Move the place in &pin mut $place when !Unpin to ensure soundness

Author: frank-king

compiler/rustc_mir_build/src/thir/cx/expr.rs

@@ -477,15 +477,39 @@ impl<'tcx> ThirBuildCx<'tcx> {
             }
 
             // Make `&pin mut $expr` and `&pin const $expr` into
-            // `Pin { __pointer: &mut $expr }` and `Pin { __pointer: &$expr }`.
-            hir::ExprKind::AddrOf(hir::BorrowKind::Pin, mutbl, arg) => match expr_ty.kind() {
-                &ty::Adt(adt_def, args)
-                    if tcx.is_lang_item(adt_def.did(), rustc_hir::LangItem::Pin) =>
-                {
-                    let arg = self.mirror_expr(arg);
+            // `Pin { __pointer: &mut { $expr } }` and `Pin { __pointer: &$expr }`.
+            hir::ExprKind::AddrOf(hir::BorrowKind::Pin, mutbl, arg_expr) => match expr_ty.kind() {
+                &ty::Adt(adt_def, args) if tcx.is_lang_item(adt_def.did(), hir::LangItem::Pin) => {
+                    let ty = args.type_at(0);
+                    let arg_ty = self.typeck_results.expr_ty(arg_expr);
+                    let mut arg = self.mirror_expr(arg_expr);
+                    // For `&pin mut $place` where `$place` is not `Unpin`, move the place
+                    // `$place` to ensure it will not be used afterwards.
+                    if mutbl.is_mut() && !arg_ty.is_unpin(self.tcx, self.typing_env) {
+                        let block = self.thir.blocks.push(Block {
+                            targeted_by_break: false,
+                            region_scope: region::Scope {
+                                local_id: arg_expr.hir_id.local_id,
+                                data: region::ScopeData::Node,
+                            },
+                            span: arg_expr.span,
+                            stmts: Box::new([]),
+                            expr: Some(arg),
+                            safety_mode: BlockSafety::Safe,
+                        });
+                        let (temp_lifetime, backwards_incompatible) = self
+                            .rvalue_scopes
+                            .temporary_scope(self.region_scope_tree, arg_expr.hir_id.local_id);
+                        arg = self.thir.exprs.push(Expr {
+                            temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
+                            ty: arg_ty,
+                            span: arg_expr.span,
+                            kind: ExprKind::Block { block },
+                        });
+                    }
                     let expr = self.thir.exprs.push(Expr {
                         temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
-                        ty: args.type_at(0),
+                        ty,
                         span: expr.span,
                         kind: ExprKind::Borrow { borrow_kind: mutbl.to_borrow_kind(), arg },
                     });

tests/pretty/pin-ergonomics.rs

@@ -30,7 +30,6 @@ fn bar() {
     foo_const(x);
 
     let x: Pin<&_> = &pin const Foo;
-
     foo_const(x);
     foo_const(x);
 }