Document the panic! hack to name resolution

In Rust PR 139493, we added a (hopefully temporary) hack to name
resolution involving `panic` to avoid breakage.  Let's document this
in an admonition.

Author: traviscross

src/names/name-resolution.md

@@ -229,6 +229,38 @@ const _: () = {
 r[names.resolution.expansion.imports.ambiguity.glob-vs-outer]
 Names in imports and macro invocations may not be resolved through glob imports when there is another candidate available in an [outer scope].
 
+r[names.resolution.expansion.imports.ambiguity.panic-hack]
+> [!NOTE]
+> When one of [std::panic!] or [core::panic!] is brought into scope due to the [standard library prelude], `rustc` currently accepts the other being resolved through a user-written [glob import]; that glob import takes precedence.
+>
+> In later editions, [core::panic!] and [std::panic!] operate identically, but we can tell the difference between them in Rust 2018 and earlier where only in [std::panic!] is a [String] accepted as the format argument.
+>
+> E.g., due to that, this is an error:
+>
+> ```rust,edition2018,compile_fail,E0308
+> extern crate core;
+> use ::core::prelude::v1::*;
+> fn main() {
+>     panic!(std::string::String::new()); // ERROR
+> }
+> ```
+>
+> And this is accepted:
+>
+> <!-- ignore: Can't test with `no_std`. -->
+> ```rust,edition2018,ignore
+> #![no_std]
+> extern crate std;
+> use ::std::prelude::v1::*;
+> fn main() {
+>     panic!(std::string::String::new()); // ERROR
+> }
+> ```
+>
+> Don't rely on this behavior; the plan is to remove it.
+>
+> For details, see [Rust issue #147319](https://github.com/rust-lang/rust/issues/147319).
+
 ```rust,compile_fail,E0659
 mod glob {
     pub mod ambig {
@@ -545,6 +577,7 @@ r[names.resolution.type-relative]
 [`use` declarations]: ../items/use-declarations.md
 [`use` glob shadowing]: ../items/use-declarations.md#r-items.use.glob.shadowing
 [derive helper scope]: ../procedural-macros.md#r-macro.proc.derive.attributes.scope
+[glob import]: items.use.glob
 [item definitions]: ../items.md
 [macro invocations]: ../macros.md#macro-invocation
 [macro textual scope shadowing]: ../macros-by-example.md#r-macro.decl.scope.textual.shadow

src/names/preludes.md

@@ -40,6 +40,11 @@ Edition | `no_std` not applied        | `no_std` applied
 >
 > [`core::prelude::rust_2015`] and [`core::prelude::rust_2018`] have the same contents as [`core::prelude::v1`].
 
+> [!NOTE]
+> When one of [std::panic!] or [core::panic!] is brought into scope due to the [standard library prelude], `rustc` currently accepts the other being resolved through a user-written [glob import]; that glob import takes precedence.
+>
+> For details, see [names.resolution.expansion.imports.ambiguity.panic-hack].
+
 r[names.preludes.extern]
 ## Extern prelude
 
@@ -203,6 +208,7 @@ r[names.preludes.no_implicit_prelude.edition2018]
 [Built-in attributes]: ../attributes.md#built-in-attributes-index
 [extern prelude]: #extern-prelude
 [floating-point types]: ../types/numeric.md#floating-point-types
+[glob import]: items.use.glob
 [Integer types]: ../types/numeric.md#integer-types
 [Language prelude]: #language-prelude
 [Machine-dependent integer types]: ../types/numeric.md#machine-dependent-integer-types