Unexpected static lifetime requirement for nested boxed closures with generic parameters

[marmeladema]

I tried this code:

type BoxedClosure<U> =
    Box<dyn for<'e> Fn(&'e Context<'e, U>) -> bool + Sync + Send + 'static>;

pub struct CompiledExpr<U>(BoxedClosure<U>);

impl<U> CompiledExpr<U> {
    pub fn new(
        closure: impl for<'e> Fn(&'e Context<'e, U>) -> bool + Sync + Send + 'static,
    ) -> Self {
        CompiledExpr(Box::new(closure))
    }
}

pub struct Context<'e, U> {
    data: &'e str,
    userdata: U,
}

pub fn compile1<U>() -> CompiledExpr<U> {
    CompiledExpr::new(|ctx| ctx.data == "rust")
}

pub fn compile2<U>() -> CompiledExpr<U> {
    let e = compile1::<U>();
    CompiledExpr::new(move |ctx| (e.0)(ctx))
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=c96c451d36a64d657cdfecc7c5185f31

I expected to see this happen: successful compilation

Instead, this happened:

error[E0310]: the parameter type `U` may not live long enough
  --> src/lib.rs:25:5
   |
25 |     CompiledExpr::new(move |ctx| (e.0)(ctx))
   |     ^^^^^^^^^^^^^^^^^
   |     |
   |     the parameter type `U` must be valid for the static lifetime...
   |     ...so that the type `U` will meet its required lifetime bounds
   |
help: consider adding an explicit lifetime bound
   |
23 | pub fn compile2<U: 'static>() -> CompiledExpr<U> {
   |                  +++++++++

I am not exactly sure its a bug per say, but from my perspective, this is relatively surprising that this code fails to build.
The 'static bound is on the boxed dyn object itself and not on the parameters of the boxed closure.
Its not clear why U should be 'static when Context which holds U is not.

Meta

rustc --version --verbose:

rustc 1.83.0 (90b35a623 2024-11-26)
binary: rustc
commit-hash: 90b35a6239c3d8bdabc530a6a0816f7ff89a0aaf
commit-date: 2024-11-26
host: x86_64-unknown-linux-gnu
release: 1.83.0
LLVM version: 19.1.1

[compiler-errors]

So a closure outlives 'static if all of its captures outlive 'static. In the second function compile2, the closure captures the type CompiledExpr<U>. CompiledExpr<U>: 'static only if U: 'static. So that's the source of the inconsistency here.

[marmeladema]

That's really unfortunate because it was never my intention to capture U since it is supposed to only be accessible / usable when calling the closure, not "building" it.

Another inconsistency is that using an impl trait in the return type of compile1 just works! I would have expected the rules for a dyn trait vs impl trait to be similar:

type BoxedClosure<U> =
    Box<dyn for<'e> Fn(&'e Context<'e, U>) -> bool + Sync + Send + 'static>;

pub struct CompiledExpr<U>(BoxedClosure<U>);

impl<U> CompiledExpr<U> {
    pub fn new(
        closure: impl for<'e> Fn(&'e Context<'e, U>) -> bool + Sync + Send + 'static,
    ) -> Self {
        CompiledExpr(Box::new(closure))
    }
}

pub struct Context<'e, U> {
    data: &'e str,
    userdata: U,
}

pub fn compile1<U>() -> impl for<'e> Fn(&'e Context<'e, U>) -> bool + Sync + Send + 'static  {
    |ctx| ctx.data == "rust"
}

pub fn compile2<U>() -> CompiledExpr<U> {
    let e = compile1::<U>();
    CompiledExpr::new(move |ctx| e(ctx))
}

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=a109e8ee8ce903af1b0fc2cdafe8c21d

[marmeladema]

Another more contrived example which exhibits the problem:

pub struct Context<'e, U> {
    data: &'e str,
    userdata: U,
}

pub fn to_opaque<U>(e: Box<dyn for<'e> Fn(&'e Context<'e, U>) -> bool + Sync + Send + 'static>) -> impl for<'e> Fn(&'e Context<'e, U>) -> bool + Sync + Send + 'static {
    e
}

Which generates the following the compilation error:

error[E0310]: the parameter type `U` may not live long enough
 --> src/lib.rs:7:5
  |
7 |     e
  |     ^
  |     |
  |     the parameter type `U` must be valid for the static lifetime...
  |     ...so that the type `U` will meet its required lifetime bounds