Document about some behaviors of const_(de)allocate and add some te…

…sts.

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);

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;
+        }
+    }
+}

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);
+        }
+    }
+}