Private tuple struct field causes confusing error message in cross-crate usage

[lnicola]

Related to, but not a duplicate of #52144.

use headers::SetCookie;

fn main() {
    let _ = SetCookie(vec![]);
}

[package]
name = "e0423-test"
version = "0.1.0"
edition = "2018"

[dependencies]
headers = "0.2.0"

    Checking e0423-test v0.1.0 (/home/grayshade/e0423-test)
error[E0423]: expected function, found struct `SetCookie`
 --> src/main.rs:4:13
  |
4 |     let _ = SetCookie(vec![]);
  |             ^^^^^^^^^ did you mean `SetCookie { /* fields */ }`?

error: aborting due to previous error

Interestingly, the following code:

mod cookie {
    pub struct HeaderValue;
    
    pub struct SetCookie(Vec<HeaderValue>);
}

use cookie::SetCookie;

fn main() {
    let _ = SetCookie(vec![]);
}

Gives a better, but still confusing error message:

error[E0423]: expected function, found struct `SetCookie`
  --> src/main.rs:10:13
   |
10 |     let _ = SetCookie(vec![]);
   |             ^^^^^^^^^ constructor is not visible here due to private fields
help: possible better candidate is found in another module, you can import it into scope
   |
7  | use crate::cookie::SetCookie;
   |

error: aborting due to previous error

Tested in rustc 1.34.0-nightly (c1c3c4e95 2019-01-29).

[cramertj]

cc @estebank

[vext01]

I was about to raise an issue for this too.

Yesterday a colleague and I spent 20 minutes trying to figure out what had gone wrong with our code. In the end it was a simple as adding pub to a field, but the error message really confused us!

[vext01]

Note also that the error message has changed slightly since this issue was raised:

error[E0423]: expected function, tuple struct or tuple variant, found struct `SetCookie`
  --> src/main.rs:4:17
   |
4  |         let _ = SetCookie(vec![]);
   |                 ^^^^^^^^^^^^^^^^^ help: use struct literal syntax instead: `SetCookie { 0: val }`
   | 
  ::: /home/vext01/.cargo/registry/src/github.com-1ecc6299db9ec823/headers-0.2.3/src/common/set_cookie.rs:56:1
   |
56 | pub struct SetCookie(Vec<::HeaderValue>);
   | ----------------------------------------- `SetCookie` defined here

[WaffleLapkin]

Since the last comment the output has changed again:

error[E0423]: cannot initialize a tuple struct which contains private fields
  --> src/main.rs:4:13
   |
4  |     let _ = SetCookie(vec![]);
   |             ^^^^^^^^^
   |
note: constructor is not visible here due to private fields
  --> /home/waffle/.cargo/registry/src/github.com-1ecc6299db9ec823/headers-0.2.3/src/common/set_cookie.rs:56:22
   |
56 | pub struct SetCookie(Vec<::HeaderValue>);
   |                      ^^^^^^^^^^^^^^^^^^ private field

I think that the output is basically perfect, so this issue can be closed 😄

I believe this diagnostic was updated by #78401.