Add try_as_dyn and try_as_dyn_mut #150033

izagawd · 2025-12-15T19:51:55Z

Tracking issue: #144361

Continuation of: #144363

rustbot · 2025-12-15T19:52:01Z

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

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

These commits modify the Cargo.lock file. Unintentional changes to Cargo.lock can be introduced when switching branches and rebasing PRs.

If this was unintentional then you should revert the changes before this PR is merged.
Otherwise, you can ignore this comment.

rustbot · 2025-12-15T19:52:03Z

r? @scottmcm

rustbot has assigned @scottmcm.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

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izagawd · 2025-12-15T19:53:34Z

r? @oli-obk

rustbot · 2025-12-15T19:53:40Z

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

rustbot · 2025-12-15T20:00:11Z

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.

oli-obk

Please squash the commits into one once ready (thx for keeping it separate initially for review purposes)

View changes since this review

oli-obk · 2025-12-16T07:54:16Z

library/core/src/intrinsics/mod.rs

+/// # Compile-time failures
+/// Determining whether `T` implements `U` requires trait resolution by the compiler.
+/// In some cases, that resolution can exceed the recursion limit,
+/// and compilation will fail *instead of* this function returning `None`.


The comment is duplicated

oli-obk · 2025-12-16T07:56:26Z

tests/ui/any/try_as_dyn_err1.rs

+    let fn_ptr: fn(&'static Box<i32>) = store;
+    let dt = try_as_dyn::<_, dyn Trait>(&fn_ptr);
+    if let Some(dt) = dt {
+        // unsound path


Panic here instead of doing UB so that we know this will reliably fail if broken.

Would an assertion work as well?

Yea, as long as the test doesn't cause UB if the thing it tests gets broken

oli-obk · 2025-12-16T07:56:43Z

tests/ui/any/try_as_dyn_err2.rs

+    let data = Box::new(Box::new(1i32));
+    let dt = try_as_dyn::<_, dyn Trait<fn(&'static Box<i32>)>>(&());
+    if let Some(dt) = dt {
+        // unsound path


Also panic here

rustbot · 2025-12-16T07:58:17Z

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

oli-obk · 2025-12-16T10:35:19Z

Please give the commit a proper description

… and `try_as_dyn_mut` for fallible coercion from `&T` / `&mut T` to `&dyn Trait`.

izagawd · 2025-12-16T10:45:51Z

Does this commit message work?

izagawd · 2025-12-16T10:47:41Z

@rustbot ready

oli-obk · 2025-12-16T11:09:18Z

Thank you! that's great

@bors r+ rollup

bors · 2025-12-16T11:09:22Z

📌 Commit d5bf1a4 has been approved by oli-obk

It is now in the queue for this repository.

Add try_as_dyn and try_as_dyn_mut Tracking issue: rust-lang#144361 Continuation of: rust-lang#144363

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Dec 15, 2025

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rustbot added has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Dec 15, 2025

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izagawd force-pushed the try_as_dyn branch from 1c25d1b to c7373ef Compare December 15, 2025 20:54

rustbot removed has-merge-commits PR has merge commits, merge with caution. S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Dec 15, 2025

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oli-obk requested changes Dec 16, 2025

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rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Dec 16, 2025

rustbot added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Dec 16, 2025

izagawd force-pushed the try_as_dyn branch from e9a1ded to ba938d7 Compare December 16, 2025 09:20

oli-obk approved these changes Dec 16, 2025

View reviewed changes

Introduce vtable_for intrinsic and use it to implement try_as_dyn…

d5bf1a4

… and `try_as_dyn_mut` for fallible coercion from `&T` / `&mut T` to `&dyn Trait`.

izagawd force-pushed the try_as_dyn branch from ba938d7 to d5bf1a4 Compare December 16, 2025 10:43

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Dec 16, 2025

bors 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 Dec 16, 2025

JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request Dec 16, 2025

Rollup merge of rust-lang#150033 - izagawd:try_as_dyn, r=oli-obk

25b73c4

Add try_as_dyn and try_as_dyn_mut Tracking issue: rust-lang#144361 Continuation of: rust-lang#144363

JonathanBrouwer mentioned this pull request Dec 16, 2025

Rollup of 11 pull requests #150068

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