ch19-05: Error with the fn type and closures

[K0n4ta13]

• I have searched open and closed issues and pull requests for duplicates, using these search terms:

  • fn type
  • 19-05

• I have checked the latest main branch to see if this has already been fixed, in this file:

  • https://github.com/rust-lang/book/blob/main/src/ch20-04-advanced-functions-and-closures.md

URL to the section(s) of the book with this problem:

https://doc.rust-lang.org/book/ch19-05-advanced-functions-and-closures.html#returning-closures

Description of the problem:

The book says that “you’re not allowed to use the function pointer fn as a return type”. However, the provided example successfully compiles using the fn type.

example in the main branch:

fn returns_closure() -> impl Fn(i32) -> i32 {
    |x| x + 1
}

does compile and is allowed to use the fn type.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=6389cacd2301160f942f838a99bbb82b

Suggested fix:

Clarify that a closure that does not capture its environment can be coerced into a function pointer.

[chriskrycho]

Thanks for flagging this up! Funky! I’ll probably tackle it by making the item actually be a closure (possibly and also adding a clarifying note). If we put on our pedantry hats, after all, the example in the code… isn’t a closure in a strict sense! It does not close over anything!