Attribute Macro extension

#[extension]

Derive an extension trait for a given impl block. The trait name goes into the parenthesized args of the macro, for greppability. For example:

use rustc_macros::extension;
#[extension(pub trait Foo)]
impl i32 { fn hello() {} }

expands to:

pub trait Foo { fn hello(); }
impl Foo for i32 { fn hello() {} }