Auto merge of #144537 - frank-king:feature/pin-drop, r=petrochenkov

Add `Drop::pin_drop` for pinned drops





This PR is part of the `pin_ergonomics` experiment (the tracking issue is #130494). It allows implementing `Drop` with a pinned `self` receiver, which is required for safe pin-projection.

Implementations:
- [x] At least and at most one of `drop` and `pin_drop` should be implemented.
- [x] No direct call of `drop` or `pin_drop`. They should only be called by the drop glue.
- [x] `pin_drop` must and must only be used with types that support pin-projection (i.e. types with `#[pin_v2]`).
- [ ] Allows writing `fn drop(&pin mut self)` and desugars to `fn pin_drop(&pin mut self)`. (Will be in the next PRs)

Author: bors

compiler/rustc_attr_parsing/src/attributes/pin_v2.rs

@@ -15,5 +15,5 @@ impl NoArgsAttributeParser for PinV2Parser {
         Allow(Target::Struct),
         Allow(Target::Union),
     ]);
-    const CREATE: fn(Span) -> AttributeKind = |_| AttributeKind::PinV2;
+    const CREATE: fn(Span) -> AttributeKind = AttributeKind::PinV2;
 }

compiler/rustc_hir/src/attrs/data_structures.rs

@@ -1212,7 +1212,7 @@ pub enum AttributeKind {
     },
 
     /// Represents `#[pin_v2]`
-    PinV2,
+    PinV2(Span),
 
     /// Represents `#[prelude_import]`
     PreludeImport,

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

@@ -86,7 +86,7 @@ impl AttributeKind {
             PatchableFunctionEntry { .. } => Yes,
             Path(..) => No,
             PatternComplexityLimit { .. } => No,
-            PinV2 => Yes,
+            PinV2(..) => Yes,
             PreludeImport => No,
             ProcMacro => No,
             ProcMacroAttribute => No,

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, TypeVisitableExt, TypingMode};
+use rustc_span::sym;
 use rustc_trait_selection::regions::InferCtxtRegionExt;
 use rustc_trait_selection::traits::{self, ObligationCtxt};
 
@@ -72,7 +73,11 @@ 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");
@@ -363,3 +368,68 @@ 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

@@ -24,6 +24,7 @@ use rustc_middle::ty::{
     TypeVisitable, TypeVisitableExt, Unnormalized, fold_regions,
 };
 use rustc_session::lint::builtin::UNINHABITED_STATIC;
+use rustc_span::sym;
 use rustc_target::spec::{AbiMap, AbiMapping};
 use rustc_trait_selection::error_reporting::InferCtxtErrorExt;
 use rustc_trait_selection::traits;
@@ -1347,6 +1348,15 @@ 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

@@ -94,7 +94,7 @@ use rustc_middle::ty::{
 use rustc_middle::{bug, span_bug};
 use rustc_session::errors::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

@@ -1968,3 +1968,35 @@ pub(crate) struct EiiDefkindMismatchStaticSafety {
     pub span: Span,
     pub eii_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_middle/src/ty/adt.rs

@@ -349,7 +349,7 @@ impl AdtDefData {
             debug!("found non-exhaustive variant list for {:?}", did);
             flags = flags | AdtFlags::IS_VARIANT_LIST_NON_EXHAUSTIVE;
         }
-        if find_attr!(tcx, did, PinV2) {
+        if find_attr!(tcx, did, PinV2(..)) {
             debug!("found pin-project type {:?}", did);
             flags |= AdtFlags::IS_PIN_PROJECT;
         }

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -461,6 +461,11 @@ 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, .. } => {
@@ -661,6 +666,8 @@ enum UnsafeOpKind {
         build_enabled: Vec<Symbol>,
     },
     UnsafeBinderCast,
+    /// Calling `Drop::drop` or `Drop::pin_drop` explicitly.
+    CallDropExplicitly(DefId),
 }
 
 use UnsafeOpKind::*;
@@ -829,6 +836,9 @@ impl UnsafeOpKind {
                     unsafe_not_inherited_note,
                 },
             ),
+            CallDropExplicitly(_) => {
+                span_bug!(span, "`Drop::drop` or `Drop::pin_drop` should not be called explicitly")
+            }
         }
     }
 
@@ -1034,6 +1044,13 @@ 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),
+                });
+            }
         }
     }
 }