Allow arbitrary value to appear in pattern

[bombless]

For now you can only use const/static value in pattern, e.g.

let x = 0;
const MAGIC: i32 = 42;
match x {
    MAGIC => {}
    _ => {}
}

First, it's confusing that if MAGIC is not already defined as constant the matching will be a binding instead.
Second, if that MAGIC value cannot be determined at compile-time this will not work.

A workaround would be

#[derive(Eq, PartialEq)]
struct NonCopy;
match NonCopy {
    ref x if *x == NonCopy => {}
    _ => {}
}

Although it works, maybe we can do better.

There are 2 solutions I can imagine:

• We cannot use block in pattern for now, so block syntax can be used here, which means

match NonCopy {
    {NonCopy} => {}
    _ => {}
}

• @ inside pattern is limited, so this is also possible IMO:

match NonCopy {
    @NonCopy => {}
    _ => {}
}

Hopefully this proposal can solve #1059 in this way,

match pair {
    (x, @x) => {}
    _ => {}
}

but that will be another discussion.

EDIT: fix typo

[bombless]

Due to the limitation of const/static values today, this will lead to new questions to answer.
I think @x => /* ... */ can be a sugar of ref y if *y == x => /* ... */ with y shadowed, I suggest we enforce an Eq comparation here, it is just sugar anyway.

[nagisa]

This has an issue of making exhaustiveness check useless. I think match
should stay as it is currently.
On Apr 17, 2015 8:37 AM, "York Xiang" [email protected] wrote:

Due to the limitation of const/static values today, this will lead to new
questions to answer.
I think @x => /* ... / can be a sugar of ref y if *x == y => / ... */
with y shadowed, I suggest we enforce a Eq comparation here, it is just
sugar anyway.

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#1067 (comment).

[Centril]

Closing in favor of #263.