No way to change mutable fields of a tag arm.

[froystig]

Suppose we have

type foo = tag(arm(mutable int));

The language binds identifiers in alt patterns as slots-local-to-the-case-block with value copied from the one sitting in the tag type. That is:

let foo farm = arm(5);
alt (farm) {
    case (arm(x)) {
        /* x is a local slot holding 5 here, assigning to
           it does not change the value in farm */
        x = 6;
    }
}

This is possibly counterintuitive. Maybe x should be an alias (caveat emptor: may require extra care). In any case, we probably want to support some kind of way of writing to these fields, if we are going to allow them to be marked mutable.

[graydon]

Shifted to rustc. May need to revisit there.

[marijnh]

Do we really want mutable tag fields? We seem to get by fine without them, and they don't really fit into any tradition of algebraic data types.

[msullivan]

It looks like we no longer allow mutable fields in tag arms. Trying to use one produces "warning: ignoring deprecated 'mutable' type constructor".