Add Drop::pin_drop for pinned drops

Fixes #122630 (accidentally).

Author: frank-king

Cargo.lock

@@ -117,6 +117,11 @@ hir_analysis_coercion_between_struct_same_note = expected coercion between the s
 
 hir_analysis_coercion_between_struct_single_note = expected a single field to be coerced, none found
 
+hir_analysis_conflict_impl_drop_and_pin_drop = conflict implementation of `Drop::drop` and `Drop::pin_drop`
+    .drop_label = `drop(&mut self)` implemented here
+    .pin_drop_label = `pin_drop(&pin mut self)` implemented here
+    .suggestion = remove this implementation
+
 hir_analysis_const_bound_for_non_const_trait = `{$modifier}` can only be applied to `const` traits
     .label = can't be applied to `{$trait_name}`
     .note = `{$trait_name}` can't be used with `{$modifier}` because it isn't `const`

compiler/rustc_hir_analysis/messages.ftl

@@ -7,11 +7,13 @@
 use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::codes::*;
 use rustc_errors::{ErrorGuaranteed, struct_span_code_err};
+use rustc_hir as hir;
 use rustc_infer::infer::{RegionResolutionError, TyCtxtInferExt};
 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 +72,11 @@ pub(crate) fn check_drop_impl(
                 drop_impl_did,
                 adt_def.did(),
                 adt_to_impl_args,
-            )
+            )?;
+
+            check_drop_xor_pin_drop(tcx, drop_impl_did)?;
+
+            Ok(())
         }
         _ => {
             span_bug!(tcx.def_span(drop_impl_did), "incoherent impl of Drop");
@@ -294,3 +300,58 @@ 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.
+fn check_drop_xor_pin_drop<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    drop_impl_did: LocalDefId,
+) -> Result<(), ErrorGuaranteed> {
+    let item_impl = tcx.hir_expect_item(drop_impl_did).expect_impl();
+    let mut drop_span = None;
+    let mut pin_drop_span = None;
+    for &impl_item_id in item_impl.items {
+        let impl_item = tcx.hir_impl_item(impl_item_id);
+        if let hir::ImplItemKind::Fn(fn_sig, _) = impl_item.kind {
+            match impl_item.ident.name {
+                sym::drop => drop_span = Some(fn_sig.span),
+                sym::pin_drop => pin_drop_span = Some(fn_sig.span),
+                _ => {}
+            }
+        }
+    }
+
+    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`"));
+            }
+        }
+        // FIXME(pin_ergonomics): reject `Drop::drop` for types that support pin-projection.
+        (Some(_span), None) => {}
+        (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",
+                ));
+            }
+            // FIXME(pin_ergonomics): reject `Drop::pin_drop` for types that don't support pin-projection.
+        }
+        (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/always_applicable.rs

@@ -26,6 +26,7 @@ use rustc_middle::ty::{
     TypeVisitable, TypeVisitableExt, 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::error_reporting::traits::on_unimplemented::OnUnimplementedDirective;
@@ -1260,6 +1261,19 @@ fn check_impl_items_against_trait<'tcx>(
                     // Unmarked default bodies are considered stable (at least for now).
                     EvalResult::Allow | EvalResult::Unmarked => {}
                 }
+
+                // FIXME(pin_ergonomics): hack for `Drop::drop` (see also the FIXME comments on top of
+                // the `Drop` trait)
+                // - When `pin_ergonomics` is not enabled, we manually check if `Drop::drop` is missing.
+                // - When `pin_ergonomics` is enabled, the presentness and exclusiveness of
+                //   `Drop::drop` and `Drop::pin_drop` will be handled in
+                //   `super::always_applicable::check_drop_xor_pin_drop`.
+                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));
+                }
             }
 
             if let Some(required_items) = &must_implement_one_of {

compiler/rustc_hir_analysis/src/check/check.rs

@@ -92,7 +92,7 @@ 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/check/mod.rs

@@ -1707,3 +1707,14 @@ pub(crate) struct AsyncDropWithoutSyncDrop {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_conflict_impl_drop_and_pin_drop)]
+pub(crate) struct ConflictImplDropAndPinDrop {
+    #[primary_span]
+    pub span: Span,
+    #[label(hir_analysis_drop_label)]
+    pub drop_span: Span,
+    #[label(hir_analysis_pin_drop_label)]
+    pub pin_drop_span: Span,
+}

compiler/rustc_hir_analysis/src/errors.rs

@@ -37,9 +37,13 @@ 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`
+        && let Some(parent) = tcx.opt_parent(body_id)
+        && !tcx.is_lang_item(parent, 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_hir_typeck/src/callee.rs

@@ -1650,6 +1650,7 @@ symbols! {
         pic,
         pie,
         pin,
+        pin_drop,
         pin_ergonomics,
         pin_macro,
         platform_intrinsics,

compiler/rustc_span/src/symbol.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.
@@ -204,6 +206,13 @@
 #[lang = "drop"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_destruct", issue = "133214")]
+// FIXME(pin_ergonomics): we don't use
+// `#[rustc_must_implement_one_of(drop, pin_drop)]` for better diagnostics, as it emits
+// `missing one of: drop, pin_drop` even without the feature `pin_ergonomics` enabled,
+// which may surprise users that don't use `pin_ergonomics`.
+//
+// Alternatively, we manually check if `Drop::drop` is missing when `pin_ergonomics`
+// is not enabled. See also `rustc_hir_analysis::check::check::check_impl_items_against_trait`.
 pub const trait Drop {
     /// Executes the destructor for this type.
     ///
@@ -237,5 +246,27 @@ 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);
+    fn drop(&mut self) {
+        // SAFETY: `self` is pinned till after dropped.
+        Drop::pin_drop(unsafe { 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.
+    ///
+    /// At least and at most one of [`Drop::drop`] and [`Drop::pin_drop`] should be implemented.
+    ///
+    // FIXME(pin_ergonomics): add requirements: `pin_drop` must be and must only be used
+    // for types that support pin-projection.
+    /// [`Drop::drop`]: crate::ops::Drop::drop
+    /// [`Unpin`]: crate::marker::Unpin
+    /// [`!Unpin`]: crate::marker::Unpin
+    /// [`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>) {}
 }