Unlike a tuple variant constructor which can be called as a function, a
struct variant constructor is not a function, so cannot be called.
If the user tries to assign the constructor to a variable, an ICE
occurs, because there is no way to use it later. So we should stop the
constructor from being used like that.
A similar mechanism already exists for a normal struct, as it prohibits
a struct from being resolved. This commit does the same for a struct
variant.
This commit also includes some changes to the existing tests.
Fixes#19452.
This breaks code that referred to variant names in the same namespace as
their enum. Reexport the variants in the old location or alter code to
refer to the new locations:
```
pub enum Foo {
A,
B
}
fn main() {
let a = A;
}
```
=>
```
pub use self::Foo::{A, B};
pub enum Foo {
A,
B
}
fn main() {
let a = A;
}
```
or
```
pub enum Foo {
A,
B
}
fn main() {
let a = Foo::A;
}
```
[breaking-change]
