proc-macros for wrapping things with normal macros

lib.rs

extern crate proc_macro;
use proc_macro::{Delimiter, Group, TokenStream, TokenTree};

/// usage:
/// ```
/// #[wrap_with(my_macro!)]
/// pub struct Blah { ... }
/// ```
///
/// becomes
///
/// ```
/// my_macro! {
///     pub struct Blah { ... }
/// }
/// ```
#[proc_macro_attribute]
pub fn wrap_with(mut attrs: TokenStream, input: TokenStream) -> TokenStream {
    if let Some(TokenTree::Punct(ref punct)) = attrs.clone().into_iter().last() {
        assert_eq!(punct.as_char(), '!', "Syntax: #[wrap_with(my_macro!)]");
    }
    let wrapper = TokenTree::Group(Group::new(Delimiter::Brace, input.into_iter().collect()));
    attrs.extend(std::iter::once(wrapper));
    attrs
}

/// usage:
/// ```
/// #[derive_with(my_macro!)]
/// pub struct Blah { ... }
/// ```
///
/// becomes
///
/// ```
/// pub struct Blah { ... }
/// my_macro! {
///     pub struct Blah { ... }
/// }
/// ```
/// same as `wrap_with` but always outputs the struct definition in addition to
/// whatever my_macro generates.
#[proc_macro_attribute]
pub fn derive_with(attrs: TokenStream, input: TokenStream) -> TokenStream {
    let mut ret = input.clone();
    ret.extend(wrap_with(attrs, input));
    ret
}