Trait bounds with lifetimes and E0392 "lifetime parameter never used"

[tonywu6]

First of all thank you for this wonderful library!

Version

> cargo tree | grep bon
├── bon v3.0.0
│   ├── bon-macros v3.0.0 (proc-macro)

Description

The following doesn't compile:

pub trait SomeTrait<'a> {
    fn borrow_stuff(stuff: &'a str) -> Self;
}

#[bon::builder]
pub fn some_func<'a, T: SomeTrait<'a>>(arg1: T) -> T { arg1 }

error[E0392]: lifetime parameter `'a` is never used
 --> src/lib.rs:6:18
  |
6 | pub fn some_func<'a, T: SomeTrait<'a>>(arg1: T) -> T {
  |                  ^^ unused lifetime parameter
  |
  = help: consider removing `'a`, referring to it in a field, or using a marker such as `std::marker::PhantomData`

As suggested by rustc, this could be worked around using PhantomData:

#[bon::builder]
pub fn some_func<'a, T: SomeTrait<'a>>(
    arg1: T,
    #[builder(default)] _lifetime: PhantomData<&'a ()>,
) -> T {
    arg1
}

Looking at the macro expansion:

pub struct SomeFuncBuilder<
    'a,
    T: SomeTrait<'a>,
    S: some_func_builder::State = some_func_builder::Empty,
> {
    __private_vinyl_phantom: ::core::marker::PhantomData<
        (fn() -> S, fn() -> ::core::marker::PhantomData<T>),
    >,
    __private_vinyl_named_members: (::core::option::Option<T>,),
}

The error (credit to this SO answer which explains it nicely) is expected: the borrow checker can't find anything in SomeFuncBuilder that is borrowing data for 'a, but it's confusing when appearing on function-like syntax (it certainly was for me!)

It'd be great if bon can detect such "unused" lifetimes and put them somewhere in the builder struct.

A note for the community from the maintainers

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[Veetaha]

Thank you for reporting this!

The nuance is that it's possible for functions and impl blocks in Rust to have unused lifetimes, while it's not possible to have unused lifetimes in structs.

For example this compiles albeit with an unused_lifetimes warning:

fn sut<'a, 'b>() {}

struct Sut;
impl<'a, 'b> Sut {}

I didn't think of this edge case. The builder's internal phantom data needs to capture all lifetime parameters as &'a (), &'b (), ... to make sure they all are used even when no function's parameter references it.

The fix was released in a 3.0.1 patch.