From aef682a89853b83c20f086fdd5a6fba2af1ebfdd Mon Sep 17 00:00:00 2001
From: Michael Goulet <michael@errs.io>
Date: Mon, 19 Feb 2024 16:07:39 +0000
Subject: [PATCH] Docs for extension proc-macro

---
 compiler/rustc_macros/src/lib.rs | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/compiler/rustc_macros/src/lib.rs b/compiler/rustc_macros/src/lib.rs
index 619f93c8a533a..e3669375c0e90 100644
--- a/compiler/rustc_macros/src/lib.rs
+++ b/compiler/rustc_macros/src/lib.rs
@@ -41,6 +41,19 @@ pub fn symbols(input: TokenStream) -> TokenStream {
     symbols::symbols(input.into()).into()
 }
 
+/// Derive an extension trait for a given impl block. The trait name
+/// goes into the parenthesized args of the macro, for greppability.
+/// For example:
+/// ```
+/// #[extension(pub trait Foo)]
+/// impl i32 { fn hello() {} }
+/// ```
+///
+/// expands to:
+/// ```
+/// pub trait Foo { fn hello(); }
+/// impl Foo for i32 { fn hello() {} }
+/// ```
 #[proc_macro_attribute]
 pub fn extension(attr: TokenStream, input: TokenStream) -> TokenStream {
     extension::extension(attr, input)