diagnostics: incorrect, redundant "expected/found" parse error

[jseyfried]

Compiling the following:

struct S(pub () ());

produces:

<anon>:1:17: 1:18 error: expected one of `+` or `,`, found `(`
<anon>:1 struct S(pub () ());
                         ^
<anon>:1:17: 1:18 error: expected one of `+`, `;`, or `where`, found `(`
<anon>:1 struct S(pub () ());
                         ^

[jseyfried]

This is not a regression (it is currently an issue on stable).

[petrochenkov]

The first one is correct, () can be followed by a next field (struct S(pub (), u8);) or operand of type sum (struct S(pub () + Send);).
The second one looks like it may be caused by some recent efforts on error recovery in parser, but I'm not sure.

[estebank]

The underlying reason is the same as in #44339, and the proposed solution can be generalized to also fix this case.

[nikomatsakis]

Even though this one is older, I am closing it as a duplicate of #44339, which has mentoring instructions.