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:

const fn f(_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.

[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.

[jakubadamw]

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

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

17    |     - value is dropped here
+    |
+ help: consider further restricting type parameter `R` with unstable trait `Destruct`
+    |
+ LL |     const fn get<R: Deref<Target = Self> + [const] Destruct>(self: R) -> u32 {
+    |                                          ++++++++++++++++++
18 
19 error[E0801]: invalid generic `self` parameter type: `R`
20   --> $DIR/arbitrary-self-from-method-substs-ice.rs:10:49


The actual stderr differed from the expected stderr
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args self/arbitrary-self-from-method-substs-ice.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/self/arbitrary-self-from-method-substs-ice.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/self/arbitrary-self-from-method-substs-ice" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers"
stdout: none
--- stderr -------------------------------
error[E0015]: cannot perform non-const deref coercion on `R` in constant functions
##[error]  --> /checkout/tests/ui/self/arbitrary-self-from-method-substs-ice.rs:13:9
   |
---

error[E0493]: destructor of `R` cannot be evaluated at compile-time
##[error]  --> /checkout/tests/ui/self/arbitrary-self-from-method-substs-ice.rs:10:43
   |
LL |     const fn get<R: Deref<Target = Self>>(self: R) -> u32 {
   |                                           ^^^^ the destructor for this type cannot be evaluated in constant functions
...
LL |     }
   |     - value is dropped here
   |
help: consider further restricting type parameter `R` with unstable trait `Destruct`
   |
LL |     const fn get<R: Deref<Target = Self> + [const] Destruct>(self: R) -> u32 {
   |                                          ++++++++++++++++++

error[E0801]: invalid generic `self` parameter type: `R`
##[error]  --> /checkout/tests/ui/self/arbitrary-self-from-method-substs-ice.rs:10:49
   |
LL |     const fn get<R: Deref<Target = Self>>(self: R) -> u32 {
   |                                                 ^
   |
   = note: type of `self` must not be a method generic parameter type
   = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0015, E0493, E0801.
For more information about an error, try `rustc --explain E0015`.