Rollup merge of #134797 - spastorino:ergonomic-ref-counting-1, r=nikomatsakis

Ergonomic ref counting

This is an experimental first version of ergonomic ref counting.

This first version implements most of the RFC but doesn't implement any of the optimizations. This was left for following iterations.

RFC: rust-lang/rfcs#3680
Tracking issue: #132290
Project goal: rust-lang/rust-project-goals#107

r? ```@nikomatsakis```

Author: matthiaskrgr

compiler/rustc_ast/src/ast.rs

@@ -1399,6 +1399,7 @@ impl Expr {
             // Never need parens
             ExprKind::Array(_)
             | ExprKind::Await(..)
+            | ExprKind::Use(..)
             | ExprKind::Block(..)
             | ExprKind::Call(..)
             | ExprKind::ConstBlock(_)
@@ -1588,6 +1589,8 @@ pub enum ExprKind {
     Gen(CaptureBy, P<Block>, GenBlockKind, Span),
     /// An await expression (`my_future.await`). Span is of await keyword.
     Await(P<Expr>, Span),
+    /// A use expression (`x.use`). Span is of use keyword.
+    Use(P<Expr>, Span),
 
     /// A try block (`try { ... }`).
     TryBlock(P<Block>),
@@ -1757,8 +1760,17 @@ pub enum CaptureBy {
         /// The span of the `move` keyword.
         move_kw: Span,
     },
-    /// `move` keyword was not specified.
+    /// `move` or `use` keywords were not specified.
     Ref,
+    /// `use |x| y + x`.
+    ///
+    /// Note that if you have a regular closure like `|| x.use`, this will *not* result
+    /// in a `Use` capture. Instead, the `ExprUseVisitor` will look at the type
+    /// of `x` and treat `x.use` as either a copy/clone/move as appropriate.
+    Use {
+        /// The span of the `use` keyword.
+        use_kw: Span,
+    },
 }
 
 /// Closure lifetime binder, `for<'a, 'b>` in `for<'a, 'b> |_: &'a (), _: &'b ()|`.

compiler/rustc_ast/src/mut_visit.rs

@@ -1745,6 +1745,10 @@ pub fn walk_expr<T: MutVisitor>(vis: &mut T, Expr { kind, id, span, attrs, token
             vis.visit_expr(expr);
             vis.visit_span(await_kw_span);
         }
+        ExprKind::Use(expr, use_kw_span) => {
+            vis.visit_expr(expr);
+            vis.visit_span(use_kw_span);
+        }
         ExprKind::Assign(el, er, span) => {
             vis.visit_expr(el);
             vis.visit_expr(er);
@@ -1895,6 +1899,9 @@ fn walk_capture_by<T: MutVisitor>(vis: &mut T, capture_by: &mut CaptureBy) {
         CaptureBy::Value { move_kw } => {
             vis.visit_span(move_kw);
         }
+        CaptureBy::Use { use_kw } => {
+            vis.visit_span(use_kw);
+        }
     }
 }
 

compiler/rustc_ast/src/util/classify.rs

@@ -108,6 +108,7 @@ pub fn leading_labeled_expr(mut expr: &ast::Expr) -> bool {
             Assign(e, _, _)
             | AssignOp(_, e, _)
             | Await(e, _)
+            | Use(e, _)
             | Binary(_, e, _)
             | Call(e, _)
             | Cast(e, _)
@@ -224,6 +225,7 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<TrailingBrace<'_>> {
             | Lit(_)
             | Type(_, _)
             | Await(_, _)
+            | Use(_, _)
             | Field(_, _)
             | Index(_, _, _)
             | Underscore

compiler/rustc_ast/src/visit.rs

@@ -1211,6 +1211,7 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) -> V
         }
         ExprKind::Gen(_capt, body, _kind, _decl_span) => try_visit!(visitor.visit_block(body)),
         ExprKind::Await(expr, _span) => try_visit!(visitor.visit_expr(expr)),
+        ExprKind::Use(expr, _span) => try_visit!(visitor.visit_expr(expr)),
         ExprKind::Assign(lhs, rhs, _span) => {
             try_visit!(visitor.visit_expr(lhs));
             try_visit!(visitor.visit_expr(rhs));

compiler/rustc_ast_lowering/src/expr.rs

@@ -207,6 +207,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     },
                 ),
                 ExprKind::Await(expr, await_kw_span) => self.lower_expr_await(*await_kw_span, expr),
+                ExprKind::Use(expr, use_kw_span) => self.lower_expr_use(*use_kw_span, expr),
                 ExprKind::Closure(box Closure {
                     binder,
                     capture_clause,
@@ -1067,6 +1068,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
         )
     }
 
+    fn lower_expr_use(&mut self, use_kw_span: Span, expr: &Expr) -> hir::ExprKind<'hir> {
+        hir::ExprKind::Use(self.lower_expr(expr), use_kw_span)
+    }
+
     fn lower_expr_closure(
         &mut self,
         binder: &ClosureBinder,

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -489,6 +489,7 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session, features: &Features) {
     gate_all!(dyn_star, "`dyn*` trait objects are experimental");
     gate_all!(const_closures, "const closures are experimental");
     gate_all!(builtin_syntax, "`builtin #` syntax is unstable");
+    gate_all!(ergonomic_clones, "ergonomic clones are experimental");
     gate_all!(explicit_tail_calls, "`become` expression is experimental");
     gate_all!(generic_const_items, "generic const items are experimental");
     gate_all!(guard_patterns, "guard patterns are experimental", "consider using match arm guards");

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

@@ -574,6 +574,14 @@ impl<'a> State<'a> {
                 );
                 self.word(".await");
             }
+            ast::ExprKind::Use(expr, _) => {
+                self.print_expr_cond_paren(
+                    expr,
+                    expr.precedence() < ExprPrecedence::Unambiguous,
+                    fixup,
+                );
+                self.word(".use");
+            }
             ast::ExprKind::Assign(lhs, rhs, _) => {
                 self.print_expr_cond_paren(
                     lhs,
@@ -885,6 +893,7 @@ impl<'a> State<'a> {
     fn print_capture_clause(&mut self, capture_clause: ast::CaptureBy) {
         match capture_clause {
             ast::CaptureBy::Value { .. } => self.word_space("move"),
+            ast::CaptureBy::Use { .. } => self.word_space("use"),
             ast::CaptureBy::Ref => {}
         }
     }

compiler/rustc_borrowck/src/borrowck_errors.rs

@@ -403,6 +403,7 @@ impl<'infcx, 'tcx> crate::MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                 .expect_closure();
             let span = match capture_clause {
                 rustc_hir::CaptureBy::Value { move_kw } => move_kw.shrink_to_lo(),
+                rustc_hir::CaptureBy::Use { use_kw } => use_kw.shrink_to_lo(),
                 rustc_hir::CaptureBy::Ref => fn_decl_span.shrink_to_lo(),
             };
             diag.span_suggestion_verbose(

compiler/rustc_borrowck/src/diagnostics/mutability_errors.rs

@@ -823,7 +823,7 @@ impl<'infcx, 'tcx> MirBorrowckCtxt<'_, 'infcx, 'tcx> {
                             ) => {
                                 capture_reason = format!("mutable borrow of `{upvar}`");
                             }
-                            ty::UpvarCapture::ByValue => {
+                            ty::UpvarCapture::ByValue | ty::UpvarCapture::ByUse => {
                                 capture_reason = format!("possible mutation of `{upvar}`");
                             }
                             _ => bug!("upvar `{upvar}` borrowed, but not mutably"),

compiler/rustc_borrowck/src/lib.rs

@@ -1490,14 +1490,20 @@ impl<'a, 'tcx> MirBorrowckCtxt<'a, '_, 'tcx> {
                         let stmt = &bbd.statements[loc.statement_index];
                         debug!("temporary assigned in: stmt={:?}", stmt);
 
-                        if let StatementKind::Assign(box (_, Rvalue::Ref(_, _, source))) = stmt.kind
-                        {
-                            propagate_closure_used_mut_place(self, source);
-                        } else {
-                            bug!(
-                                "closures should only capture user variables \
+                        match stmt.kind {
+                            StatementKind::Assign(box (
+                                _,
+                                Rvalue::Ref(_, _, source)
+                                | Rvalue::Use(Operand::Copy(source) | Operand::Move(source)),
+                            )) => {
+                                propagate_closure_used_mut_place(self, source);
+                            }
+                            _ => {
+                                bug!(
+                                    "closures should only capture user variables \
                                  or references to user variables"
-                            );
+                                );
+                            }
                         }
                     }
                     _ => propagate_closure_used_mut_place(self, place),