4857: Fix invalid shorthand initialization diagnostic for tuple structs r=jonas-schievink a=OptimalStrategy
Initializing tuple structs explicitly, like in the example below, produces a "Shorthand struct initialization" diagnostic that leads to a compilation error when applied:
```rust
struct S(usize);
fn main() {
let s = S { 0: 0 }; // OK, but triggers the diagnostic
// let s = S { 0 }; // Compilation error
}
```
This PR adds a check that the field name is not a literal.
Co-authored-by: OptimalStrategy <george@usan-podgornov.com>
Co-authored-by: OptimalStrategy <17456182+OptimalStrategy@users.noreply.github.com>
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