Better error message when attempting to instantiate tuple structs with private fields

[estebank]

Given

mod a {
    pub struct Error(usize);
}

fn main(){
    let x = a::Error(3);
}

we emit

error[E0603]: tuple struct `Error` is private
 --> src/main.rs:6:16
  |
6 |     let x = a::Error(3);
  |                ^^^^^

which is confusing and inconsistent with the same error in a regular struct:

error[E0451]: field `a` of struct `a::Error` is private
 --> src/main.rs:6:23
  |
6 |     let x = a::Error {a: 3};
  |                       ^^^^ field `a` is private

This has confused users in the past. CC #57951 and #52144, which are somewhat related.

[Centril]

How about?:

error[E0451]: field `0` of tuple struct `a::Error` is private

[csmoe]

in addition to @Centril 's proposal, it should locate the exact position of the private fields:

struct Foo(pub i32, i32, pub i32, i32);
                    ^^^           ^^^

[lnicola]

Is there a good reason why the three test cases give completely different error messages?

[estebank]

This is actually a regression from 1.16:

error[E0450]: cannot invoke tuple struct constructor with private fields
 --> <source>:6:13
  |
2 |     pub struct Error(usize);
  |                      ------ private field declared here
...
6 |     let x = a::Error(3);
  |             ^^^^^^^^ cannot construct with a private field

[cramertj]

ping on this, which still reproduces.

[estebank]

cc @petrochenkov as this overlaps with a lot of the work he's been doing.

[pnkfelix]

triage: I personally don't think this should be P-high, as in I don't think it should be part of the triage docket for T-compiler every week.

But I won't downgrade it yet.

First, lets tag it as E-needs-mentor, and see if anyone steps up to provide instructions for a solution here.