Improve syntax diagnostics or suggestions with `enum` vs `struct`

[rossmacarthur]

I changed an enum to a struct but forgot to change the enum keyword to a struct keyword, it took me a while to realize what the problem was because the error is unclear.

enum used where code looks like a struct

pub enum NotEnum {
    field: u8   
}

The error is

error: expected one of `(`, `,`, `=`, `{`, or `}`, found `:`
 --> src/lib.rs:2:10
  |
2 |     field: u8,    
  |          ^ expected one of `(`, `,`, `=`, `{`, or `}`

error: expected item, found `:`
 --> src/lib.rs:2:10
  |
2 |     field: u8,    
  |          ^ expected item

A suggestion saying help: change `enum` to `struct` would be useful.

struct used where code looks like an enum

pub struct NotStruct {
    Variant
}

error: expected `:`, found `}`
 --> src/lib.rs:3:1
  |
2 |     Variant
  |            - expected `:`
3 | }
  | ^ unexpected token

A suggestion saying help: change `struct` to `enum` would be useful.

[rossmacarthur]

@rustbot modify labels: +A-diagnostics +A-suggestion-diagnostics

[fmease]

Current output for the first snippet:

error: expected one of `(`, `,`, `=`, `{`, or `}`, found `:`
 --> src/lib.rs:2:10
  |
1 | pub enum NotEnum {
  |          ------- while parsing this enum
2 |     field: u8   
  |          ^ expected one of `(`, `,`, `=`, `{`, or `}`
  |
  = help: enum variants can be `Variant`, `Variant = <integer>`, `Variant(Type, ..., TypeN)` or `Variant { fields: Types }`
help: perhaps you meant to use `struct` here
  |
1 | pub struct NotEnum {
  |     ~~~~~~

→ Clearly fixed.

Current output for the second snippet:

error: expected `:`, found `}`
 --> src/lib.rs:3:1
  |
1 | pub struct NotStruct {
  |            --------- while parsing this struct
2 |     Variant
  |            - expected `:`
3 | }
  | ^ unexpected token

→ Slightly improved (due to the label).

[rossmacarthur]

Looks reasonably fixed