Gate const drop behind const_destruct feature, and fix const_precise_live_drops post-drop-elaboration check

Author: compiler-errors

compiler/rustc_const_eval/src/check_consts/check.rs

@@ -23,7 +23,7 @@ use rustc_span::{Span, Symbol, sym};
 use rustc_trait_selection::traits::{
     Obligation, ObligationCause, ObligationCauseCode, ObligationCtxt,
 };
-use tracing::{debug, instrument, trace};
+use tracing::{instrument, trace};
 
 use super::ops::{self, NonConstOp, Status};
 use super::qualifs::{self, HasMutInterior, NeedsDrop, NeedsNonConstDrop};
@@ -46,7 +46,7 @@ impl<'mir, 'tcx> Qualifs<'mir, 'tcx> {
     /// Returns `true` if `local` is `NeedsDrop` at the given `Location`.
     ///
     /// Only updates the cursor if absolutely necessary
-    fn needs_drop(
+    pub(crate) fn needs_drop(
         &mut self,
         ccx: &'mir ConstCx<'mir, 'tcx>,
         local: Local,
@@ -314,6 +314,14 @@ impl<'mir, 'tcx> Checker<'mir, 'tcx> {
         }
     }
 
+    /// Emits an error at the given `span` if an expression cannot be evaluated in the current
+    /// context. This is meant for use in a post-const-checker pass such as the const precise
+    /// // live drops lint.
+    pub fn check_op_spanned_post<O: NonConstOp<'tcx>>(mut self, op: O, span: Span) {
+        self.check_op_spanned(op, span);
+        assert!(self.secondary_errors.is_empty());
+    }
+
     fn check_static(&mut self, def_id: DefId, span: Span) {
         if self.tcx.is_thread_local_static(def_id) {
             self.tcx.dcx().span_bug(span, "tls access is checked in `Rvalue::ThreadLocalRef`");
@@ -839,12 +847,13 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 let mut err_span = self.span;
                 let ty_of_dropped_place = dropped_place.ty(self.body, self.tcx).ty;
 
-                let ty_needs_non_const_drop =
-                    qualifs::NeedsNonConstDrop::in_any_value_of_ty(self.ccx, ty_of_dropped_place);
-
-                debug!(?ty_of_dropped_place, ?ty_needs_non_const_drop);
-
-                if !ty_needs_non_const_drop {
+                let needs_drop = if let Some(local) = dropped_place.as_local() {
+                    self.qualifs.needs_drop(self.ccx, local, location)
+                } else {
+                    qualifs::NeedsDrop::in_any_value_of_ty(self.ccx, ty_of_dropped_place)
+                };
+                // If this type doesn't need a drop at all, then there's nothing to enforce.
+                if !needs_drop {
                     return;
                 }
 
@@ -853,18 +862,17 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                     err_span = self.body.local_decls[local].source_info.span;
                     self.qualifs.needs_non_const_drop(self.ccx, local, location)
                 } else {
-                    true
+                    qualifs::NeedsNonConstDrop::in_any_value_of_ty(self.ccx, ty_of_dropped_place)
                 };
 
-                if needs_non_const_drop {
-                    self.check_op_spanned(
-                        ops::LiveDrop {
-                            dropped_at: Some(terminator.source_info.span),
-                            dropped_ty: ty_of_dropped_place,
-                        },
-                        err_span,
-                    );
-                }
+                self.check_op_spanned(
+                    ops::LiveDrop {
+                        dropped_at: Some(terminator.source_info.span),
+                        dropped_ty: ty_of_dropped_place,
+                        needs_non_const_drop,
+                    },
+                    err_span,
+                );
             }
 
             TerminatorKind::InlineAsm { .. } => self.check_op(ops::InlineAsm),

compiler/rustc_const_eval/src/check_consts/ops.rs

@@ -452,15 +452,40 @@ impl<'tcx> NonConstOp<'tcx> for InlineAsm {
 pub(crate) struct LiveDrop<'tcx> {
     pub dropped_at: Option<Span>,
     pub dropped_ty: Ty<'tcx>,
+    pub needs_non_const_drop: bool,
 }
 impl<'tcx> NonConstOp<'tcx> for LiveDrop<'tcx> {
+    fn status_in_item(&self, _ccx: &ConstCx<'_, 'tcx>) -> Status {
+        if self.needs_non_const_drop {
+            Status::Forbidden
+        } else {
+            Status::Unstable {
+                gate: sym::const_destruct,
+                safe_to_expose_on_stable: false,
+                is_function_call: false,
+            }
+        }
+    }
+
     fn build_error(&self, ccx: &ConstCx<'_, 'tcx>, span: Span) -> Diag<'tcx> {
-        ccx.dcx().create_err(errors::LiveDrop {
-            span,
-            dropped_ty: self.dropped_ty,
-            kind: ccx.const_kind(),
-            dropped_at: self.dropped_at,
-        })
+        if self.needs_non_const_drop {
+            ccx.dcx().create_err(errors::LiveDrop {
+                span,
+                dropped_ty: self.dropped_ty,
+                kind: ccx.const_kind(),
+                dropped_at: self.dropped_at,
+            })
+        } else {
+            ccx.tcx.sess.create_feature_err(
+                errors::LiveDrop {
+                    span,
+                    dropped_ty: self.dropped_ty,
+                    kind: ccx.const_kind(),
+                    dropped_at: self.dropped_at,
+                },
+                sym::const_destruct,
+            )
+        }
     }
 }
 

compiler/rustc_const_eval/src/check_consts/post_drop_elaboration.rs

@@ -1,14 +1,15 @@
 use rustc_middle::mir::visit::Visitor;
 use rustc_middle::mir::{self, BasicBlock, Location};
-use rustc_middle::ty::{Ty, TyCtxt};
-use rustc_span::Span;
+use rustc_middle::ty::TyCtxt;
 use rustc_span::symbol::sym;
 use tracing::trace;
 
 use super::ConstCx;
 use super::check::Qualifs;
-use super::ops::{self, NonConstOp};
+use super::ops::{self};
 use super::qualifs::{NeedsNonConstDrop, Qualif};
+use crate::check_consts::check::Checker;
+use crate::check_consts::qualifs::NeedsDrop;
 use crate::check_consts::rustc_allow_const_fn_unstable;
 
 /// Returns `true` if we should use the more precise live drop checker that runs after drop
@@ -66,12 +67,6 @@ impl<'mir, 'tcx> std::ops::Deref for CheckLiveDrops<'mir, 'tcx> {
     }
 }
 
-impl<'tcx> CheckLiveDrops<'_, 'tcx> {
-    fn check_live_drop(&self, span: Span, dropped_ty: Ty<'tcx>) {
-        ops::LiveDrop { dropped_at: None, dropped_ty }.build_error(self.ccx, span).emit();
-    }
-}
-
 impl<'tcx> Visitor<'tcx> for CheckLiveDrops<'_, 'tcx> {
     fn visit_basic_block_data(&mut self, bb: BasicBlock, block: &mir::BasicBlockData<'tcx>) {
         trace!("visit_basic_block_data: bb={:?} is_cleanup={:?}", bb, block.is_cleanup);
@@ -89,28 +84,39 @@ impl<'tcx> Visitor<'tcx> for CheckLiveDrops<'_, 'tcx> {
 
         match &terminator.kind {
             mir::TerminatorKind::Drop { place: dropped_place, .. } => {
-                let dropped_ty = dropped_place.ty(self.body, self.tcx).ty;
-
-                if !NeedsNonConstDrop::in_any_value_of_ty(self.ccx, dropped_ty) {
-                    // Instead of throwing a bug, we just return here. This is because we have to
-                    // run custom `const Drop` impls.
+                let ty_of_dropped_place = dropped_place.ty(self.body, self.tcx).ty;
+
+                let needs_drop = if let Some(local) = dropped_place.as_local() {
+                    self.qualifs.needs_drop(self.ccx, local, location)
+                } else {
+                    NeedsDrop::in_any_value_of_ty(self.ccx, ty_of_dropped_place)
+                };
+                // If this type doesn't need a drop at all, then there's nothing to enforce.
+                if !needs_drop {
                     return;
                 }
 
-                if dropped_place.is_indirect() {
-                    self.check_live_drop(terminator.source_info.span, dropped_ty);
-                    return;
-                }
-
-                // Drop elaboration is not precise enough to accept code like
-                // `tests/ui/consts/control-flow/drop-pass.rs`; e.g., when an `Option<Vec<T>>` is
-                // initialized with `None` and never changed, it still emits drop glue.
-                // Hence we additionally check the qualifs here to allow more code to pass.
-                if self.qualifs.needs_non_const_drop(self.ccx, dropped_place.local, location) {
-                    // Use the span where the dropped local was declared for the error.
-                    let span = self.body.local_decls[dropped_place.local].source_info.span;
-                    self.check_live_drop(span, dropped_ty);
-                }
+                let mut err_span = terminator.source_info.span;
+
+                let needs_non_const_drop = if let Some(local) = dropped_place.as_local() {
+                    // Use the span where the local was declared as the span of the drop error.
+                    err_span = self.body.local_decls[local].source_info.span;
+                    self.qualifs.needs_non_const_drop(self.ccx, local, location)
+                } else {
+                    NeedsNonConstDrop::in_any_value_of_ty(self.ccx, ty_of_dropped_place)
+                };
+
+                // I know it's not great to be creating a new const checker, but I'd
+                // rather use it so we can deduplicate the error emitting logic that
+                // it contains.
+                Checker::new(self.ccx).check_op_spanned_post(
+                    ops::LiveDrop {
+                        dropped_at: Some(terminator.source_info.span),
+                        dropped_ty: ty_of_dropped_place,
+                        needs_non_const_drop,
+                    },
+                    err_span,
+                );
             }
 
             mir::TerminatorKind::UnwindTerminate(_)

compiler/rustc_const_eval/src/check_consts/qualifs.rs

@@ -136,6 +136,7 @@ pub struct NeedsDrop;
 impl Qualif for NeedsDrop {
     const ANALYSIS_NAME: &'static str = "flow_needs_drop";
     const IS_CLEARED_ON_MOVE: bool = true;
+    const ALLOW_PROMOTED: bool = true;
 
     fn in_qualifs(qualifs: &ConstQualifs) -> bool {
         qualifs.needs_drop
@@ -175,25 +176,22 @@ impl Qualif for NeedsNonConstDrop {
         // that the components of this type are also `~const Destruct`. This
         // amounts to verifying that there are no values in this ADT that may have
         // a non-const drop.
-        if cx.tcx.features().const_trait_impl() {
-            let destruct_def_id = cx.tcx.require_lang_item(LangItem::Destruct, Some(cx.body.span));
-            let infcx = cx.tcx.infer_ctxt().build(TypingMode::from_param_env(cx.param_env));
-            let ocx = ObligationCtxt::new(&infcx);
-            ocx.register_obligation(Obligation::new(
-                cx.tcx,
-                ObligationCause::misc(cx.body.span, cx.def_id()),
-                cx.param_env,
-                ty::Binder::dummy(ty::TraitRef::new(cx.tcx, destruct_def_id, [ty]))
-                    .to_host_effect_clause(cx.tcx, match cx.const_kind() {
-                        rustc_hir::ConstContext::ConstFn => ty::BoundConstness::Maybe,
-                        rustc_hir::ConstContext::Static(_)
-                        | rustc_hir::ConstContext::Const { .. } => ty::BoundConstness::Const,
-                    }),
-            ));
-            !ocx.select_all_or_error().is_empty()
-        } else {
-            NeedsDrop::in_any_value_of_ty(cx, ty)
-        }
+        let destruct_def_id = cx.tcx.require_lang_item(LangItem::Destruct, Some(cx.body.span));
+        let infcx = cx.tcx.infer_ctxt().build(TypingMode::from_param_env(cx.param_env));
+        let ocx = ObligationCtxt::new(&infcx);
+        ocx.register_obligation(Obligation::new(
+            cx.tcx,
+            ObligationCause::misc(cx.body.span, cx.def_id()),
+            cx.param_env,
+            ty::Binder::dummy(ty::TraitRef::new(cx.tcx, destruct_def_id, [ty]))
+                .to_host_effect_clause(cx.tcx, match cx.const_kind() {
+                    rustc_hir::ConstContext::ConstFn => ty::BoundConstness::Maybe,
+                    rustc_hir::ConstContext::Static(_) | rustc_hir::ConstContext::Const { .. } => {
+                        ty::BoundConstness::Const
+                    }
+                }),
+        ));
+        !ocx.select_all_or_error().is_empty()
     }
 
     fn is_structural_in_adt<'tcx>(cx: &ConstCx<'_, 'tcx>, adt: AdtDef<'tcx>) -> bool {

compiler/rustc_feature/src/unstable.rs

@@ -424,6 +424,8 @@ declare_features! (
     (unstable, const_async_blocks, "1.53.0", Some(85368)),
     /// Allows `const || {}` closures in const contexts.
     (incomplete, const_closures, "1.68.0", Some(106003)),
+    /// Uwu
+    (unstable, const_destruct, "CURRENT_RUSTC_VERSION", Some(10)),
     /// Allows `for _ in _` loops in const contexts.
     (unstable, const_for, "1.56.0", Some(87575)),
     /// Be more precise when looking for live drops in a const context.

compiler/rustc_span/src/symbol.rs

@@ -609,6 +609,7 @@ symbols! {
         const_compare_raw_pointers,
         const_constructor,
         const_deallocate,
+        const_destruct,
         const_eval_limit,
         const_eval_select,
         const_evaluatable_checked,

library/core/src/marker.rs

@@ -953,7 +953,7 @@ marker_impls! {
 ///
 /// This should be used for `~const` bounds,
 /// as non-const bounds will always hold for every type.
-#[unstable(feature = "const_trait_impl", issue = "67792")]
+#[unstable(feature = "const_destruct", issue = "10")]
 #[lang = "destruct"]
 #[rustc_on_unimplemented(message = "can't drop `{Self}`", append_const_msg)]
 #[rustc_deny_explicit_impl(implement_via_object = false)]

tests/ui/consts/const-block-const-bound.stderr

@@ -1,3 +1,23 @@
+error[E0658]: use of unstable library feature `const_destruct`
+  --> $DIR/const-block-const-bound.rs:6:5
+   |
+LL | use std::marker::Destruct;
+   |     ^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #10 <https://github.com/rust-lang/rust/issues/10> for more information
+   = help: add `#![feature(const_destruct)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: use of unstable library feature `const_destruct`
+  --> $DIR/const-block-const-bound.rs:8:22
+   |
+LL | const fn f<T: ~const Destruct>(x: T) {}
+   |                      ^^^^^^^^
+   |
+   = note: see issue #10 <https://github.com/rust-lang/rust/issues/10> for more information
+   = help: add `#![feature(const_destruct)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
 error: `~const` can only be applied to `#[const_trait]` traits
   --> $DIR/const-block-const-bound.rs:8:15
    |
@@ -20,6 +40,7 @@ LL | const fn f<T: ~const Destruct>(x: T) {}
    |                                |
    |                                the destructor for this type cannot be evaluated in constant functions
 
-error: aborting due to 3 previous errors
+error: aborting due to 5 previous errors
 
-For more information about this error, try `rustc --explain E0493`.
+Some errors have detailed explanations: E0493, E0658.
+For more information about an error, try `rustc --explain E0493`.

tests/ui/consts/control-flow/drop-fail.precise.stderr

@@ -1,14 +1,20 @@
 error[E0493]: destructor of `Option<Vec<i32>>` cannot be evaluated at compile-time
-  --> $DIR/drop-fail.rs:8:9
+  --> $DIR/drop-fail.rs:9:9
    |
 LL |     let x = Some(Vec::new());
    |         ^ the destructor for this type cannot be evaluated in constants
+...
+LL | };
+   | - value is dropped here
 
 error[E0493]: destructor of `Option<Vec<i32>>` cannot be evaluated at compile-time
-  --> $DIR/drop-fail.rs:39:9
+  --> $DIR/drop-fail.rs:40:9
    |
 LL |     let mut tmp = None;
    |         ^^^^^^^ the destructor for this type cannot be evaluated in constants
+...
+LL | };
+   | - value is dropped here
 
 error: aborting due to 2 previous errors
 

tests/ui/consts/control-flow/drop-fail.rs

@@ -1,5 +1,6 @@
 //@ revisions: stock precise
 
+#![feature(const_destruct)]
 #![cfg_attr(precise, feature(const_precise_live_drops))]
 
 // `x` is *not* always moved into the final value and may be dropped inside the initializer.