Bad generic error for incorrectly placed `where` clause for tuple struct

[CAD97]

Given the following code: [playground]

struct Example
where
    (): Sized,
(usize);

The current output is:

error: expected one of `:`, `==`, or `=`, found `;`
 --> src/lib.rs:4:8
  |
4 | (usize);
  |        ^ expected one of `:`, `==`, or `=`

What appears to be happening is that we're trying to parse this as a unit struct (which can have a where clause), and thus trying to parse the struct tuple as a type to bound.

With a tuple visibility:

struct Example
where
    (): Sized,
(pub usize, usize);

error: expected type, found keyword `pub`
 --> src/lib.rs:4:2
  |
4 | (pub usize, usize);
  |  ^^^ expected type

Ideally,

• In the former case (easier?) we can notice that we parsed a tuple type and then got ; instead of : (or a currently-just-for-diagnostics =) and then reinterpret the tuple type as a tuple struct tuple, printing a more useful error;
• In the latter case (harder?) we would likely need to backtrack when seeing a pub visibility marker and retry to parse as a tuple struct definition tuple in order to provide the nicer error.

[fmease]

printing a more useful error ("tuple structs cannot have a where clause")

Note that tuple structs can indeed have a where clause albeit after the fields not before them:

- struct Example
- where
-     (): Sized,
- (usize);
+ struct Example(usize)
+ where
+     (): Sized;

The proposed diagnostic should suggest to move the where clause.

[vincenzopalazzo]

@rustbot claim

[fmease]

@rustbot claim

[fmease]

Fyi, PR is up: #106537. CC @gamma-delta.