Error for instantiating tuple struct with private field has regressed on nightly

[jonas-schievink]

fn main() {
    module::TupleStruct(0);
}

pub mod module {
    pub struct TupleStruct(u8);
}

On stable/beta, the error message is very good:

error[E0450]: cannot invoke tuple struct constructor with private fields
 --> <anon>:2:5
  |
2 |     module::TupleStruct(0);
  |     ^^^^^^^^^^^^^^^^^^^ cannot construct with a private field
...
6 |     pub struct TupleStruct(u8);
  |                            --- private field declared here

error: aborting due to previous error

On nightly, it's flat out lying to the user:

error: tuple struct `TupleStruct` is private
 --> <anon>:2:5
  |
2 |     module::TupleStruct(0);
  |     ^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

[petrochenkov]

The error was detected by a special code in rustc_privacy previously, now it's caught by a generic privacy check in resolve.
The code is here: https://github.com/rust-lang/rust/blob/master/src/librustc_resolve/lib.rs#L3147
It should probably detect the case def is Def::StructCtor(..) and add a note about constructor visibility being determined by visibility of its fields.

[Restioson]

Made a couple example playgrounds in a duplicate I made (#46407):

Here we see that when using an absolute path to the struct we get the misleading error: playground.

Here we see that adding a use for the struct gives us a better error, though imo the 'better candidate' section is wrong as we already do what it suggests: playground

It's also worth noting that this misleading error is not only on nightly -- it is also on stable.

[estebank]

Current output:

error[E0603]: tuple struct `Foo` is private
  --> src/main.rs:12:14
   |
12 |         ::A::Foo(0)
   |              ^^^

error[E0423]: expected function, found struct `Foo`
  --> src/main.rs:13:9
   |
13 |         Foo(0)
   |         ^^^ constructor is not visible here due to private fields

[nilslice]

FWIW, this caught me just now and although just a short google away, it still had me scratching my head for a bit!

error[E0603]: tuple struct `Link` is private
  --> sandbox/src/main.rs:24:25
   |
24 |         address: derp::Link(Address{
   |           

rustc 1.36.0 (a53f9df 2019-07-03)

---

For those who reach this issue and still have not resolved the error, you must add pub prefixed to your type parameter within the tuple, as such:

pub struct Link(pub Address);