Suggest the [const] Destruct bound for type parameters in const functions when missing

[jakubadamw]

When a const function drops a value whose type is a type parameter without a [const] Destruct bound, suggest adding it so that the destructor can be evaluated at compile-time.

Closes#103270.

Example:

constfnf(_x:impl std::fmt::Debug){}

Before:

error[E0493]: destructor of `impl std::fmt::Debug` cannot be evaluated at compile-time
--> $DIR/min_const_fn.rs:1:11
|
LL | const fn f(_x: impl std::fmt::Debug) {}
| ^^ - value is dropped here
| |
| the destructor for this type cannot be evaluated in constant functions

After:

error[E0493]: destructor of `impl std::fmt::Debug` cannot be evaluated at compile-time
--> $DIR/min_const_fn.rs:1:11
|
LL | const fn f(_x: impl std::fmt::Debug) {}
| ^^ - value is dropped here
| |
| the destructor for this type cannot be evaluated in constant functions
|
help: consider restricting opaque type `impl std::fmt::Debug` with unstable trait `Destruct`
|
LL | const fn f(_x: impl std::fmt::Debug + [const] Destruct) {}
| ++++++++++++++++++

No new tests as the existing ones had their results updated with the new suggestion.

[rustbot]

Some changes occurred to constck

cc @fee1-dead

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

[rustbot]

r? @wesleywiser

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[Kivooeo]

thanks for the contribution, but this is experimental syntax which means syntax may change in future, so I think it should be held until stabilization

[wesleywiser]

I think we would probably only want to offer this suggestion if the user is on a nightly compiler.

View changes since this review

[rustbot]

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

[jakubadamw]

@wesleywiser, thank you, I made the suggestion trigger on nightly only. 🙂

[rustbot]

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]

r? @oli-obk

@bors r+ rollup

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[rust-bors]

📌 Commit 2f961b8 has been approved by oli-obk

It is now in the queue for this repository.