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Add try_as_dyn and try_as_dyn_mut #150033

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@izagawd izagawd commented Dec 15, 2025

Tracking issue: #144361

Continuation of: #144363

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rustbot commented Dec 15, 2025

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 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 commented Dec 15, 2025

r? @scottmcm

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@rustbot 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 commented Dec 15, 2025

r? @oli-obk

@rustbot rustbot assigned oli-obk and unassigned scottmcm Dec 15, 2025
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rustbot commented Dec 15, 2025

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

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rustbot commented Dec 15, 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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@rustbot 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
oli-obk requested changes Dec 16, 2025
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Please squash the commits into one once ready (thx for keeping it separate initially for review purposes)

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library/core/src/intrinsics/mod.rs Outdated
/// # 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`.
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The comment is duplicated

tests/ui/any/try_as_dyn_err1.rs Outdated
let fn_ptr: fn(&'static Box<i32>) = store;
let dt = try_as_dyn::<_, dyn Trait>(&fn_ptr);
if let Some(dt) = dt {
// unsound path
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Panic here instead of doing UB so that we know this will reliably fail if broken.

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@izagawd izagawd Dec 16, 2025

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Would an assertion work as well?

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

tests/ui/any/try_as_dyn_err2.rs Outdated
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
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Also panic here

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

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

@rustbot 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
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oli-obk commented Dec 16, 2025

Please give the commit a proper description

@ivarflakstad @izagawd
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`.
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izagawd commented Dec 16, 2025

Does this commit message work?

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izagawd commented Dec 16, 2025

@rustbot ready

@rustbot 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
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oli-obk commented Dec 16, 2025

Thank you! that's great

@bors r+ rollup

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bors commented Dec 16, 2025

📌 Commit d5bf1a4 has been approved by oli-obk

It is now in the queue for this repository.

@bors 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
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