Better error message for comma after base struct
#41834
This adds a better error for commas after the base struct:
```
let foo = Foo {
one: 111,
..Foo::default(), // This comma is a syntax error
};
```
The current error is a generic `expected one of ...` which isn't beginner-friendly. My error looks like this:
```
error: cannot use a comma after the base struct
--> tmp/example.rs:26:9
|
26 | ..Foo::default(),
| ^^^^^^^^^^^^^^^^- help: remove this comma
|
= note: the base struct expansion must always be the last field
```
I even added a note for people who don't know why this isn't allowed.
e6a6d980e0