[error message][regression] Private fields in tuple-struct give unhelpful error message

[TyOverby]

I had a tuple-struct defined pub Line(f32, f32, f32, f32) and tried using it from another module without remembering to put pub on the fields.

Nightly and Stable have very different error messages, and the stable message is much better.

Nightly and Beta

error: tuple struct `Line` is private
  --> testsuite/src/main.rs:44:95
   |
44 |     let lines = evaluator.get_polylines(&result, &ctx).into_iter().map(|((x1, y1), (x2, y2))| formats::lines::Line(x1, y1, x2, y2));
   |                                                                                               ^^^^^^^^^^^^^^^^^^^^

Stable

error[E0450]: cannot invoke tuple struct constructor with private fields
  --> testsuite/src/main.rs:44:95
   |
44 |     let lines = evaluator.get_polylines(&result, &ctx).into_iter().map(|((x1, y1), (x2, y2))| formats::lines::Line(x1, y1, x2, y2));
   |                                                                                               ^^^^^^^^^^^^^^^^^^^^ cannot construct with a private field
   |
  ::: testsuite/src/formats.rs
   |
72 |     pub struct Line(f32, f32, f32, f32);
   |                     ---- ---- ---- ---- private field declared here
   |                     |    |    |
   |                     |    |    private field declared here
   |                     |    private field declared here
   |                     private field declared here

[petrochenkov]

Duplicate of #39703