Add Drop::pin_drop for structually pinned types

Author: frank-king

compiler/rustc_hir_analysis/src/check/always_applicable.rs

@@ -12,6 +12,7 @@ use rustc_infer::traits::{ObligationCause, ObligationCauseCode};
 use rustc_middle::span_bug;
 use rustc_middle::ty::util::CheckRegions;
 use rustc_middle::ty::{self, GenericArgsRef, Ty, TyCtxt, TypingMode};
+use rustc_span::sym;
 use rustc_trait_selection::regions::InferCtxtRegionExt;
 use rustc_trait_selection::traits::{self, ObligationCtxt};
 
@@ -70,7 +71,11 @@ pub(crate) fn check_drop_impl(
                 drop_impl_did,
                 adt_def.did(),
                 adt_to_impl_args,
-            )
+            )?;
+
+            check_drop_xor_pin_drop(tcx, adt_def.did(), drop_impl_did)?;
+
+            Ok(())
         }
         _ => {
             span_bug!(tcx.def_span(drop_impl_did), "incoherent impl of Drop");
@@ -291,3 +296,68 @@ fn ensure_impl_predicates_are_implied_by_item_defn<'tcx>(
 
     Ok(())
 }
+
+/// This function checks at least and at most one of `Drop::drop` and `Drop::pin_drop` is implemented.
+/// It also checks that `Drop::pin_drop` must be implemented if `#[pin_v2]` is present on the type.
+fn check_drop_xor_pin_drop<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    adt_def_id: DefId,
+    drop_impl_did: LocalDefId,
+) -> Result<(), ErrorGuaranteed> {
+    let mut drop_span = None;
+    let mut pin_drop_span = None;
+    for item in tcx.associated_items(drop_impl_did).in_definition_order() {
+        match item.kind {
+            ty::AssocKind::Fn { name: sym::drop, .. } => {
+                drop_span = Some(tcx.def_span(item.def_id))
+            }
+            ty::AssocKind::Fn { name: sym::pin_drop, .. } => {
+                pin_drop_span = Some(tcx.def_span(item.def_id))
+            }
+            _ => {}
+        }
+    }
+
+    match (drop_span, pin_drop_span) {
+        (None, None) => {
+            if tcx.features().pin_ergonomics() {
+                return Err(tcx.dcx().emit_err(crate::errors::MissingOneOfTraitItem {
+                    span: tcx.def_span(drop_impl_did),
+                    note: None,
+                    missing_items_msg: "drop`, `pin_drop".to_string(),
+                }));
+            } else {
+                return Err(tcx
+                    .dcx()
+                    .span_delayed_bug(tcx.def_span(drop_impl_did), "missing `Drop::drop`"));
+            }
+        }
+        (Some(span), None) => {
+            if tcx.adt_def(adt_def_id).is_pin_project() {
+                let pin_v2_span = rustc_hir::find_attr!(tcx, adt_def_id, PinV2(attr) => *attr);
+                let adt_name = tcx.item_name(adt_def_id);
+                return Err(tcx.dcx().emit_err(crate::errors::PinV2WithoutPinDrop {
+                    span,
+                    pin_v2_span,
+                    adt_name,
+                }));
+            }
+        }
+        (None, Some(span)) => {
+            if !tcx.features().pin_ergonomics() {
+                return Err(tcx.dcx().span_delayed_bug(
+                    span,
+                    "`Drop::pin_drop` should be guarded by the library feature gate",
+                ));
+            }
+        }
+        (Some(drop_span), Some(pin_drop_span)) => {
+            return Err(tcx.dcx().emit_err(crate::errors::ConflictImplDropAndPinDrop {
+                span: tcx.def_span(drop_impl_did),
+                drop_span,
+                pin_drop_span,
+            }));
+        }
+    }
+    Ok(())
+}

compiler/rustc_hir_analysis/src/check/check.rs

@@ -25,6 +25,7 @@ use rustc_middle::ty::{
 };
 use rustc_session::lint::builtin::UNINHABITED_STATIC;
 use rustc_span::source_map::Spanned;
+use rustc_span::sym;
 use rustc_target::spec::{AbiMap, AbiMapping};
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
 use rustc_trait_selection::traits;
@@ -1321,6 +1322,15 @@ fn check_impl_items_against_trait<'tcx>(
             if !is_implemented_here {
                 let full_impl_span = tcx.hir_span_with_body(tcx.local_def_id_to_hir_id(impl_id));
                 match tcx.eval_default_body_stability(trait_item_id, full_impl_span) {
+                    // When the feature `pin_ergonomics` is disabled, we report `Drop::drop` is missing,
+                    // instead of `Drop::drop` is unstable that might be confusing.
+                    EvalResult::Deny { .. }
+                        if !tcx.features().pin_ergonomics()
+                            && tcx.is_lang_item(trait_ref.def_id, hir::LangItem::Drop)
+                            && tcx.item_name(trait_item_id) == sym::drop =>
+                    {
+                        missing_items.push(tcx.associated_item(trait_item_id));
+                    }
                     EvalResult::Deny { feature, reason, issue, .. } => default_body_is_unstable(
                         tcx,
                         full_impl_span,

compiler/rustc_hir_analysis/src/check/mod.rs

@@ -93,7 +93,7 @@ use rustc_middle::ty::{
 use rustc_middle::{bug, span_bug};
 use rustc_session::parse::feature_err;
 use rustc_span::def_id::CRATE_DEF_ID;
-use rustc_span::{BytePos, DUMMY_SP, Ident, Span, Symbol, kw, sym};
+use rustc_span::{BytePos, DUMMY_SP, Ident, Span, Symbol, kw};
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
 use rustc_trait_selection::error_reporting::infer::ObligationCauseExt as _;
 use rustc_trait_selection::error_reporting::traits::suggestions::ReturnsVisitor;

compiler/rustc_hir_analysis/src/errors.rs

@@ -1908,3 +1908,35 @@ pub(crate) struct ImplUnpinForPinProjectedType {
     pub adt_span: Span,
     pub adt_name: Symbol,
 }
+
+#[derive(Diagnostic)]
+#[diag("conflicting implementations of `Drop::drop` and `Drop::pin_drop`")]
+pub(crate) struct ConflictImplDropAndPinDrop {
+    #[primary_span]
+    pub span: Span,
+    #[label("`drop(&mut self)` implemented here")]
+    pub drop_span: Span,
+    #[label("`pin_drop(&pin mut self)` implemented here")]
+    pub pin_drop_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag("`{$adt_name}` must implement `pin_drop`")]
+#[help("structurally pinned types must keep `Pin`'s safety contract")]
+pub(crate) struct PinV2WithoutPinDrop {
+    #[primary_span]
+    #[suggestion(
+        "implement `pin_drop` instead",
+        code = "fn pin_drop(&pin mut self)",
+        applicability = "maybe-incorrect"
+    )]
+    pub span: Span,
+    #[note("`{$adt_name}` is marked `#[pin_v2]` here")]
+    #[suggestion(
+        "remove the `#[pin_v2]` attribute if it is not intended for structurally pinning",
+        code = "",
+        applicability = "maybe-incorrect"
+    )]
+    pub pin_v2_span: Option<Span>,
+    pub adt_name: Symbol,
+}

compiler/rustc_hir_typeck/src/callee.rs

@@ -37,9 +37,12 @@ pub(crate) fn check_legal_trait_for_method_call(
     receiver: Option<Span>,
     expr_span: Span,
     trait_id: DefId,
-    _body_id: DefId,
+    body_id: DefId,
 ) -> Result<(), ErrorGuaranteed> {
-    if tcx.is_lang_item(trait_id, LangItem::Drop) {
+    if tcx.is_lang_item(trait_id, LangItem::Drop)
+        // Allow calling `Drop::pin_drop` in `Drop::drop`
+        && !tcx.is_lang_item(tcx.parent(body_id), LangItem::Drop)
+    {
         let sugg = if let Some(receiver) = receiver.filter(|s| !s.is_empty()) {
             errors::ExplicitDestructorCallSugg::Snippet {
                 lo: expr_span.shrink_to_lo(),

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -515,6 +515,11 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                             CallToFunctionWith { function: func_did, missing, build_enabled },
                         );
                     }
+                    if let Some(trait_did) = self.tcx.trait_of_assoc(func_did)
+                        && self.tcx.is_lang_item(trait_did, hir::LangItem::Drop)
+                    {
+                        self.requires_unsafe(expr.span, CallDropExplicitly(func_did));
+                    }
                 }
             }
             ExprKind::RawBorrow { arg, .. } => {
@@ -743,6 +748,8 @@ enum UnsafeOpKind {
         build_enabled: Vec<Symbol>,
     },
     UnsafeBinderCast,
+    /// Calling `Drop::drop` or `Drop::pin_drop` explicitly.
+    CallDropExplicitly(DefId),
 }
 
 use UnsafeOpKind::*;
@@ -920,6 +927,9 @@ impl UnsafeOpKind {
                     unsafe_not_inherited_note,
                 },
             ),
+            CallDropExplicitly(_) => {
+                span_bug!(span, "`Drop::drop` or `Drop::pin_drop` should not be called explicitly")
+            }
         }
     }
 
@@ -1137,6 +1147,13 @@ impl UnsafeOpKind {
             UnsafeBinderCast => {
                 dcx.emit_err(UnsafeBinderCastRequiresUnsafe { span, unsafe_not_inherited_note });
             }
+            CallDropExplicitly(did) => {
+                dcx.emit_err(CallDropExplicitlyRequiresUnsafe {
+                    span,
+                    unsafe_not_inherited_note,
+                    function: tcx.def_path_str(*did),
+                });
+            }
         }
     }
 }

compiler/rustc_mir_build/src/errors.rs

@@ -603,6 +603,16 @@ pub(crate) struct UnsafeBinderCastRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
     pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
 }
 
+#[derive(Diagnostic)]
+#[diag("call `{$function}` explicitly is unsafe and requires unsafe block", code = E0133)]
+pub(crate) struct CallDropExplicitlyRequiresUnsafe {
+    #[primary_span]
+    pub(crate) span: Span,
+    pub(crate) function: String,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
+}
+
 #[derive(Subdiagnostic)]
 #[label("items do not inherit unsafety from separate enclosing items")]
 pub(crate) struct UnsafeNotInheritedNote {

compiler/rustc_span/src/symbol.rs

@@ -1493,6 +1493,7 @@ symbols! {
         pic,
         pie,
         pin,
+        pin_drop,
         pin_ergonomics,
         pin_v2,
         platform_intrinsics,

library/core/src/ops/drop.rs

@@ -1,3 +1,5 @@
+use crate::pin::Pin;
+
 /// Custom code within the destructor.
 ///
 /// When a value is no longer needed, Rust will run a "destructor" on that value.
@@ -237,5 +239,30 @@ pub const trait Drop {
     /// [`mem::drop`]: drop
     /// [`ptr::drop_in_place`]: crate::ptr::drop_in_place
     #[stable(feature = "rust1", since = "1.0.0")]
-    fn drop(&mut self);
+    #[rustc_default_body_unstable(feature = "pin_ergonomics", issue = "130494")]
+    fn drop(&mut self) {
+        // SAFETY: `self` is pinned till after dropped.
+        unsafe { Drop::pin_drop(Pin::new_unchecked(self)) }
+    }
+
+    /// Execute the destructor for this type, but different to [`Drop::drop`], it requires `self`
+    /// to be pinned.
+    ///
+    /// By implementing this method instead of [`Drop::drop`], the receiver type [`Pin<&mut Self>`]
+    /// makes sure that the value is pinned until it is deallocated (See [`std::pin` module docs] for
+    /// more details), which enables us to support field projections of `Self` type safely.
+    ///
+    /// For types that support pin-projection (i.e., marked with `#[pin_v2]`), this method must be used.
+    ///
+    /// For any type, at least and at most one of [`Drop::drop`] and [`Drop::pin_drop`] should be
+    /// implemented.
+    ///
+    /// See also [`Drop::drop`] for more details.
+    ///
+    /// [`Drop::drop`]: crate::ops::Drop::drop
+    /// [`Drop::pin_drop`]: crate::ops::Drop::pin_drop
+    /// [`Pin<&mut Self>`]: crate::pin::Pin
+    /// [`std::pin` module docs]: crate::pin
+    #[unstable(feature = "pin_ergonomics", issue = "130494")]
+    fn pin_drop(self: Pin<&mut Self>) {}
 }

tests/codegen-units/item-collection/drop-glue-eager.rs

@@ -10,6 +10,7 @@ struct StructWithDrop {
 
 impl Drop for StructWithDrop {
     //~ MONO_ITEM fn <StructWithDrop as std::ops::Drop>::drop
+    //~ MONO_ITEM fn <StructWithDrop as std::ops::Drop>::pin_drop
     fn drop(&mut self) {}
 }
 
@@ -24,6 +25,7 @@ enum EnumWithDrop {
 
 impl Drop for EnumWithDrop {
     //~ MONO_ITEM fn <EnumWithDrop as std::ops::Drop>::drop
+    //~ MONO_ITEM fn <EnumWithDrop as std::ops::Drop>::pin_drop
     fn drop(&mut self) {}
 }
 
@@ -34,6 +36,7 @@ enum EnumNoDrop {
 // We should be able to monomorphize drops for struct with lifetimes.
 impl<'a> Drop for StructWithDropAndLt<'a> {
     //~ MONO_ITEM fn <StructWithDropAndLt<'_> as std::ops::Drop>::drop
+    //~ MONO_ITEM fn <StructWithDropAndLt<'_> as std::ops::Drop>::pin_drop
     fn drop(&mut self) {}
 }
 
@@ -52,5 +55,6 @@ struct StructWithLtAndPredicate<'a: 'a> {
 // We should be able to monomorphize drops for struct with lifetimes.
 impl<'a> Drop for StructWithLtAndPredicate<'a> {
     //~ MONO_ITEM fn <StructWithLtAndPredicate<'_> as std::ops::Drop>::drop
+    //~ MONO_ITEM fn <StructWithLtAndPredicate<'_> as std::ops::Drop>::pin_drop
     fn drop(&mut self) {}
 }