Auto merge of #100036 - DrMeepster:box_free_free_box, r=oli-obk

Remove `box_free` lang item

This PR removes the `box_free` lang item, replacing it with `Box`'s `Drop` impl. Box dropping is still slightly magic because the contained value is still dropped by the compiler.

4 files changed, 11 insertions, 11 deletions

diff --git a/src/doc/unstable-book/src/language-features/lang-items.md b/src/doc/unstable-book/src/language-features/lang-items.md
index 6adb3506e64..f4bc18bc7da 100644
--- a/src/doc/unstable-book/src/language-features/lang-items.md
+++ b/src/doc/unstable-book/src/language-features/lang-items.md
@@ -11,8 +11,8 @@ it exists. The marker is the attribute `#[lang = "..."]` and there are
 various different values of `...`, i.e. various different 'lang
 items'.
 
-For example, `Box` pointers require two lang items, one for allocation
-and one for deallocation. A freestanding program that uses the `Box`
+For example, `Box` pointers require a lang item for allocation.
+A freestanding program that uses the `Box`
 sugar for dynamic allocations via `malloc` and `free`:
 
 ```rust,ignore (libc-is-finicky)
@@ -48,9 +48,10 @@ unsafe fn allocate(size: usize, _align: usize) -> *mut u8 {
     p
 }
 
-#[lang = "box_free"]
-unsafe fn box_free<T: ?Sized>(ptr: *mut T) {
-    libc::free(ptr as *mut libc::c_void)
+impl<T> Drop for Box<T> {
+    fn drop(&mut self) {
+      libc::free(self.0.0.0 as *mut libc::c_void)
+    }
 }
 
 #[start]
@@ -84,8 +85,8 @@ Other features provided by lang items include:
   `contravariant_lifetime`, etc.
 
 Lang items are loaded lazily by the compiler; e.g. if one never uses
-`Box` then there is no need to define functions for `exchange_malloc`
-and `box_free`. `rustc` will emit an error when an item is needed
+`Box` then there is no need to define a function for `exchange_malloc`.
+`rustc` will emit an error when an item is needed
 but not found in the current crate or any that it depends on.
 
 Most lang items are defined by `libcore`, but if you're trying to build
@@ -250,7 +251,6 @@ the source code.
 - Allocations
   - `owned_box`: `liballoc/boxed.rs`
   - `exchange_malloc`: `liballoc/heap.rs`
-  - `box_free`: `liballoc/heap.rs`
 - Operands
   - `not`: `libcore/ops/bit.rs`
   - `bitand`: `libcore/ops/bit.rs`
diff --git a/src/tools/miri/tests/fail/alloc/stack_free.stderr b/src/tools/miri/tests/fail/alloc/stack_free.stderr
index b1636050a78..7c14d372f0c 100644
--- a/src/tools/miri/tests/fail/alloc/stack_free.stderr
+++ b/src/tools/miri/tests/fail/alloc/stack_free.stderr
@@ -9,7 +9,7 @@ LL |     unsafe { __rust_dealloc(ptr, layout.size(), layout.align()) }
    = note: BACKTRACE:
    = note: inside `std::alloc::dealloc` at RUSTLIB/alloc/src/alloc.rs:LL:CC
    = note: inside `<std::alloc::Global as std::alloc::Allocator>::deallocate` at RUSTLIB/alloc/src/alloc.rs:LL:CC
-   = note: inside `alloc::alloc::box_free::<i32, std::alloc::Global>` at RUSTLIB/alloc/src/alloc.rs:LL:CC
+   = note: inside `<std::boxed::Box<i32> as std::ops::Drop>::drop` at RUSTLIB/alloc/src/boxed.rs:LL:CC
    = note: inside `std::ptr::drop_in_place::<std::boxed::Box<i32>> - shim(Some(std::boxed::Box<i32>))` at RUSTLIB/core/src/ptr/mod.rs:LL:CC
    = note: inside `std::mem::drop::<std::boxed::Box<i32>>` at RUSTLIB/core/src/mem/mod.rs:LL:CC
 note: inside `main`
diff --git a/src/tools/miri/tests/fail/stacked_borrows/deallocate_against_protector1.stderr b/src/tools/miri/tests/fail/stacked_borrows/deallocate_against_protector1.stderr
index 516964b9a4e..8ebb35450e5 100644
--- a/src/tools/miri/tests/fail/stacked_borrows/deallocate_against_protector1.stderr
+++ b/src/tools/miri/tests/fail/stacked_borrows/deallocate_against_protector1.stderr
@@ -9,7 +9,7 @@ LL |     unsafe { __rust_dealloc(ptr, layout.size(), layout.align()) }
    = note: BACKTRACE:
    = note: inside `std::alloc::dealloc` at RUSTLIB/alloc/src/alloc.rs:LL:CC
    = note: inside `<std::alloc::Global as std::alloc::Allocator>::deallocate` at RUSTLIB/alloc/src/alloc.rs:LL:CC
-   = note: inside `alloc::alloc::box_free::<i32, std::alloc::Global>` at RUSTLIB/alloc/src/alloc.rs:LL:CC
+   = note: inside `<std::boxed::Box<i32> as std::ops::Drop>::drop` at RUSTLIB/alloc/src/boxed.rs:LL:CC
    = note: inside `std::ptr::drop_in_place::<std::boxed::Box<i32>> - shim(Some(std::boxed::Box<i32>))` at RUSTLIB/core/src/ptr/mod.rs:LL:CC
    = note: inside `std::mem::drop::<std::boxed::Box<i32>>` at RUSTLIB/core/src/mem/mod.rs:LL:CC
 note: inside closure
diff --git a/src/tools/miri/tests/fail/tree-borrows/strongly-protected.stderr b/src/tools/miri/tests/fail/tree-borrows/strongly-protected.stderr
index 97088d5854c..55665e63e8a 100644
--- a/src/tools/miri/tests/fail/tree-borrows/strongly-protected.stderr
+++ b/src/tools/miri/tests/fail/tree-borrows/strongly-protected.stderr
@@ -20,7 +20,7 @@ LL | fn inner(x: &mut i32, f: fn(&mut i32)) {
    = note: BACKTRACE (of the first span):
    = note: inside `std::alloc::dealloc` at RUSTLIB/alloc/src/alloc.rs:LL:CC
    = note: inside `<std::alloc::Global as std::alloc::Allocator>::deallocate` at RUSTLIB/alloc/src/alloc.rs:LL:CC
-   = note: inside `alloc::alloc::box_free::<i32, std::alloc::Global>` at RUSTLIB/alloc/src/alloc.rs:LL:CC
+   = note: inside `<std::boxed::Box<i32> as std::ops::Drop>::drop` at RUSTLIB/alloc/src/boxed.rs:LL:CC
    = note: inside `std::ptr::drop_in_place::<std::boxed::Box<i32>> - shim(Some(std::boxed::Box<i32>))` at RUSTLIB/core/src/ptr/mod.rs:LL:CC
    = note: inside `std::mem::drop::<std::boxed::Box<i32>>` at RUSTLIB/core/src/mem/mod.rs:LL:CC
 note: inside closure