Add Drop::pin_drop for pinned drops

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 +300,50 @@ 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) => {
+            return Err(tcx
+                .dcx()
+                .span_delayed_bug(tcx.def_span(drop_impl_did), "unexpected empty impl of `Drop`"));
+        }
+        // FIXME(pin_ergonomics): reject `Drop::drop` for types that support pin-projection.
+        (Some(_span), None) => {}
+        (None, Some(span)) => {
+            if !tcx.features().pin_ergonomics() {
+                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

@@ -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 +206,7 @@
 #[lang = "drop"]
 #[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_const_unstable(feature = "const_destruct", issue = "133214")]
+#[rustc_must_implement_one_of(drop, pin_drop)]
 pub const trait Drop {
     /// Executes the destructor for this type.
     ///
@@ -237,5 +240,27 @@ 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>) {}
 }

library/core/src/ops/drop.rs

@@ -8,7 +8,7 @@ trait Get {
 }
 
 trait Other {
-    fn uhoh<U: Get>(&self, foo: U, bar: <Self as Get>::Value) where Self: Sized, Self: Get {}
+    fn uhoh<U: Get>(&self, foo: U, bar: <Self as Get>::Value) where Self: Sized, Self: Get, Self: Get {}
     //~^ ERROR the trait bound `Self: Get` is not satisfied
     //~| ERROR the trait bound `Self: Get` is not satisfied
 }

tests/ui/associated-types/associated-types-for-unimpl-trait.fixed

@@ -6,10 +6,12 @@
 #![feature(async_drop)]
 use std::future::AsyncDrop;
 struct a;
-impl Drop for a { //~ ERROR: not all trait items implemented, missing: `drop`
+impl Drop for a {
+    //~^ ERROR: not all trait items implemented, missing one of: `drop`, `pin_drop`
     fn b() {} //~ ERROR: method `b` is not a member of trait `Drop`
 }
-impl AsyncDrop for a { //~ ERROR: not all trait items implemented, missing: `drop`
+impl AsyncDrop for a {
+    //~^ ERROR: not all trait items implemented, missing: `drop`
     type c = ();
     //~^ ERROR: type `c` is not a member of trait `AsyncDrop`
 }

tests/ui/async-await/async-drop/unexpected-sort.rs

@@ -1,25 +1,23 @@
 error[E0407]: method `b` is not a member of trait `Drop`
-  --> $DIR/unexpected-sort.rs:10:5
+  --> $DIR/unexpected-sort.rs:11:5
    |
 LL |     fn b() {}
    |     ^^^^^^^^^ not a member of trait `Drop`
 
 error[E0437]: type `c` is not a member of trait `AsyncDrop`
-  --> $DIR/unexpected-sort.rs:13:5
+  --> $DIR/unexpected-sort.rs:15:5
    |
 LL |     type c = ();
    |     ^^^^^^^^^^^^ not a member of trait `AsyncDrop`
 
-error[E0046]: not all trait items implemented, missing: `drop`
+error[E0046]: not all trait items implemented, missing one of: `drop`, `pin_drop`
   --> $DIR/unexpected-sort.rs:9:1
    |
 LL | impl Drop for a {
-   | ^^^^^^^^^^^^^^^ missing `drop` in implementation
-   |
-   = help: implement the missing item: `fn drop(&mut self) { todo!() }`
+   | ^^^^^^^^^^^^^^^ missing one of `drop`, `pin_drop` in implementation
 
 error[E0046]: not all trait items implemented, missing: `drop`
-  --> $DIR/unexpected-sort.rs:12:1
+  --> $DIR/unexpected-sort.rs:13:1
    |
 LL | impl AsyncDrop for a {
    | ^^^^^^^^^^^^^^^^^^^^ missing `drop` in implementation