Auto merge of #108471 - clubby789:unbox-the-syntax, r=Nilstrieb,est31

Remove `box_syntax`

r? `@Nilstrieb`

This removes the feature `box_syntax`, which allows the use of `box <expr>` to create a Box, and finalises removing use of the feature from the compiler. `box_patterns` (allowing the use of `box <pat>` in a pattern) is unaffected.
It also removes `ast::ExprKind::Box` - the only way to create a 'box' expression now is with the rustc-internal `#[rustc_box]` attribute.
As a temporary measure to help users move away, `box <expr>` now parses the inner expression, and emits a `MachineApplicable` lint to replace it with `Box::new`

Closes #49733

compiler/rustc_ast/src/ast.rs

@@ -1230,7 +1230,6 @@ impl Expr {
 
     pub fn precedence(&self) -> ExprPrecedence {
         match self.kind {
-            ExprKind::Box(_) => ExprPrecedence::Box,
             ExprKind::Array(_) => ExprPrecedence::Array,
             ExprKind::ConstBlock(_) => ExprPrecedence::ConstBlock,
             ExprKind::Call(..) => ExprPrecedence::Call,
@@ -1291,8 +1290,7 @@ impl Expr {
     /// To a first-order approximation, is this a pattern?
     pub fn is_approximately_pattern(&self) -> bool {
         match &self.peel_parens().kind {
-            ExprKind::Box(_)
-            | ExprKind::Array(_)
+            ExprKind::Array(_)
             | ExprKind::Call(_, _)
             | ExprKind::Tup(_)
             | ExprKind::Lit(_)
@@ -1363,8 +1361,6 @@ pub struct StructExpr {
 
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub enum ExprKind {
-    /// A `box x` expression.
-    Box(P<Expr>),
     /// An array (`[a, b, c, d]`)
     Array(ThinVec<P<Expr>>),
     /// Allow anonymous constants from an inline `const` block

compiler/rustc_ast/src/mut_visit.rs

@@ -1316,7 +1316,6 @@ pub fn noop_visit_expr<T: MutVisitor>(
     vis: &mut T,
 ) {
     match kind {
-        ExprKind::Box(expr) => vis.visit_expr(expr),
         ExprKind::Array(exprs) => visit_thin_exprs(exprs, vis),
         ExprKind::ConstBlock(anon_const) => {
             vis.visit_anon_const(anon_const);

compiler/rustc_ast/src/util/classify.rs

@@ -35,7 +35,6 @@ pub fn expr_trailing_brace(mut expr: &ast::Expr) -> Option<&ast::Expr> {
             | Assign(_, e, _)
             | AssignOp(_, _, e)
             | Binary(_, _, e)
-            | Box(e)
             | Break(_, Some(e))
             | Let(_, e, _)
             | Range(_, Some(e), _)

compiler/rustc_ast/src/visit.rs

@@ -772,7 +772,6 @@ pub fn walk_expr<'a, V: Visitor<'a>>(visitor: &mut V, expression: &'a Expr) {
     walk_list!(visitor, visit_attribute, expression.attrs.iter());
 
     match &expression.kind {
-        ExprKind::Box(subexpression) => visitor.visit_expr(subexpression),
         ExprKind::Array(subexpressions) => {
             walk_list!(visitor, visit_expr, subexpressions);
         }

compiler/rustc_ast_lowering/src/expr.rs

@@ -70,7 +70,6 @@ impl<'hir> LoweringContext<'_, 'hir> {
             self.lower_attrs(hir_id, &e.attrs);
 
             let kind = match &e.kind {
-                ExprKind::Box(inner) => hir::ExprKind::Box(self.lower_expr(inner)),
                 ExprKind::Array(exprs) => hir::ExprKind::Array(self.lower_exprs(exprs)),
                 ExprKind::ConstBlock(anon_const) => {
                     let anon_const = self.lower_anon_const(anon_const);

compiler/rustc_ast_passes/src/feature_gate.rs

@@ -392,14 +392,6 @@ impl<'a> Visitor<'a> for PostExpansionVisitor<'a> {
 
     fn visit_expr(&mut self, e: &'a ast::Expr) {
         match e.kind {
-            ast::ExprKind::Box(_) => {
-                gate_feature_post!(
-                    &self,
-                    box_syntax,
-                    e.span,
-                    "box expression syntax is experimental; you can call `Box::new` instead"
-                );
-            }
             ast::ExprKind::Type(..) => {
                 if self.sess.parse_sess.span_diagnostic.err_count() == 0 {
                     // To avoid noise about type ascription in common syntax errors,
@@ -604,7 +596,6 @@ pub fn check_crate(krate: &ast::Crate, sess: &Session) {
     gate_all!(box_patterns, "box pattern syntax is experimental");
     gate_all!(exclusive_range_pattern, "exclusive range pattern syntax is experimental");
     gate_all!(try_blocks, "`try` blocks are unstable");
-    gate_all!(box_syntax, "box expression syntax is experimental; you can call `Box::new` instead");
     gate_all!(type_ascription, "type ascription is experimental");
 
     visit::walk_crate(&mut visitor, krate);

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

@@ -296,10 +296,6 @@ impl<'a> State<'a> {
         self.ibox(INDENT_UNIT);
         self.ann.pre(self, AnnNode::Expr(expr));
         match &expr.kind {
-            ast::ExprKind::Box(expr) => {
-                self.word_space("box");
-                self.print_expr_maybe_paren(expr, parser::PREC_PREFIX);
-            }
             ast::ExprKind::Array(exprs) => {
                 self.print_expr_vec(exprs);
             }

compiler/rustc_builtin_macros/src/assert/context.rs

@@ -290,7 +290,6 @@ impl<'cx, 'a> Context<'cx, 'a> {
             | ExprKind::Async(_, _, _)
             | ExprKind::Await(_)
             | ExprKind::Block(_, _)
-            | ExprKind::Box(_)
             | ExprKind::Break(_, _)
             | ExprKind::Closure(_)
             | ExprKind::ConstBlock(_)

compiler/rustc_codegen_cranelift/example/alloc_example.rs

@@ -1,4 +1,4 @@
-#![feature(start, core_intrinsics, alloc_error_handler, box_syntax)]
+#![feature(start, core_intrinsics, alloc_error_handler)]
 #![no_std]
 
 extern crate alloc;
@@ -29,7 +29,7 @@ fn alloc_error_handler(_: alloc::alloc::Layout) -> ! {
 
 #[start]
 fn main(_argc: isize, _argv: *const *const u8) -> isize {
-    let world: Box<&str> = box "Hello World!\0";
+    let world: Box<&str> = Box::new("Hello World!\0");
     unsafe {
         puts(*world as *const str as *const u8);
     }

compiler/rustc_codegen_cranelift/example/mini_core_hello_world.rs

@@ -1,4 +1,4 @@
-#![feature(no_core, lang_items, never_type, linkage, extern_types, thread_local, box_syntax)]
+#![feature(no_core, lang_items, never_type, linkage, extern_types, thread_local)]
 #![no_core]
 #![allow(dead_code, non_camel_case_types)]
 
@@ -178,7 +178,7 @@ fn main() {
         let ptr: *const i8 = hello as *const [u8] as *const i8;
         puts(ptr);
 
-        let world: Box<&str> = box "World!\0";
+        let world: Box<&str> = Box::new("World!\0");
         puts(*world as *const str as *const i8);
         world as Box<dyn SomeTrait>;
 
@@ -238,10 +238,10 @@ fn main() {
         }
     }
 
-    let _ = box NoisyDrop {
+    let _ = Box::new(NoisyDrop {
         text: "Boxed outer got dropped!\0",
         inner: NoisyDropInner,
-    } as Box<dyn SomeTrait>;
+    }) as Box<dyn SomeTrait>;
 
     const FUNC_REF: Option<fn()> = Some(main);
     match FUNC_REF {

compiler/rustc_codegen_gcc/example/alloc_example.rs

@@ -1,4 +1,4 @@
-#![feature(start, box_syntax, core_intrinsics, alloc_error_handler, lang_items)]
+#![feature(start, core_intrinsics, alloc_error_handler, lang_items)]
 #![no_std]
 
 extern crate alloc;
@@ -38,7 +38,7 @@ unsafe extern "C" fn _Unwind_Resume() {
 
 #[start]
 fn main(_argc: isize, _argv: *const *const u8) -> isize {
-    let world: Box<&str> = box "Hello World!\0";
+    let world: Box<&str> = Box::new("Hello World!\0");
     unsafe {
         puts(*world as *const str as *const u8);
     }

compiler/rustc_codegen_gcc/example/mini_core_hello_world.rs

@@ -1,7 +1,7 @@
 // Adapted from https://github.com/sunfishcode/mir2cranelift/blob/master/rust-examples/nocore-hello-world.rs
 
 #![feature(
-    no_core, unboxed_closures, start, lang_items, box_syntax, never_type, linkage,
+    no_core, unboxed_closures, start, lang_items, never_type, linkage,
     extern_types, thread_local
 )]
 #![no_core]
@@ -163,7 +163,7 @@ fn main() {
         let ptr: *const u8 = hello as *const [u8] as *const u8;
         puts(ptr);
 
-        let world: Box<&str> = box "World!\0";
+        let world: Box<&str> = Box::new("World!\0");
         puts(*world as *const str as *const u8);
         world as Box<dyn SomeTrait>;
 
@@ -223,10 +223,10 @@ fn main() {
         }
     }
 
-    let _ = box NoisyDrop {
+    let _ = Box::new(NoisyDrop {
         text: "Boxed outer got dropped!\0",
         inner: NoisyDropInner,
-    } as Box<dyn SomeTrait>;
+    }) as Box<dyn SomeTrait>;
 
     const FUNC_REF: Option<fn()> = Some(main);
     #[allow(unreachable_code)]

compiler/rustc_codegen_gcc/example/mod_bench.rs

@@ -1,4 +1,4 @@
-#![feature(start, box_syntax, core_intrinsics, lang_items)]
+#![feature(start, core_intrinsics, lang_items)]
 #![no_std]
 
 #[link(name = "c")]

compiler/rustc_error_codes/src/error_codes/E0010.md

@@ -5,7 +5,5 @@ the heap at runtime, and therefore cannot be done at compile time.
 Erroneous code example:
 
 ```compile_fail,E0010
-#![feature(box_syntax)]
-
-const CON : Box<i32> = box 0;
+const CON : Vec<i32> = vec![1, 2, 3];
 ```

compiler/rustc_feature/src/active.rs

@@ -200,8 +200,6 @@ declare_features! (
     (active, auto_traits, "1.50.0", Some(13231), None),
     /// Allows using `box` in patterns (RFC 469).
     (active, box_patterns, "1.0.0", Some(29641), None),
-    /// Allows using the `box $expr` syntax.
-    (active, box_syntax, "1.0.0", Some(49733), None),
     /// Allows `#[doc(notable_trait)]`.
     /// Renamed from `doc_spotlight`.
     (active, doc_notable_trait, "1.52.0", Some(45040), None),

compiler/rustc_feature/src/removed.rs

@@ -52,6 +52,8 @@ declare_features! (
     (removed, allow_fail, "1.19.0", Some(46488), None, Some("removed due to no clear use cases")),
     (removed, await_macro, "1.38.0", Some(50547), None,
      Some("subsumed by `.await` syntax")),
+    /// Allows using the `box $expr` syntax.
+    (removed, box_syntax, "CURRENT_RUSTC_VERSION", Some(49733), None, Some("replaced with `#[rustc_box]`")),
     /// Allows capturing disjoint fields in a closure/generator (RFC 2229).
     (removed, capture_disjoint_fields, "1.49.0", Some(53488), None, Some("stabilized in Rust 2021")),
     /// Allows comparing raw pointers during const eval.

compiler/rustc_hir_pretty/src/lib.rs

@@ -1367,8 +1367,8 @@ impl<'a> State<'a> {
         self.ann.pre(self, AnnNode::Expr(expr));
         match expr.kind {
             hir::ExprKind::Box(expr) => {
-                self.word_space("box");
-                self.print_expr_maybe_paren(expr, parser::PREC_PREFIX);
+                self.word_space("Box::new");
+                self.print_call_post(std::slice::from_ref(expr));
             }
             hir::ExprKind::Array(exprs) => {
                 self.print_expr_vec(exprs);

compiler/rustc_lint/src/unused.rs

@@ -1371,7 +1371,6 @@ declare_lint_pass!(UnusedAllocation => [UNUSED_ALLOCATION]);
 impl<'tcx> LateLintPass<'tcx> for UnusedAllocation {
     fn check_expr(&mut self, cx: &LateContext<'_>, e: &hir::Expr<'_>) {
         match e.kind {
-            hir::ExprKind::Box(_) => {}
             hir::ExprKind::Call(path_expr, [_])
                 if let hir::ExprKind::Path(qpath) = &path_expr.kind
                 && let Some(did) = cx.qpath_res(qpath, path_expr.hir_id).opt_def_id()

compiler/rustc_mir_transform/src/large_enums.rs

@@ -158,10 +158,12 @@ impl EnumSizeOpt {
                             tmp_ty,
                         ),
                     };
-                    let rval = Rvalue::Use(Operand::Constant(box (constant_vals)));
+                    let rval = Rvalue::Use(Operand::Constant(Box::new(constant_vals)));
 
-                    let const_assign =
-                        Statement { source_info, kind: StatementKind::Assign(box (place, rval)) };
+                    let const_assign = Statement {
+                        source_info,
+                        kind: StatementKind::Assign(Box::new((place, rval))),
+                    };
 
                     let discr_place = Place::from(
                         local_decls
@@ -170,48 +172,51 @@ impl EnumSizeOpt {
 
                     let store_discr = Statement {
                         source_info,
-                        kind: StatementKind::Assign(box (discr_place, Rvalue::Discriminant(*rhs))),
+                        kind: StatementKind::Assign(Box::new((
+                            discr_place,
+                            Rvalue::Discriminant(*rhs),
+                        ))),
                     };
 
                     let discr_cast_place =
                         Place::from(local_decls.push(LocalDecl::new(tcx.types.usize, span)));
 
                     let cast_discr = Statement {
                         source_info,
-                        kind: StatementKind::Assign(box (
+                        kind: StatementKind::Assign(Box::new((
                             discr_cast_place,
                             Rvalue::Cast(
                                 CastKind::IntToInt,
                                 Operand::Copy(discr_place),
                                 tcx.types.usize,
                             ),
-                        )),
+                        ))),
                     };
 
                     let size_place =
                         Place::from(local_decls.push(LocalDecl::new(tcx.types.usize, span)));
 
                     let store_size = Statement {
                         source_info,
-                        kind: StatementKind::Assign(box (
+                        kind: StatementKind::Assign(Box::new((
                             size_place,
                             Rvalue::Use(Operand::Copy(Place {
                                 local: size_array_local,
                                 projection: tcx
                                     .mk_place_elems(&[PlaceElem::Index(discr_cast_place.local)]),
                             })),
-                        )),
+                        ))),
                     };
 
                     let dst =
                         Place::from(local_decls.push(LocalDecl::new(tcx.mk_mut_ptr(ty), span)));
 
                     let dst_ptr = Statement {
                         source_info,
-                        kind: StatementKind::Assign(box (
+                        kind: StatementKind::Assign(Box::new((
                             dst,
                             Rvalue::AddressOf(Mutability::Mut, *lhs),
-                        )),
+                        ))),
                     };
 
                     let dst_cast_ty = tcx.mk_mut_ptr(tcx.types.u8);
@@ -220,21 +225,21 @@ impl EnumSizeOpt {
 
                     let dst_cast = Statement {
                         source_info,
-                        kind: StatementKind::Assign(box (
+                        kind: StatementKind::Assign(Box::new((
                             dst_cast_place,
                             Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(dst), dst_cast_ty),
-                        )),
+                        ))),
                     };
 
                     let src =
                         Place::from(local_decls.push(LocalDecl::new(tcx.mk_imm_ptr(ty), span)));
 
                     let src_ptr = Statement {
                         source_info,
-                        kind: StatementKind::Assign(box (
+                        kind: StatementKind::Assign(Box::new((
                             src,
                             Rvalue::AddressOf(Mutability::Not, *rhs),
-                        )),
+                        ))),
                     };
 
                     let src_cast_ty = tcx.mk_imm_ptr(tcx.types.u8);
@@ -243,24 +248,24 @@ impl EnumSizeOpt {
 
                     let src_cast = Statement {
                         source_info,
-                        kind: StatementKind::Assign(box (
+                        kind: StatementKind::Assign(Box::new((
                             src_cast_place,
                             Rvalue::Cast(CastKind::PtrToPtr, Operand::Copy(src), src_cast_ty),
-                        )),
+                        ))),
                     };
 
                     let deinit_old =
-                        Statement { source_info, kind: StatementKind::Deinit(box dst) };
+                        Statement { source_info, kind: StatementKind::Deinit(Box::new(dst)) };
 
                     let copy_bytes = Statement {
                         source_info,
-                        kind: StatementKind::Intrinsic(
-                            box NonDivergingIntrinsic::CopyNonOverlapping(CopyNonOverlapping {
+                        kind: StatementKind::Intrinsic(Box::new(
+                            NonDivergingIntrinsic::CopyNonOverlapping(CopyNonOverlapping {
                                 src: Operand::Copy(src_cast_place),
                                 dst: Operand::Copy(dst_cast_place),
                                 count: Operand::Copy(size_place),
                             }),
-                        ),
+                        )),
                     };
 
                     let store_dead = Statement {

compiler/rustc_mir_transform/src/lib.rs

@@ -1,7 +1,6 @@
 #![allow(rustc::potential_query_instability)]
 #![feature(box_patterns)]
 #![feature(drain_filter)]
-#![feature(box_syntax)]
 #![feature(let_chains)]
 #![feature(map_try_insert)]
 #![feature(min_specialization)]

compiler/rustc_parse/messages.ftl

@@ -731,3 +731,6 @@ parse_unknown_start_of_token = unknown start of token: {$escaped}
         [one] once more
         *[other] {$repeats} more times
     }
+
+parse_box_syntax_removed = `box_syntax` has been removed
+    .suggestion = use `Box::new()` instead