Update for 2021 edition standard library prelude changes

Author: mattheww

src/names/preludes.md

@@ -18,9 +18,22 @@ There are several different preludes:
 
 ## Standard library prelude
 
-The standard library prelude includes names from the [`std::prelude::v1`]
-module. If the [`no_std` attribute] is used, then it instead uses the names
-from the [`core::prelude::v1`] module.
+Each crate has a standard library prelude, which consists of the names from a single standard library module. The module used depends on the crate's edition, and on whether the [`no_std` attribute] is applied to the crate:
+
+Edition | `no_std` not applied        | `no_std` applied
+--------| --------------------------- | ----------------------------
+2015    | [`std::prelude::rust_2015`] | [`core::prelude::rust_2015`]
+2018    | [`std::prelude::rust_2018`] | [`core::prelude::rust_2018`]
+2021    | [`std::prelude::rust_2021`] | [`core::prelude::rust_2021`]
+
+This choice of prelude is in effect throughout the crate, including code generated by macros defined in crates with a different edition or `no_std` attribute.
+
+
+> **Note**:
+>
+> [`std::prelude::rust_2015`] and [`std::prelude::rust_2018`] have the same contents as [`std::prelude::v1`].
+>
+> [`core::prelude::rust_2015`] and [`core::prelude::rust_2018`] have the same contents as [`core::prelude::v1`].
 
 ## Extern prelude
 
@@ -63,15 +76,13 @@ By default, the standard library is automatically included in the crate root
 module. The [`std`] crate is added to the root, along with an implicit
 [`macro_use` attribute] pulling in all macros exported from `std` into the
 [`macro_use` prelude]. Both [`core`] and [`std`] are added to the [extern
-prelude]. The [standard library prelude] includes everything from the
-[`std::prelude::v1`] module.
+prelude].
 
 The *`no_std` [attribute]* may be applied at the crate level to prevent the
 [`std`] crate from being automatically added into scope. It does three things:
 
 * Prevents `std` from being added to the [extern prelude](#extern-prelude).
-* Uses [`core::prelude::v1`] in the [standard library prelude] instead of
-  [`std::prelude::v1`].
+* Affects which module is used to make up the [standard library prelude] (as described above).
 * Injects the [`core`] crate into the crate root instead of [`std`], and pulls
   in all macros exported from `core` in the [`macro_use` prelude].
 
@@ -130,13 +141,19 @@ This attribute does not affect the [language prelude].
 [`alloc`]: ../../alloc/index.html
 [`Box`]: ../../std/boxed/struct.Box.html
 [`core::prelude::v1`]: ../../core/prelude/index.html
+[`core::prelude::rust_2015`]: ../../core/prelude/index.html
+[`core::prelude::rust_2018`]: ../../core/prelude/index.html
+[`core::prelude::rust_2021`]: ../../core/prelude/index.html
 [`core`]: ../../core/index.html
 [`extern crate`]: ../items/extern-crates.md
 [`macro_use` attribute]: ../macros-by-example.md#the-macro_use-attribute
 [`macro_use` prelude]: #macro_use-prelude
 [`no_std` attribute]: #the-no_std-attribute
 [`no_std` attribute]: #the-no_std-attribute
 [`std::prelude::v1`]: ../../std/prelude/index.html
+[`std::prelude::rust_2015`]: ../../std/prelude/index.html
+[`std::prelude::rust_2018`]: ../../std/prelude/index.html
+[`std::prelude::rust_2021`]: ../../std/prelude/index.html
 [`std`]: ../../std/index.html
 [`test`]: ../../test/index.html
 [attribute]: ../attributes.md