turn rustc_box into an intrinsic

Author: RalfJung

compiler/rustc_feature/src/builtin_attrs.rs

@@ -933,11 +933,6 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         "#[rustc_has_incoherent_inherent_impls] allows the addition of incoherent inherent impls for \
          the given type by annotating all impl items with #[rustc_allow_incoherent_impl]."
     ),
-    rustc_attr!(
-        rustc_box, AttributeType::Normal, template!(Word), ErrorFollowing, EncodeCrossCrate::No,
-        "#[rustc_box] allows creating boxes \
-        and it is only intended to be used in `alloc`."
-    ),
 
     BuiltinAttribute {
         name: sym::rustc_diagnostic_item,

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -86,6 +86,7 @@ pub fn intrinsic_operation_unsafety(tcx: TyCtxt<'_>, intrinsic_id: LocalDefId) -
         | sym::assert_inhabited
         | sym::assert_zero_valid
         | sym::assert_mem_uninitialized_valid
+        | sym::box_new
         | sym::breakpoint
         | sym::size_of
         | sym::min_align_of
@@ -606,6 +607,8 @@ pub fn check_intrinsic_type(
 
             sym::ub_checks => (0, 0, Vec::new(), tcx.types.bool),
 
+            sym::box_new => (1, 0, vec![param(0)], Ty::new_box(tcx, param(0))),
+
             sym::simd_eq
             | sym::simd_ne
             | sym::simd_lt

compiler/rustc_mir_build/messages.ftl

@@ -287,11 +287,6 @@ mir_build_privately_uninhabited = pattern `{$witness_1}` is currently uninhabite
 
 mir_build_rust_2024_incompatible_pat = this pattern relies on behavior which may change in edition 2024
 
-mir_build_rustc_box_attribute_error = `#[rustc_box]` attribute used incorrectly
-    .attributes = no other attributes may be applied
-    .not_box = `#[rustc_box]` may only be applied to a `Box::new()` call
-    .missing_box = `#[rustc_box]` requires the `owned_box` lang item
-
 mir_build_static_in_pattern = statics cannot be referenced in patterns
     .label = can't be used in patterns
 mir_build_static_in_pattern_def = `static` defined here

compiler/rustc_mir_build/src/errors.rs

@@ -1067,25 +1067,6 @@ pub(crate) enum MiscPatternSuggestion {
     },
 }
 
-#[derive(Diagnostic)]
-#[diag(mir_build_rustc_box_attribute_error)]
-pub(crate) struct RustcBoxAttributeError {
-    #[primary_span]
-    pub(crate) span: Span,
-    #[subdiagnostic]
-    pub(crate) reason: RustcBoxAttrReason,
-}
-
-#[derive(Subdiagnostic)]
-pub(crate) enum RustcBoxAttrReason {
-    #[note(mir_build_attributes)]
-    Attributes,
-    #[note(mir_build_not_box)]
-    NotBoxNew,
-    #[note(mir_build_missing_box)]
-    MissingBox,
-}
-
 #[derive(LintDiagnostic)]
 #[diag(mir_build_rust_2024_incompatible_pat)]
 pub(crate) struct Rust2024IncompatiblePat<'a> {

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

@@ -20,7 +20,6 @@ use rustc_middle::{bug, span_bug};
 use rustc_span::{Span, sym};
 use tracing::{debug, info, instrument, trace};
 
-use crate::errors;
 use crate::thir::cx::Cx;
 use crate::thir::util::UserAnnotatedTyHelpers;
 
@@ -380,45 +379,25 @@ impl<'tcx> Cx<'tcx> {
                         from_hir_call: true,
                         fn_span: expr.span,
                     }
-                } else {
-                    let attrs = tcx.hir().attrs(expr.hir_id);
-                    if attrs.iter().any(|a| a.name_or_empty() == sym::rustc_box) {
-                        if attrs.len() != 1 {
-                            tcx.dcx().emit_err(errors::RustcBoxAttributeError {
-                                span: attrs[0].span,
-                                reason: errors::RustcBoxAttrReason::Attributes,
-                            });
-                        } else if let Some(box_item) = tcx.lang_items().owned_box() {
-                            if let hir::ExprKind::Path(hir::QPath::TypeRelative(ty, fn_path)) =
-                                fun.kind
-                                && let hir::TyKind::Path(hir::QPath::Resolved(_, path)) = ty.kind
-                                && path.res.opt_def_id().is_some_and(|did| did == box_item)
-                                && fn_path.ident.name == sym::new
-                                && let [value] = args
-                            {
-                                return Expr {
-                                    temp_lifetime: TempLifetime {
-                                        temp_lifetime,
-                                        backwards_incompatible,
-                                    },
-                                    ty: expr_ty,
-                                    span: expr.span,
-                                    kind: ExprKind::Box { value: self.mirror_expr(value) },
-                                };
-                            } else {
-                                tcx.dcx().emit_err(errors::RustcBoxAttributeError {
-                                    span: expr.span,
-                                    reason: errors::RustcBoxAttrReason::NotBoxNew,
-                                });
-                            }
-                        } else {
-                            tcx.dcx().emit_err(errors::RustcBoxAttributeError {
-                                span: attrs[0].span,
-                                reason: errors::RustcBoxAttrReason::MissingBox,
-                            });
-                        }
+                } else if let ty::FnDef(def_id, _) = self.typeck_results().expr_ty(fun).kind()
+                    && let Some(intrinsic) = self.tcx.intrinsic(def_id)
+                    && intrinsic.name == sym::box_new
+                {
+                    // We don't actually evaluate `fun` here, so make sure that doesn't miss any side-effects.
+                    if !matches!(fun.kind, hir::ExprKind::Path(_)) {
+                        span_bug!(
+                            expr.span,
+                            "`box_new` intrinsic can only be called via path expression"
+                        );
                     }
-
+                    let value = &args[0];
+                    return Expr {
+                        temp_lifetime: TempLifetime { temp_lifetime, backwards_incompatible },
+                        ty: expr_ty,
+                        span: expr.span,
+                        kind: ExprKind::Box { value: self.mirror_expr(value) },
+                    };
+                } else {
                     // Tuple-like ADTs are represented as ExprKind::Call. We convert them here.
                     let adt_data = if let hir::ExprKind::Path(ref qpath) = fun.kind
                         && let Some(adt_def) = expr_ty.ty_adt_def()

compiler/rustc_span/src/symbol.rs

@@ -1696,7 +1696,6 @@ symbols! {
         rustc_as_ptr,
         rustc_attrs,
         rustc_autodiff,
-        rustc_box,
         rustc_builtin_macro,
         rustc_capture_analysis,
         rustc_clean,

library/alloc/src/alloc.rs

@@ -339,7 +339,7 @@ unsafe impl Allocator for Global {
     }
 }
 
-/// The allocator for unique pointers.
+/// The allocator for `Box`.
 #[cfg(all(not(no_global_oom_handling), not(test)))]
 #[lang = "exchange_malloc"]
 #[inline]

library/alloc/src/boxed.rs

@@ -233,6 +233,27 @@ pub struct Box<
     #[unstable(feature = "allocator_api", issue = "32838")] A: Allocator = Global,
 >(Unique<T>, A);
 
+/// Constructs a `Box<T>` by calling the `exchange_malloc` lang item and moving the argument into
+/// the newly allocated memory.
+///
+/// This is the surface syntax for `box <expr>` expressions.
+#[cfg(not(bootstrap))]
+#[rustc_intrinsic]
+#[rustc_intrinsic_must_be_overridden]
+#[unstable(feature = "liballoc_internals", issue = "none")]
+pub fn box_new<T>(_x: T) -> Box<T> {
+    unreachable!()
+}
+
+/// Transition function for the next bootstrap bump.
+#[cfg(bootstrap)]
+#[unstable(feature = "liballoc_internals", issue = "none")]
+#[inline(always)]
+pub fn box_new<T>(x: T) -> Box<T> {
+    #[rustc_box]
+    Box::new(x)
+}
+
 impl<T> Box<T> {
     /// Allocates memory on the heap and then places `x` into it.
     ///
@@ -250,8 +271,7 @@ impl<T> Box<T> {
     #[rustc_diagnostic_item = "box_new"]
     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
     pub fn new(x: T) -> Self {
-        #[rustc_box]
-        Box::new(x)
+        return box_new(x);
     }
 
     /// Constructs a new box with uninitialized contents.

library/alloc/src/lib.rs

@@ -168,6 +168,7 @@
 #![feature(dropck_eyepatch)]
 #![feature(fundamental)]
 #![feature(hashmap_internals)]
+#![feature(intrinsics)]
 #![feature(lang_items)]
 #![feature(min_specialization)]
 #![feature(multiple_supertrait_upcastable)]

library/alloc/src/macros.rs

@@ -48,10 +48,9 @@ macro_rules! vec {
     );
     ($($x:expr),+ $(,)?) => (
         <[_]>::into_vec(
-            // This rustc_box is not required, but it produces a dramatic improvement in compile
+            // Using the intrinsic produces a dramatic improvement in compile
             // time when constructing arrays with many elements.
-            #[rustc_box]
-            $crate::boxed::Box::new([$($x),+])
+            $crate::boxed::box_new([$($x),+])
         )
     );
 }

tests/ui/attributes/rustc-box.rs

@@ -2,7 +2,7 @@
 #![feature(coroutines)]
 #![feature(coroutine_clone)]
 #![feature(coroutine_trait)]
-#![feature(rustc_attrs, stmt_expr_attributes)]
+#![feature(rustc_attrs, stmt_expr_attributes, liballoc_internals)]
 
 use std::ops::Coroutine;
 use std::pin::Pin;
@@ -19,8 +19,7 @@ fn main() {
         // - create a Box that is ignored for trait computations;
         // - compute fields (and yields);
         // - assign to `t`.
-        let t = #[rustc_box]
-        Box::new((5, yield));
+        let t = std::boxed::box_new((5, yield));
         drop(t);
     };
 

tests/ui/attributes/rustc-box.stderr

@@ -1,5 +1,5 @@
 error[E0382]: borrow of moved value: `g`
-  --> $DIR/issue-105084.rs:39:14
+  --> $DIR/issue-105084.rs:38:14
    |
 LL |     let mut g = #[coroutine]
    |         ----- move occurs because `g` has type `{coroutine@$DIR/issue-105084.rs:16:5: 16:7}`, which does not implement the `Copy` trait
@@ -23,7 +23,7 @@ LL |     let mut h = copy(g.clone());
    |                       ++++++++
 
 error[E0277]: the trait bound `Box<(i32, ())>: Copy` is not satisfied in `{coroutine@$DIR/issue-105084.rs:16:5: 16:7}`
-  --> $DIR/issue-105084.rs:33:17
+  --> $DIR/issue-105084.rs:32:17
    |
 LL |     || {
    |     -- within this `{coroutine@$DIR/issue-105084.rs:16:5: 16:7}`
@@ -32,13 +32,13 @@ LL |     let mut h = copy(g);
    |                 ^^^^^^^ within `{coroutine@$DIR/issue-105084.rs:16:5: 16:7}`, the trait `Copy` is not implemented for `Box<(i32, ())>`
    |
 note: coroutine does not implement `Copy` as this value is used across a yield
-  --> $DIR/issue-105084.rs:23:22
+  --> $DIR/issue-105084.rs:22:41
    |
-LL |         Box::new((5, yield));
-   |         -------------^^^^^--
-   |         |            |
-   |         |            yield occurs here, with `Box::new((5, yield))` maybe used later
-   |         has type `Box<(i32, ())>` which does not implement `Copy`
+LL |         let t = std::boxed::box_new((5, yield));
+   |                 ------------------------^^^^^--
+   |                 |                       |
+   |                 |                       yield occurs here, with `std::boxed::box_new((5, yield))` maybe used later
+   |                 has type `Box<(i32, ())>` which does not implement `Copy`
 note: required by a bound in `copy`
   --> $DIR/issue-105084.rs:10:12
    |

tests/ui/coroutine/issue-105084.rs

@@ -1,7 +1,5 @@
-#![feature(rustc_attrs, stmt_expr_attributes)]
 #![deny(unused_allocation)]
 
 fn main() {
-    _ = (#[rustc_box] Box::new([1])).len(); //~ error: unnecessary allocation, use `&` instead
     _ = Box::new([1]).len(); //~ error: unnecessary allocation, use `&` instead
 }

tests/ui/coroutine/issue-105084.stderr

@@ -1,20 +1,14 @@
 error: unnecessary allocation, use `&` instead
-  --> $DIR/unused-allocation.rs:5:9
+  --> $DIR/unused-allocation.rs:4:9
    |
-LL |     _ = (#[rustc_box] Box::new([1])).len();
-   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+LL |     _ = Box::new([1]).len();
+   |         ^^^^^^^^^^^^^
    |
 note: the lint level is defined here
-  --> $DIR/unused-allocation.rs:2:9
+  --> $DIR/unused-allocation.rs:1:9
    |
 LL | #![deny(unused_allocation)]
    |         ^^^^^^^^^^^^^^^^^
 
-error: unnecessary allocation, use `&` instead
-  --> $DIR/unused-allocation.rs:6:9
-   |
-LL |     _ = Box::new([1]).len();
-   |         ^^^^^^^^^^^^^
-
-error: aborting due to 2 previous errors
+error: aborting due to 1 previous error
 

tests/ui/lint/unused/unused-allocation.rs

@@ -4,7 +4,9 @@
 
 fn main() {
     match Some(vec![42]) {
-        Some(vec![43]) => {} //~ ERROR expected pattern, found `#`
+        Some(vec![43]) => {} //~ ERROR expected a pattern, found a function call
+        //~| ERROR found associated function
+        //~| ERROR usage of qualified paths in this context is experimental
         _ => {}
     }
 }