Forbid manual Unpin impls for structurally pinned types

compiler/rustc_hir_analysis/messages.ftl

@@ -222,6 +222,10 @@ hir_analysis_impl_not_marked_default = `{$ident}` specializes an item from a par
 hir_analysis_impl_not_marked_default_err = `{$ident}` specializes an item from a parent `impl`, but that item is not marked `default`
     .note = parent implementation is in crate `{$cname}`
 
+hir_analysis_impl_unpin_for_pin_projected_type = explicit impls for the `Unpin` trait are not permitted for structurally pinned types
+    .label = impl of `Unpin` not allowed
+    .help = `{$adt_name}` is structurally pinned because it is marked as `#[pin_v2]`
+
 hir_analysis_inherent_dyn = cannot define inherent `impl` for a dyn auto trait
     .label = impl requires at least one non-auto trait
     .note = define and implement a new trait or type instead

compiler/rustc_hir_analysis/src/coherence/mod.rs

@@ -90,6 +90,20 @@ fn enforce_trait_manually_implementable(
         return Err(err.emit());
     }
 
+    // Disallow explicit impls of the `Unpin` trait for structurally pinned types
+    if tcx.features().pin_ergonomics()
+        && tcx.is_lang_item(trait_def_id, LangItem::Unpin)
+        && let Some(adt) =
+            tcx.impl_trait_ref(impl_def_id).instantiate_identity().self_ty().ty_adt_def()
+        && adt.is_pin_project()
+    {
+        return Err(tcx.dcx().emit_err(crate::errors::ImplUnpinForPinProjectedType {
+            span: impl_header_span,
+            adt_span: tcx.def_span(adt.did()),
+            adt_name: tcx.item_name(adt.did()),
+        }));
+    }
+
     if let ty::trait_def::TraitSpecializationKind::AlwaysApplicable = trait_def.specialization_kind
     {
         if !tcx.features().specialization()

compiler/rustc_hir_analysis/src/errors.rs

@@ -1708,3 +1708,14 @@ pub(crate) struct AsyncDropWithoutSyncDrop {
     #[primary_span]
     pub span: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(hir_analysis_impl_unpin_for_pin_projected_type)]
+pub(crate) struct ImplUnpinForPinProjectedType {
+    #[primary_span]
+    #[label]
+    pub span: Span,
+    #[help]
+    pub adt_span: Span,
+    pub adt_name: Symbol,
+}

tests/ui/pin-ergonomics/impl-unpin.rs

@@ -0,0 +1,11 @@
+#![feature(pin_ergonomics)]
+#![allow(incomplete_features)]
+
+#[pin_v2]
+struct Foo;
+struct Bar;
+
+impl Unpin for Foo {} //~ ERROR explicit impls for the `Unpin` trait are not permitted for structurally pinned types
+impl Unpin for Bar {} // ok
+
+fn main() {}

tests/ui/pin-ergonomics/impl-unpin.stderr

@@ -0,0 +1,14 @@
+error: explicit impls for the `Unpin` trait are not permitted for structurally pinned types
+  --> $DIR/impl-unpin.rs:8:1
+   |
+LL | impl Unpin for Foo {}
+   | ^^^^^^^^^^^^^^^^^^ impl of `Unpin` not allowed
+   |
+help: `Foo` is structurally pinned because it is marked as `#[pin_v2]`
+  --> $DIR/impl-unpin.rs:5:1
+   |
+LL | struct Foo;
+   | ^^^^^^^^^^
+
+error: aborting due to 1 previous error
+