Try to recover more from struct field parse error

[chenyukang]

Partially fixes #126344

I tried to recover more from parsing struct field error, instead of skipping the whole struct, we may continue to parse the other fields, this can help to reduce some noises when there is only one struct field is not parsed correctly, we will not report unknown field for all the other fields.

But this PR can not remove all later diagnostics from the struct, I'm not sure whether is worth to solve it, I think the main noise comes from too many fields are ignored, while the left one is trivial.

r? @estebank

[chenyukang]

I tested on some local projects, the typical change is like this:

Current:

 mini-redis git:(master) ✗ cargo build      
   Compiling mini-redis v0.4.1 (/Users/yukang/code/playground/mini-redis)
error: struct fields are separated by `,`
  --> src/server.rs:29:26
   |
18 | struct Listener {
   |        -------- while parsing this struct
...
29 |     listener: TcpListener;
   |                          ^ help: replace `;` with `,`

error[E0609]: no field `limit_connections` on type `&mut Listener`
   --> src/server.rs:229:18
    |
229 |                 .limit_connections
    |                  ^^^^^^^^^^^^^^^^^ unknown field
    |
    = note: available field is: `db_holder`

error[E0609]: no field `notify_shutdown` on type `&mut Listener`
   --> src/server.rs:250:46
    |
250 |                 shutdown: Shutdown::new(self.notify_shutdown.subscribe()),
    |                                              ^^^^^^^^^^^^^^^ unknown field
    |
    = note: available field is: `db_holder`

error[E0609]: no field `shutdown_complete_tx` on type `&mut Listener`
   --> src/server.rs:254:42
    |
254 |                 _shutdown_complete: self.shutdown_complete_tx.clone(),
    |                                          ^^^^^^^^^^^^^^^^^^^^ unknown field
    |
    = note: available field is: `db_holder`

error[E0609]: no field `listener` on type `&mut Listener`
   --> src/server.rs:285:24
    |
285 |             match self.listener.accept().await {
    |                        ^^^^^^^^ unknown field
    |
    = note: available field is: `db_holder`

For more information about this error, try `rustc --explain E0609`.
error: could not compile `mini-redis` (lib) due to 5 previous errors

With fix:

➜  mini-redis git:(master) ✗ cargo +dev2 build
   Compiling mini-redis v0.4.1 (/Users/yukang/code/playground/mini-redis)
error: struct fields are separated by `,`
  --> src/server.rs:29:26
   |
18 | struct Listener {
   |        -------- while parsing this struct
...
29 |     listener: TcpListener;
   |                          ^ help: replace `;` with `,`

error[E0609]: no field `listener` on type `&mut Listener`
   --> src/server.rs:285:24
    |
285 |             match self.listener.accept().await {
    |                        ^^^^^^^^ unknown field
    |
    = note: available fields are: `db_holder`, `limit_connections`, `notify_shutdown`, `shutdown_complete_tx`

For more information about this error, try `rustc --explain E0609`.
error: could not compile `mini-redis` (lib) due to 2 previous errors

[chenyukang]

Got a better solution