 6, 2026

Stabilize the Try trait rust-lang/goals#654

Open

rustbot assigned tmandry Oct 6, 2026

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rust-bors Bot pushed a commit that referenced this pull request Oct 6, 2026

Auto merge of #163637 - MusicalNinjaDad:try_poll, r=<try>

8773d2c

Proof of Concept: enforced homogeneity for Try (& Poll)

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 7, 2026

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MusicalNinjaDad commented Oct 2, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

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Requesting a crater run to validate the following points

homogeneous Poll

Iterator::try_fold

enforced homogeneity

TODOs - this is a draft PR for the following reasons

LLM statement

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tmandry commented Oct 5, 2026

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MusicalNinjaDad commented Oct 2, 2026 •

edited by rustbot

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homogeneous `Poll`

`Iterator::try_fold`