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Make PinCoerceUnsized require Deref #149218

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Make PinCoerceUnsized require Deref #149218

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@theemathas theemathas commented Nov 22, 2025 •
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Also, delete impls on non-Deref types.

Pin doesn't do anything useful for non-Deref types, so PinCoerceUnsized on such types makes no sense.

This is a breaking change, since stable code can observe the deleted PinCoerceUnsized impls by uselessly coercing between such types inside a Pin.

There is still some strange behavior, such as Pin<&mut i32> being able to coerce to Pin<&dyn Send>, but not being able to coerce to Pin<&i32>. However, I don't think it's possible to fix this.

Fixes #145081

@theemathas theemathas added T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. A-coercions Area: implicit and explicit `expr as Type` coercions needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. A-pin Area: Pin T-types Relevant to the types team, which will review and decide on the PR/issue. needs-crater This change needs a crater run to check for possible breakage in the ecosystem. labels Nov 22, 2025
@rustbot rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Nov 22, 2025
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@theemathas theemathas force-pushed the pin-coerce-unsized branch 2 times, most recently from f921d32 to f4fa919 Compare November 23, 2025 03:35
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jdonszelmann commented Nov 23, 2025

@bors try

rust-bors bot added a commit that referenced this pull request Nov 23, 2025
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Auto merge of #149218 - theemathas:pin-coerce-unsized, r=<try>
94e8487 
Make PinCoerceUnsized require Deref
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rust-bors bot commented Nov 23, 2025

☀️ Try build successful (CI)
Build commit: 94e8487 (94e848735e78b58442e0b82e9c60f8f430840389, parent: e0e204f3e97ad5f79524b9c259dc38df606ed82c)

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saethlin commented Nov 24, 2025

@craterbot check

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craterbot commented Nov 24, 2025

👌 Experiment pr-149218 created and queued.
🤖 Automatically detected try build 94e8487
🔍 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

@craterbot craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Nov 24, 2025
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craterbot commented Dec 1, 2025

🚧 Experiment pr-149218 is now running

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Darksonn commented Dec 1, 2025

I added these impls because otherwise the introduction of PinCoerceUnsized would have been a breaking change. I'm in favor of removing them.

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craterbot commented Dec 2, 2025

🎉 Experiment pr-149218 is completed!
📊 6 regressed and 5 fixed (743358 total)
📊 1966 spurious results on the retry-regessed-list.txt, consider a retry1 if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Footnotes

  1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-149218/retry-regressed-list.txt ↩

@craterbot craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Dec 2, 2025
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workingjubilee commented Dec 3, 2025

@craterbot check crates=https://crater-reports.s3.amazonaws.com/pr-149218/retry-regressed-list.txt

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craterbot commented Dec 3, 2025

👌 Experiment pr-149218-1 created and queued.
🤖 Automatically detected try build 94e8487
🔍 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

@craterbot craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Dec 3, 2025
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craterbot commented Dec 9, 2025

🚧 Experiment pr-149218-1 is now running

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craterbot commented Dec 9, 2025

🎉 Experiment pr-149218-1 is completed!
📊 0 regressed and 0 fixed (1799 total)
📊 128 spurious results on the retry-regessed-list.txt, consider a retry1 if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

Footnotes

  1. re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-149218-1/retry-regressed-list.txt ↩

@craterbot craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Dec 9, 2025
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theemathas commented Dec 9, 2025

Crater results are clean.

Which team does this need an FCP with?

@theemathas theemathas removed the needs-crater This change needs a crater run to check for possible breakage in the ecosystem. label Dec 10, 2025
@Darksonn Darksonn mentioned this pull request Dec 17, 2025
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bors commented Dec 18, 2025

☔ The latest upstream changes (presumably #150115) made this pull request unmergeable. Please resolve the merge conflicts.

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Darksonn commented Dec 18, 2025

Probably T-lang?

@Darksonn Darksonn added the I-lang-nominated Nominated for discussion during a lang team meeting. label Dec 18, 2025
@theemathas
Make PinCoerceUnsized require Deref
d884e9b 
Also, delete impls on non-Deref types.

Pin doesn't do anything useful for non-Deref types, so PinCoerceUnsized
on such types makes no sense.

This is a breaking change, since stable code can observe the deleted
`PinCoerceUnsized` impls by uselessly coercing between such types
inside a `Pin`.

There is still some strange behavior, such as `Pin<&mut i32>` being
able to coerce to `Pin<&dyn Send>`, but not being able to coerce to
`Pin<&i32>`. However, I don't think it's possible to fix this.

Fixes rust-lang#145081
@theemathas
Rewrite safety comment for PinCoerceUnsized
969c3df
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rustbot commented Dec 19, 2025

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.

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A-coercions Area: implicit and explicit `expr as Type` coercions A-pin Area: Pin I-lang-nominated Nominated for discussion during a lang team meeting. needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. T-types Relevant to the types team, which will review and decide on the PR/issue.

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PinCoerceUnsized has strange impls

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