Initialization of @mustuse member variable causes spurious error

[pbackus]

As of DMD 2.109.1, the following program fails to compile:

import core.attribute;

@mustuse struct Inner
{
    this(int n) {}
}

struct Outer
{
    Inner inner;
    this(int n)
    {
        this.inner = n;
    }
}

The error message is:

bug.d(13): Error: ignored value of `@mustuse` type `bug.Inner`; prepend a `cast(void)` if intentional

That is, the initialization of this.inner is incorrectly flagged as a violation of @mustuse.

[pbackus]

The error happens because this.inner = n; is lowered by the DMD frontend to (roughly) the following:

(this.inner = typeof(this.inner).init, this.inner.__ctor(n));

And the function that checks for @mustuse violations in DMD does not recognize the lowered code as an assignment expression.