[Issue 12901] `in`/`out` contracts on struct constructor must require function body

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=12901

--- Comment #4 from hsteoh at quickfur.ath.cx ---
Arguably, the original fix was invalid: it doesn't stop you from writing:

````
struct S
{
    int a;
    this(int n) in { a = n; } body {} // or do {}
}
````

The problem really is allowing assignment to struct members from inside a
contract, which is abuse of the contract feature.  Perhaps the solution should
be to enforce purity on the contract, or, less intrusively, make the contract
body const (i.e., `this` is const inside the contract body).

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