Document built-in macros and no_implicit_prelude

Macros are part of the standard library prelude.  One might expect
these to not be in scope when `no_implicit_prelude` is used.  But
certain macros still are.  The history is that these macros were once
built in to the language; when this was changed, in Rust PR 62086, a
hack was added.  Then in Rust PR 139493 this list grew to include
`panic!` and `unreachable!`.

Author: traviscross

src/names/preludes.md

@@ -192,6 +192,41 @@ The `no_implicit_prelude` attribute may be used any number of times on a form.
 r[names.preludes.no_implicit_prelude.excluded-preludes]
 The `no_implicit_prelude` attribute prevents the [standard library prelude], [extern prelude], [`macro_use` prelude], and the [tool prelude] from being brought into scope for the module and its descendants.
 
+r[names.preludes.no_implicit_prelude.implicitly-imported-macros]
+> [!NOTE]
+> Despite `#![no_implicit_prelude]`, `rustc` currently brings certain macros implicitly into scope.  Those macros are:
+>
+> - [assert!]
+> - [cfg!]
+> - [cfg_select!]
+> - [column!]
+> - [compile_error!]
+> - [concat!]
+> - [concat_bytes!]
+> - [env!]
+> - [file!]
+> - [format_args!]
+> - [include!]
+> - [include_bytes!]
+> - [include_str!]
+> - [line!]
+> - [module_path!]
+> - [option_env!]
+> - [panic!]
+> - [stringify!]
+> - [unreachable!]
+>
+> E.g., this works:
+>
+> ```rust
+> #![no_implicit_prelude]
+> fn main() { assert!(true); }
+> ```
+>
+> Don't rely on this behavior; it may be removed in the future.  Always bring the items you need into scope explicitly when using `#![no_implicit_prelude]`.
+>
+> For details, see [Rust PR #62086](https://github.com/rust-lang/rust/pull/62086) and [Rust PR #139493](https://github.com/rust-lang/rust/pull/139493).
+
 r[names.preludes.no_implicit_prelude.lang]
 The `no_implicit_prelude` attribute does not affect the [language prelude].