Initial implementation of unsafe binder types

compiler/rustc_ast/src/ast.rs

@@ -1329,6 +1329,7 @@ impl Expr {
             | ExprKind::Type(..)
             | ExprKind::OffsetOf(..)
             | ExprKind::FormatArgs(..)
+            | ExprKind::UnsafeBinderCast(..)
             | ExprKind::MacCall(..) => ExprPrecedence::Mac,
             ExprKind::Err(_) | ExprKind::Dummy => ExprPrecedence::Err,
         }
@@ -1455,7 +1456,13 @@ pub enum ExprKind {
     /// `'label: for await? pat in iter { block }`
     ///
     /// This is desugared to a combination of `loop` and `match` expressions.
-    ForLoop { pat: P<Pat>, iter: P<Expr>, body: P<Block>, label: Option<Label>, kind: ForLoopKind },
+    ForLoop {
+        pat: P<Pat>,
+        iter: P<Expr>,
+        body: P<Block>,
+        label: Option<Label>,
+        kind: ForLoopKind,
+    },
     /// Conditionless loop (can be exited with `break`, `continue`, or `return`).
     ///
     /// `'label: loop { block }`
@@ -1560,6 +1567,8 @@ pub enum ExprKind {
     /// A `format_args!()` expression.
     FormatArgs(P<FormatArgs>),
 
+    UnsafeBinderCast(UnsafeBinderCastKind, P<Expr>, Option<P<Ty>>),
+
     /// Placeholder for an expression that wasn't syntactically well formed in some way.
     Err(ErrorGuaranteed),
 
@@ -1598,6 +1607,16 @@ impl GenBlockKind {
     }
 }
 
+/// Whether we're unwrapping or wrapping an unsafe binder
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Hash)]
+#[derive(Encodable, Decodable, HashStable_Generic)]
+pub enum UnsafeBinderCastKind {
+    // e.g. `&i32` -> `unsafe<'a> &'a i32`
+    Wrap,
+    // e.g. `unsafe<'a> &'a i32` -> `&i32`
+    Unwrap,
+}
+
 /// The explicit `Self` type in a "qualified path". The actual
 /// path, including the trait and the associated item, is stored
 /// separately. `position` represents the index of the associated
@@ -2148,6 +2167,12 @@ pub struct BareFnTy {
     pub decl_span: Span,
 }
 
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct UnsafeBinderTy {
+    pub generic_params: ThinVec<GenericParam>,
+    pub inner_ty: P<Ty>,
+}
+
 /// The various kinds of type recognized by the compiler.
 //
 // Adding a new variant? Please update `test_ty` in `tests/ui/macros/stringify.rs`.
@@ -2163,6 +2188,8 @@ pub enum TyKind {
     Ref(Option<Lifetime>, MutTy),
     /// A bare function (e.g., `fn(usize) -> bool`).
     BareFn(P<BareFnTy>),
+    /// An unsafe existential lifetime binder (e.g., `unsafe<'a> &'a ()`).
+    UnsafeBinder(P<UnsafeBinderTy>),
     /// The never type (`!`).
     Never,
     /// A tuple (`(A, B, C, D,...)`).

compiler/rustc_ast/src/mut_visit.rs

@@ -496,6 +496,11 @@ pub fn walk_ty<T: MutVisitor>(vis: &mut T, ty: &mut P<Ty>) {
             vis.visit_fn_decl(decl);
             vis.visit_span(decl_span);
         }
+        TyKind::UnsafeBinder(binder) => {
+            let UnsafeBinderTy { generic_params, inner_ty } = binder.deref_mut();
+            generic_params.flat_map_in_place(|param| vis.flat_map_generic_param(param));
+            vis.visit_ty(inner_ty);
+        }
         TyKind::Tup(tys) => visit_thin_vec(tys, |ty| vis.visit_ty(ty)),
         TyKind::Paren(ty) => vis.visit_ty(ty),
         TyKind::Pat(ty, pat) => {
@@ -1615,6 +1620,12 @@ pub fn walk_expr<T: MutVisitor>(vis: &mut T, Expr { kind, id, span, attrs, token
         ExprKind::TryBlock(body) => vis.visit_block(body),
         ExprKind::Lit(_token) => {}
         ExprKind::IncludedBytes(_bytes) => {}
+        ExprKind::UnsafeBinderCast(_kind, expr, ty) => {
+            vis.visit_expr(expr);
+            if let Some(ty) = ty {
+                vis.visit_ty(ty);
+            }
+        }
         ExprKind::Err(_guar) => {}
         ExprKind::Dummy => {}
     }

compiler/rustc_ast/src/util/classify.rs

@@ -152,6 +152,7 @@ pub fn leading_labeled_expr(mut expr: &ast::Expr) -> bool {
             | Underscore
             | Yeet(..)
             | Yield(..)
+            | UnsafeBinderCast(..)
             | Err(..)
             | Dummy => return false,
         }
@@ -232,6 +233,7 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<TrailingBrace<'_>> {
             | Paren(_)
             | Try(_)
             | Yeet(None)
+            | UnsafeBinderCast(..)
             | Err(_)
             | Dummy => break None,
         }
@@ -251,6 +253,10 @@ fn type_trailing_braced_mac_call(mut ty: &ast::Ty) -> Option<&ast::MacCall> {
                 ty = &mut_ty.ty;
             }
 
+            ast::TyKind::UnsafeBinder(binder) => {
+                ty = &binder.inner_ty;
+            }
+
             ast::TyKind::BareFn(fn_ty) => match &fn_ty.decl.output {
                 ast::FnRetTy::Default(_) => break None,
                 ast::FnRetTy::Ty(ret) => ty = ret,

compiler/rustc_ast/src/visit.rs

@@ -512,6 +512,10 @@ pub fn walk_ty<'a, V: Visitor<'a>>(visitor: &mut V, typ: &'a Ty) -> V::Result {
             walk_list!(visitor, visit_generic_param, generic_params);
             try_visit!(walk_fn_decl(visitor, decl));
         }
+        TyKind::UnsafeBinder(binder) => {
+            walk_list!(visitor, visit_generic_param, &binder.generic_params);
+            try_visit!(visitor.visit_ty(&binder.inner_ty));
+        }
         TyKind::Path(maybe_qself, path) => {
             try_visit!(walk_qself(visitor, maybe_qself));
             try_visit!(visitor.visit_path(path, *id));
@@ -1183,6 +1187,10 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) -> V
         ExprKind::TryBlock(body) => try_visit!(visitor.visit_block(body)),
         ExprKind::Lit(_token) => {}
         ExprKind::IncludedBytes(_bytes) => {}
+        ExprKind::UnsafeBinderCast(_kind, expr, ty) => {
+            try_visit!(visitor.visit_expr(expr));
+            visit_opt!(visitor, visit_ty, ty);
+        }
         ExprKind::Err(_guar) => {}
         ExprKind::Dummy => {}
     }

compiler/rustc_ast_lowering/src/expr.rs

@@ -341,6 +341,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 ExprKind::Yield(opt_expr) => self.lower_expr_yield(e.span, opt_expr.as_deref()),
                 ExprKind::Err(guar) => hir::ExprKind::Err(*guar),
 
+                ExprKind::UnsafeBinderCast(kind, expr, ty) => hir::ExprKind::UnsafeBinderCast(
+                    *kind,
+                    self.lower_expr(expr),
+                    ty.as_ref().map(|ty| {
+                        self.lower_ty(ty, ImplTraitContext::Disallowed(ImplTraitPosition::Cast))
+                    }),
+                ),
+
                 ExprKind::Dummy => {
                     span_bug!(e.span, "lowered ExprKind::Dummy")
                 }

compiler/rustc_ast_lowering/src/index.rs

@@ -285,6 +285,12 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         self.insert(ty.span, ty.hir_id, Node::Ty(ty));
 
         self.with_parent(ty.hir_id, |this| {
+            // FIXME(unsafe_binders): Should we split this out into a separate
+            // visit function?
+            if let TyKind::UnsafeBinder(binder) = ty.kind {
+                this.insert(ty.span, binder.hir_id, Node::UnsafeBinder(binder));
+            };
+
             intravisit::walk_ty(this, ty);
         });
     }

compiler/rustc_ast_lowering/src/lib.rs

@@ -1332,6 +1332,15 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
                     param_names: self.lower_fn_params_to_names(&f.decl),
                 }))
             }
+            TyKind::UnsafeBinder(f) => {
+                let generic_params = self.lower_lifetime_binder(t.id, &f.generic_params);
+                hir::TyKind::UnsafeBinder(self.arena.alloc(hir::UnsafeBinderTy {
+                    span: self.lower_span(t.span),
+                    hir_id: self.next_id(),
+                    generic_params,
+                    inner_ty: self.lower_ty(&f.inner_ty, itctx),
+                }))
+            }
             TyKind::Never => hir::TyKind::Never,
             TyKind::Tup(tys) => hir::TyKind::Tup(
                 self.arena.alloc_from_iter(tys.iter().map(|ty| self.lower_ty_direct(ty, itctx))),

compiler/rustc_ast_lowering/src/lifetime_collector.rs

@@ -95,6 +95,11 @@ impl<'ast> Visitor<'ast> for LifetimeCollectVisitor<'ast> {
                 visit::walk_ty(self, t);
                 self.current_binders.pop();
             }
+            TyKind::UnsafeBinder(_) => {
+                self.current_binders.push(t.id);
+                visit::walk_ty(self, t);
+                self.current_binders.pop();
+            }
             TyKind::Ref(None, _) => {
                 self.record_elided_anchor(t.id, t.span);
                 visit::walk_ty(self, t);

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -1190,6 +1190,14 @@ impl<'a> State<'a> {
             ast::TyKind::BareFn(f) => {
                 self.print_ty_fn(f.ext, f.safety, &f.decl, None, &f.generic_params);
             }
+            ast::TyKind::UnsafeBinder(f) => {
+                self.ibox(INDENT_UNIT);
+                self.word("unsafe");
+                self.print_generic_params(&f.generic_params);
+                self.nbsp();
+                self.print_type(&f.inner_ty);
+                self.end();
+            }
             ast::TyKind::Path(None, path) => {
                 self.print_path(path, false, 0);
             }

compiler/rustc_ast_pretty/src/pprust/state/expr.rs

@@ -726,6 +726,25 @@ impl<'a> State<'a> {
                 self.word_nbsp("try");
                 self.print_block_with_attrs(blk, attrs)
             }
+            ast::ExprKind::UnsafeBinderCast(kind, expr, ty) => {
+                self.word("builtin # ");
+                match kind {
+                    ast::UnsafeBinderCastKind::Wrap => self.word("wrap_unsafe_binder"),
+                    ast::UnsafeBinderCastKind::Unwrap => self.word("unwrap_unsafe_binder"),
+                }
+                self.popen();
+                self.ibox(0);
+                self.print_expr(expr, FixupContext::default());
+
+                if let Some(ty) = ty {
+                    self.word(",");
+                    self.space();
+                    self.print_type(ty);
+                }
+
+                self.end();
+                self.pclose();
+            }
             ast::ExprKind::Err(_) => {
                 self.popen();
                 self.word("/*ERROR*/");

compiler/rustc_borrowck/src/diagnostics/conflict_errors.rs

@@ -3950,7 +3950,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                         ProjectionElem::ConstantIndex { .. }
                         | ProjectionElem::Subslice { .. }
                         | ProjectionElem::Subtype(_)
-                        | ProjectionElem::Index(_) => kind,
+                        | ProjectionElem::Index(_)
+                        | ProjectionElem::UnsafeBinderCast(..) => kind,
                     },
                     place_ty.projection_ty(tcx, elem),
                 )

compiler/rustc_borrowck/src/diagnostics/mod.rs

@@ -243,6 +243,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 ProjectionElem::Downcast(..) => (),
                 ProjectionElem::OpaqueCast(..) => (),
                 ProjectionElem::Subtype(..) => (),
+                ProjectionElem::UnsafeBinderCast(..) => (),
                 ProjectionElem::Field(field, _ty) => {
                     // FIXME(project-rfc_2229#36): print capture precisely here.
                     if let Some(field) = self.is_upvar_field_projection(PlaceRef {
@@ -323,9 +324,9 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                     PlaceRef { local, projection: proj_base }.ty(self.body, self.infcx.tcx)
                 }
                 ProjectionElem::Downcast(..) => place.ty(self.body, self.infcx.tcx),
-                ProjectionElem::Subtype(ty) | ProjectionElem::OpaqueCast(ty) => {
-                    PlaceTy::from_ty(*ty)
-                }
+                ProjectionElem::Subtype(ty)
+                | ProjectionElem::OpaqueCast(ty)
+                | ProjectionElem::UnsafeBinderCast(_, ty) => PlaceTy::from_ty(*ty),
                 ProjectionElem::Field(_, field_type) => PlaceTy::from_ty(*field_type),
             },
         };

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -168,7 +168,8 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                         | ProjectionElem::ConstantIndex { .. }
                         | ProjectionElem::OpaqueCast { .. }
                         | ProjectionElem::Subslice { .. }
-                        | ProjectionElem::Downcast(..),
+                        | ProjectionElem::Downcast(..)
+                        | ProjectionElem::UnsafeBinderCast(..),
                     ],
             } => bug!("Unexpected immutable place."),
         }

compiler/rustc_borrowck/src/lib.rs

@@ -1634,6 +1634,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                     | ty::Slice(_)
                     | ty::FnDef(_, _)
                     | ty::FnPtr(..)
+                    | ty::UnsafeBinder(_)
                     | ty::Dynamic(_, _, _)
                     | ty::Closure(_, _)
                     | ty::CoroutineClosure(_, _)
@@ -1679,6 +1680,7 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                     | ty::Ref(_, _, _)
                     | ty::FnDef(_, _)
                     | ty::FnPtr(..)
+                    | ty::UnsafeBinder(_)
                     | ty::Dynamic(_, _, _)
                     | ty::CoroutineWitness(..)
                     | ty::Never
@@ -1722,7 +1724,8 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                 // So it's safe to skip these.
                 ProjectionElem::OpaqueCast(_)
                 | ProjectionElem::Subtype(_)
-                | ProjectionElem::Downcast(_, _) => (),
+                | ProjectionElem::Downcast(_, _)
+                | ProjectionElem::UnsafeBinderCast(_, _) => (),
             }
 
             place_ty = place_ty.projection_ty(tcx, elem);
@@ -1956,6 +1959,10 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                 // FIXME: is this true even if P is an adt with a dtor?
                 { }
 
+                ProjectionElem::UnsafeBinderCast(..) => {
+                    check_parent_of_field(self, location, place_base, span, state);
+                }
+
                 // assigning to (*P) requires P to be initialized
                 ProjectionElem::Deref => {
                     self.check_if_full_path_is_moved(
@@ -2339,7 +2346,8 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                     | ProjectionElem::Subslice { .. }
                     | ProjectionElem::Subtype(..)
                     | ProjectionElem::OpaqueCast { .. }
-                    | ProjectionElem::Downcast(..) => {
+                    | ProjectionElem::Downcast(..)
+                    | ProjectionElem::UnsafeBinderCast(..) => {
                         let upvar_field_projection = self.is_upvar_field_projection(place);
                         if let Some(field) = upvar_field_projection {
                             let upvar = &self.upvars[field.index()];

compiler/rustc_borrowck/src/places_conflict.rs

@@ -250,7 +250,8 @@ fn place_components_conflict<'tcx>(
                 | (ProjectionElem::Subslice { .. }, _, _)
                 | (ProjectionElem::OpaqueCast { .. }, _, _)
                 | (ProjectionElem::Subtype(_), _, _)
-                | (ProjectionElem::Downcast { .. }, _, _) => {
+                | (ProjectionElem::Downcast { .. }, _, _)
+                | (ProjectionElem::UnsafeBinderCast(..), _, _) => {
                     // Recursive case. This can still be disjoint on a
                     // further iteration if this a shallow access and
                     // there's a deref later on, e.g., a borrow
@@ -519,5 +520,9 @@ fn place_projection_conflict<'tcx>(
             pi1_elem,
             pi2_elem
         ),
+
+        (ProjectionElem::UnsafeBinderCast(..), _) => {
+            todo!()
+        }
     }
 }

compiler/rustc_borrowck/src/prefixes.rs

@@ -66,6 +66,10 @@ impl<'tcx> Iterator for Prefixes<'tcx> {
                             self.next = Some(cursor_base);
                             return Some(cursor);
                         }
+                        ProjectionElem::UnsafeBinderCast(..) => {
+                            self.next = Some(cursor_base);
+                            return Some(cursor);
+                        }
                         ProjectionElem::Downcast(..)
                         | ProjectionElem::Subslice { .. }
                         | ProjectionElem::OpaqueCast { .. }