alternative ways to indicate method not provided

[mppf]

Spin-off from #8065.

There are currently a few stylized ways to indicate a type is not copyable or assignable (using where false on the init= or a compilerError in the init=). However we might prefer to have alternative ways to do this. This issue discusses other alternatives.

new syntax

e.g. proc init=(other) = none (or something)

Pros:

• new syntax can give us a general structure to provide pure virtual methods (should Chapel allow pure virtual methods? #8566) and opt-in to compiler-generated default initializer or init=
• while the new syntax might not come up often for users, it will be reasonably clear to read and won't contain things like superfluous method bodies

Cons:

• it requires new syntax
• perhaps pure virtual methods and opt-in to compiler-generated default initializer are different enough problems that they should have different solutions

attributes

use attributes on the type, with a generalized attribute syntax (see #14141), to indicate it; e.g. @attribute(no copy init) record R { }

Pros:

• attribute can be reasonably clear to read and apply to type
• able to also indicate things like opting in to default initializer or init=

Cons:

• what functions compiler generates might be too fundamental for attributes