Rust error E0687: #### Note: this error code is no longer emitted by the compiler

The compiler team's explanation

Note: this error code is no longer emitted by the compiler.

In-band lifetimes cannot be used in fn/Fn syntax.

Erroneous code examples:

#![feature(in_band_lifetimes)]

fn foo(x: fn(&'a u32)) {} // error!

fn bar(x: &Fn(&'a u32)) {} // error!

fn baz(x: fn(&'a u32), y: &'a u32) {} // error!

struct Foo<'a> { x: &'a u32 }

impl Foo<'a> {
    fn bar(&self, x: fn(&'a u32)) {} // error!
}

Lifetimes used in fn or Fn syntax must be explicitly declared using <...> binders. For example:

fn foo<'a>(x: fn(&'a u32)) {} // ok!

fn bar<'a>(x: &Fn(&'a u32)) {} // ok!

fn baz<'a>(x: fn(&'a u32), y: &'a u32) {} // ok!

struct Foo<'a> { x: &'a u32 }

impl<'a> Foo<'a> {
    fn bar(&self, x: fn(&'a u32)) {} // ok!
}

From rustc --explain E0687 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.