Improve diagnostics when attempting to match tuple enum variant with struct pattern

[thombles]

Adds an extra note as below to explain that a tuple pattern was probably intended.

error[E0026]: variant `X::Y` does not have a field named `data`
  --> src/main.rs:18:16
   |
18 |         X::Y { data } => println!("The data is {}", data)
   |                ^^^^ variant `X::Y` does not have field `data`

error[E0027]: pattern does not mention field `0`
  --> src/main.rs:18:9
   |
18 |         X::Y { data } => println!("The data is {}", data)
   |         ^^^^^^^^^^^^^ missing field `0`
   |
   = note: trying to match a tuple variant with a struct variant pattern

Fixes #41314.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @arielb1 (or someone else) soon.

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[petrochenkov]

Thanks!
@bors r+

[bors]

📌 Commit def660c has been approved by petrochenkov

[bors]

⌛️ Testing commit def660c with merge 6f90787...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: petrochenkov
Pushing 6f90787 to master...