dead code lint to say "never constructed" for variants

[zackmdavis]

As reported in #19140, #44083, and #44565, some users were confused when
the dead-code lint reported an enum variant to be "unused" when it was
matched on (but not constructed). This wording change makes it clearer
that the lint is in fact checking for construction.

We continue to say "used" for all other items (it's tempting to say
"called" for functions and methods, but this turns out not to be
correct: functions can be passed as arguments and the dead-code lint
isn't special-casing that or anything).

Resolves #19140.

r? @pnkfelix

[nagisa]

We continue to say "used" for all other items (it's tempting to say
"called" for functions and methods, but this turns out not to be
correct: functions can be passed as arguments and the dead-code lint
isn't special-casing that or anything).

Same also is true for enum variants though. Much like with functions:

enum Foo {
    A(usize),
}

fn main() {
    drop::<fn(usize)->Foo>(Foo::A);
}

will not warn, despite the enum variant never having been constructed. It might be worthwhile to find some other wording.

e.g. rather than saying that enum variant is never used, say that the enum variant constructor is never used or something?

[arielb1]

@nagisa

This is a static analysis, so it can never be 100% precise (see if false { Foo::A(0) }).

I think "variant is never constructed" is a fluent way of saying "variant constructor is never used".

@bors r+

[bors]

📌 Commit 1a9dc2e has been approved by arielb1