Update docs regarding the default global allocator

Author: SimonSapin

src/liballoc_system/lib.rs

@@ -48,25 +48,9 @@ use core::ptr::NonNull;
 /// This is based on `malloc` on Unix platforms and `HeapAlloc` on Windows,
 /// plus related functions.
 ///
-/// This type can be used in a `static` item
-/// with the `#[global_allocator]` attribute
-/// to force the global allocator to be the system’s one.
-/// (The default is jemalloc for executables, on some platforms.)
-///
-/// ```rust
-/// use std::alloc::System;
-///
-/// #[global_allocator]
-/// static A: System = System;
-///
-/// fn main() {
-///     let a = Box::new(4); // Allocates from the system allocator.
-///     println!("{}", a);
-/// }
-/// ```
-///
-/// It can also be used directly to allocate memory
-/// independently of the standard library’s global allocator.
+/// This type can be used directly to allocate memory
+/// independently of the standard library’s global allocator,
+/// which may have been changed with the `#[global_allocator]` attribute.
 #[stable(feature = "alloc_system_type", since = "1.28.0")]
 pub struct System;
 

src/libstd/alloc.rs

@@ -12,14 +12,8 @@
 //!
 //! In a given program, the standard library has one “global” memory allocator
 //! that is used for example by `Box<T>` and `Vec<T>`.
+//! The default global allocator is [`System`].
 //!
-//! Currently the default global allocator is unspecified.
-//! The compiler may link to a version of [jemalloc] on some platforms,
-//! but this is not guaranteed.
-//! Libraries, however, like `cdylib`s and `staticlib`s are guaranteed
-//! to use the [`System`] by default.
-//!
-//! [jemalloc]: https://github.com/jemalloc/jemalloc
 //! [`System`]: struct.System.html
 //!
 //! # The `#[global_allocator]` attribute