Rollup merge of #106584 - kpreid:vec-allocator, r=JohnTitor

Document that `Vec::from_raw_parts[_in]` must be given a pointer from the correct allocator.

Currently, the documentation of `Vec::from_raw_parts` and `Vec::from_raw_parts_in` says nothing about what allocator the pointer must come from. This PR adds that missing information explicitly.

library/alloc/src/vec/mod.rs

@@ -490,6 +490,8 @@ pub fn with_capacity(capacity: usize) -> Self {
     /// This is highly unsafe, due to the number of invariants that aren't
     /// checked:
     ///
+    /// * `ptr` must have been allocated using the global allocator, such as via
+    ///   the [`alloc::alloc`] function.
     /// * `T` needs to have the same alignment as what `ptr` was allocated with.
     ///   (`T` having a less strict alignment is not sufficient, the alignment really
     ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
@@ -526,6 +528,7 @@ pub fn with_capacity(capacity: usize) -> Self {
     /// function.
     ///
     /// [`String`]: crate::string::String
+    /// [`alloc::alloc`]: crate::alloc::alloc
     /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
     ///
     /// # Examples
@@ -681,6 +684,7 @@ pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
     /// This is highly unsafe, due to the number of invariants that aren't
     /// checked:
     ///
+    /// * `ptr` must be [*currently allocated*] via the given allocator `alloc`.
     /// * `T` needs to have the same alignment as what `ptr` was allocated with.
     ///   (`T` having a less strict alignment is not sufficient, the alignment really
     ///   needs to be equal to satisfy the [`dealloc`] requirement that memory must be
@@ -714,6 +718,7 @@ pub fn with_capacity_in(capacity: usize, alloc: A) -> Self {
     ///
     /// [`String`]: crate::string::String
     /// [`dealloc`]: crate::alloc::GlobalAlloc::dealloc
+    /// [*currently allocated*]: crate::alloc::Allocator#currently-allocated-memory
     /// [*fit*]: crate::alloc::Allocator#memory-fitting
     ///
     /// # Examples