From 9728cc4e26c936e17a107ceeccf029bde4e7e1f0 Mon Sep 17 00:00:00 2001
From: woppopo <woppopo@protonmail.com>
Date: Sat, 29 Jan 2022 19:13:23 +0900
Subject: [PATCH] Document about some behaviors of `const_(de)allocate` and add
 some tests.

---
 library/core/src/intrinsics.rs                | 22 +++++++++++++++----
 .../heap/alloc_intrinsic_zero_sized.rs        | 16 ++++++++++++++
 .../heap/dealloc_intrinsic_zero_sized.rs      | 17 ++++++++++++++
 3 files changed, 51 insertions(+), 4 deletions(-)
 create mode 100644 src/test/ui/consts/const-eval/heap/alloc_intrinsic_zero_sized.rs
 create mode 100644 src/test/ui/consts/const-eval/heap/dealloc_intrinsic_zero_sized.rs

diff --git a/library/core/src/intrinsics.rs b/library/core/src/intrinsics.rs
index b03f1268e36..b5228397f0a 100644
--- a/library/core/src/intrinsics.rs
+++ b/library/core/src/intrinsics.rs
@@ -1914,13 +1914,27 @@ extern "rust-intrinsic" {
     #[rustc_const_unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
     pub fn ptr_guaranteed_ne<T>(ptr: *const T, other: *const T) -> bool;
 
-    /// Allocate at compile time.
-    /// Returns a null pointer at runtime.
+    /// Allocates a block of memory at compile time.
+    /// At runtime, just returns a null pointer.
+    ///
+    /// # Safety
+    ///
+    /// - The `align` argument must be a power of two.
+    ///    - At compile time, a compile error occurs if this constraint is violated.
+    ///    - At runtime, it is not checked.
     #[rustc_const_unstable(feature = "const_heap", issue = "79597")]
     pub fn const_allocate(size: usize, align: usize) -> *mut u8;
 
-    /// Deallocate a memory which allocated by `intrinsics::const_allocate` at compile time.
-    /// Does nothing at runtime.
+    /// Deallocates a memory which allocated by `intrinsics::const_allocate` at compile time.
+    /// At runtime, does nothing.
+    ///
+    /// # Safety
+    ///
+    /// - The `align` argument must be a power of two.
+    ///    - At compile time, a compile error occurs if this constraint is violated.
+    ///    - At runtime, it is not checked.
+    /// - If the `ptr` is created in an another const, this intrinsic doesn't deallocate it.
+    /// - If the `ptr` is pointing to a local variable, this intrinsic doesn't deallocate it.
     #[rustc_const_unstable(feature = "const_heap", issue = "79597")]
     #[cfg(not(bootstrap))]
     pub fn const_deallocate(ptr: *mut u8, size: usize, align: usize);
diff --git a/src/test/ui/consts/const-eval/heap/alloc_intrinsic_zero_sized.rs b/src/test/ui/consts/const-eval/heap/alloc_intrinsic_zero_sized.rs
new file mode 100644
index 00000000000..407e69d41a0
--- /dev/null
+++ b/src/test/ui/consts/const-eval/heap/alloc_intrinsic_zero_sized.rs
@@ -0,0 +1,16 @@
+// run-pass
+#![feature(core_intrinsics)]
+#![feature(const_heap)]
+#![feature(inline_const)]
+
+use std::intrinsics;
+
+struct ZST;
+
+fn main() {
+    const {
+        unsafe {
+            let _ = intrinsics::const_allocate(0, 0) as *mut ZST;
+        }
+    }
+}
diff --git a/src/test/ui/consts/const-eval/heap/dealloc_intrinsic_zero_sized.rs b/src/test/ui/consts/const-eval/heap/dealloc_intrinsic_zero_sized.rs
new file mode 100644
index 00000000000..84fb4d2ea87
--- /dev/null
+++ b/src/test/ui/consts/const-eval/heap/dealloc_intrinsic_zero_sized.rs
@@ -0,0 +1,17 @@
+// run-pass
+#![feature(core_intrinsics)]
+#![feature(const_heap)]
+#![feature(inline_const)]
+
+use std::intrinsics;
+
+fn main() {
+    const {
+        unsafe {
+            let ptr1 = intrinsics::const_allocate(0, 0);
+            let ptr2 = intrinsics::const_allocate(0, 0);
+            intrinsics::const_deallocate(ptr1, 0, 0);
+            intrinsics::const_deallocate(ptr2, 0, 0);
+        }
+    }
+}