Lifetimes not added to inner fn in proc macro

[coadler]

Given this example function

use cached::proc_macro::cached;
use cached::UnboundCache;

#[cached(
    type = "UnboundCache<usize, CreateMessage>",
    create = "{ UnboundCache::with_capacity(1) }",
    convert = r#"{ cmds.len() }"#
)]
fn generate_help<'a>(cmds: &[Arc<dyn Command>]) -> CreateMessage<'a> {
    CreateMessage::default()
}

Expands to

use cached::proc_macro::cached;
use cached::UnboundCache;
static GENERATE_HELP: ::cached::once_cell::sync::Lazy<
    std::sync::Mutex<UnboundCache<usize, CreateMessage>>,
> = ::cached::once_cell::sync::Lazy::new(|| {
    std::sync::Mutex::new({ UnboundCache::with_capacity(1) })
});
fn generate_help<'a>(cmds: &[Arc<dyn Command>]) -> CreateMessage<'a> {
    use cached::Cached;
    let key = { cmds.len() };
    {
        let mut cache = GENERATE_HELP.lock().unwrap();
        if let Some(result) = cache.cache_get(&key) {
            return result.clone();
        }
    }
    fn inner(cmds: &[Arc<dyn Command>]) -> CreateMessage<'a> {
        CreateMessage::default()
    }
    let result = inner(cmds);
    let mut cache = GENERATE_HELP.lock().unwrap();
    cache.cache_set(key, result.clone());
    result
}

Resulting in

error[E0261]: use of undeclared lifetime name `'a`
  --> src/<file>.rs:67:66
   |
66 | )]
   |   - help: consider introducing lifetime `'a` here: `<'a>`
67 | fn generate_help<'a>(cmds: &[Arc<dyn Command>]) -> CreateMessage<'a> {
   |                                                                  ^^ undeclared lifetime

Since the lifetime specifiers aren't copied into the inner func, we're not able to return types that require specifiers. The same problem exists with the cached! macro, except that it completely fails to parse the lifetimes instead of translating them wrong.