Function type of ctor of tuple struct with ref

help
1

Let's have this tuple struct:

struct Holder<'a>(&'a u8);

This tuple struct has the constructor with the same name:

let h: Holder = Holder(&123);

But I cannot figure out what type this constructor has (except it is a function). The following code compiles, but I want to substitute gaps with real types.

let ctor: fn(_) -> _ = Holder;
let h: Holder = ctor(&123);

I tried to use for<'a>, but it doesn't work the way I thought:

let ctor: for<'a> fn(&'a u8) -> Holder<'a> = Holder;
let h: Holder = ctor(&123);

The error hints that "concrete lifetime" != "bound lifetime":

  |
6 |     let ctor: for<'a> fn(&'a u8) -> Holder<'a> = Holder;
  |                                                  ^^^^^^ expected concrete lifetime, found bound lifetime parameter 'a
  |
  = note: expected type `for<'a> fn(&'a u8) -> Holder<'a>`
             found type `fn(&u8) -> Holder<'_> {Holder::<'_>}`

But I still don't have a clue how to solve the puzzle.

The playground

Initially, I wanted to accept the ctor as a (generic) function parameter, so I needed to fill Fn(_) -> _ with some types too. I'm pretty sure that doesn't differ from function pointer significantly, does it?

2

First time I've seen such use of constructor as function, think it is on edge of language.

Anyhow it is the inferred lifetime your after.

    fn foo<'a>(x:&'a u8) {
        let ctor = Holder::<'a> as fn(&'a u8) -> Holder<'a>;
        let h: Holder = ctor(x);
        println!("{:?}", h)
    }
    foo(&123);

For completeness note the function is its own zero sized type to begin with and the cast is then to a more regular function pointer.

3

Note that this also appears when casting closures to function pointers:

fn main() {
    let f = |x: &u32| x;
    let _: fn(&u32)->&u32 = f;
}

error[E0308]: mismatched types
 --> src/main.rs:3:29
  |
3 |     let _: fn(&u32)->&u32 = f;
  |                             ^ expected concrete lifetime, found bound lifetime parameter
  |
  = note: expected type `for<'r> fn(&'r u32) -> &'r u32`
             found type `[closure@src/main.rs:2:13: 2:24]`
4

FYI:

https://words.steveklabnik.com/structure-literals-vs-constructors-in-rust

i.e. Holder(&123) would be considered "struct literal syntax" rather than a constructor.

The nomicon on the other hand uses the term "The One True Constructor".

Alternately, in Rust, "constructors" are just a convention:

Constructors are static (no self ) inherent methods for the type that they construct.

5

The constructor will like all other functions and closures have its own unique type. As for the for<'a> conversion, normally these unique types will coerce into function pointers, but the coercion doesn't seem to properly work when for<'a> is involved.

6

Initially I needed a way to wrap a reference into a generic holder so that

  • holder's lifetime ≠ referenced object's lifetime
  • holder constructor (aka struct initializer) is a generic function parameter

That's why I was trying to figure out the type of that function parameter. And faced with a lifetime problem.

From @jonh's answer I got the idea that I had to introduce an extra function specifically to solve this lifetime stuck, since "function with lifetime parameters" is exactly the thing to specify lifetimes =)

So I can introduce the function:

fn foo<'a, F>(x: &'a u8, ctor: F)
where F: FnOnce(&'a u8) -> Holder<'a> {
    let h = ctor(x);
    // use this "h" somehow
}

and then replace

let ctor: fn(_) -> _ = Holder;
let h: Holder = ctor(&123);

with

foo(&123, Holder);

Now I see that I had to call this function with both reference and struct ctor, otherwise compiler won't guess the (elided) lifetime of that second argument Holder of foo.

As far as I understand, there is no way to do something like this:

fn foo<F>(ctor: F)
where F: for<'a> FnOnce(&'a u8) -> Holder<'a> {
    let x = 123;
    let h = ctor(&x);
}
fn main() {
    foo(Holder);  // mean some Holder<'_> instead of Holder<'concrete>
}

Or it is possible somehow?

7

There happens to be a very similar question over here.

8

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