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

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -594,7 +594,7 @@ impl<'tcx> NonConstOp<'tcx> for LiveDrop<'tcx> {
     }
 
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
-        if self.needs_non_const_drop {
+        let mut err = if self.needs_non_const_drop {
             ccx.dcx().create_err(diagnostics::LiveDrop {
                 span,
                 dropped_ty: self.dropped_ty,
@@ -611,7 +611,30 @@ impl<'tcx> NonConstOp<'tcx> for LiveDrop<'tcx> {
                 },
                 sym::const_destruct,
             )
+        };
+
+        // If the dropped type is a type parameter, suggest adding a `[const] Destruct` bound.
+        // The suggestion is only offered on nightly, since `[const]` bounds are unstable.
+        if let Param(param_ty) = self.dropped_ty.kind()
+            && ccx.tcx.sess.is_nightly_build()
+        {
+            let tcx = ccx.tcx;
+            let caller = ccx.def_id();
+            if let Some(generics) = tcx.hir_node_by_def_id(caller).generics() {
+                let destruct_def_id = tcx.lang_items().destruct_trait();
+                suggest_constraining_type_param(
+                    tcx,
+                    generics,
+                    &mut err,
+                    param_ty.name.as_str(),
+                    "[const] Destruct",
+                    destruct_def_id,
+                    None,
+                );
+            }
         }
+
+        err
     }
 }
 

tests/run-make/const-destruct-stable-toolchain/const-drop-nightly.stderr

@@ -0,0 +1,16 @@
+error[E0493]: destructor of `T` cannot be evaluated at compile-time
+  --> const-drop.rs:1:24
+   |
+LL | const fn const_drop<T>(_: T) {}
+   |                        ^      - value is dropped here
+   |                        |
+   |                        the destructor for this type cannot be evaluated in constant functions
+   |
+help: consider restricting type parameter `T` with unstable trait `Destruct`
+   |
+LL | const fn const_drop<T: [const] Destruct>(_: T) {}
+   |                      ++++++++++++++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0493`.

tests/run-make/const-destruct-stable-toolchain/const-drop-stable.stderr

@@ -0,0 +1,11 @@
+error[E0493]: destructor of `T` cannot be evaluated at compile-time
+ --> const-drop.rs:1:24
+  |
+1 | const fn const_drop<T>(_: T) {}
+  |                        ^      - value is dropped here
+  |                        |
+  |                        the destructor for this type cannot be evaluated in constant functions
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0493`.

tests/run-make/const-destruct-stable-toolchain/const-drop.rs

@@ -0,0 +1,3 @@
+const fn const_drop<T>(_: T) {}
+
+fn main() {}

tests/run-make/const-destruct-stable-toolchain/rmake.rs

@@ -0,0 +1,26 @@
+//@ needs-target-std
+//
+// Test that the suggestion to constrain a type parameter that is dropped in a const
+// function with a `[const] Destruct` bound is only offered on nightly, since the bound
+// requires an unstable feature.
+
+use run_make_support::{diff, rustc};
+
+fn main() {
+    let out = rustc()
+        .input("const-drop.rs")
+        .env("RUSTC_BOOTSTRAP", "-1")
+        .run_fail()
+        .assert_stderr_not_contains("consider restricting type parameter `T`")
+        .stderr_utf8();
+    diff().expected_file("const-drop-stable.stderr").actual_text("(rustc)", &out).run();
+    let out = rustc()
+        .input("const-drop.rs")
+        .ui_testing()
+        .run_fail()
+        .assert_stderr_contains(
+            "consider restricting type parameter `T` with unstable trait `Destruct`",
+        )
+        .stderr_utf8();
+    diff().expected_file("const-drop-nightly.stderr").actual_text("(rustc)", &out).run();
+}

tests/ui/consts/min_const_fn/min_const_fn.stderr

@@ -73,6 +73,11 @@ LL | const fn no_apit(_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 no_apit(_x: impl std::fmt::Debug + [const] Destruct) {}
+   |                                           ++++++++++++++++++
 
 error: aborting due to 9 previous errors
 

tests/ui/consts/unstable-const-fn-in-libcore.stderr

@@ -6,6 +6,11 @@ LL |     const fn unwrap_or_else<F: [const] FnOnce() -> T>(self, f: F) -> T {
 ...
 LL |     }
    |     - value is dropped here
+   |
+help: consider further restricting type parameter `F` with unstable trait `Destruct`
+   |
+LL |     const fn unwrap_or_else<F: [const] FnOnce() -> T + [const] Destruct>(self, f: F) -> T {
+   |                                                      ++++++++++++++++++
 
 error[E0493]: destructor of `Opt<T>` cannot be evaluated at compile-time
   --> $DIR/unstable-const-fn-in-libcore.rs:19:55

tests/ui/self/arbitrary-self-from-method-substs-ice.stderr

@@ -15,6 +15,11 @@ LL |     const fn get<R: Deref<Target = Self>>(self: R) -> u32 {
 ...
 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`
   --> $DIR/arbitrary-self-from-method-substs-ice.rs:10:49

tests/ui/static/static-drop-scope.stderr

@@ -37,6 +37,11 @@ LL | const fn const_drop<T>(_: T) {}
    |                        ^      - value is dropped here
    |                        |
    |                        the destructor for this type cannot be evaluated in constant functions
+   |
+help: consider restricting type parameter `T` with unstable trait `Destruct`
+   |
+LL | const fn const_drop<T: [const] Destruct>(_: T) {}
+   |                      ++++++++++++++++++
 
 error[E0493]: destructor of `(T, ())` cannot be evaluated at compile-time
   --> $DIR/static-drop-scope.rs:17:5

tests/ui/traits/const-traits/minicore-drop-fail.stderr

@@ -30,6 +30,11 @@ LL | const fn drop_arbitrary<T>(_: T) {
 LL |
 LL | }
    | - value is dropped here
+   |
+help: consider restricting type parameter `T` with trait `Destruct`
+   |
+LL | const fn drop_arbitrary<T: [const] Destruct>(_: T) {
+   |                          ++++++++++++++++++
 
 error: aborting due to 4 previous errors