not clear what "base expression" is

[matthiaskrgr]

Code

struct Struct<S, T> {
    a: S,
    b: T,
}

fn main() {
    Struct { .. };
}

Current output

error: base expression required after `..`
 --> src/lib.rs:7:16
  |
7 |     Struct { .. };
  |                ^ add a base expression here

Desired output

No response

Rationale and extra context

rustc does not explain what it actually mean with "base expression".
Unfortunately there also is no error number so we can't look this up with --explain

Other cases

No response

Anything else?

No response