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

Author: bors

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 {