static tuple used as a match arm yields weird error

[alan-andrade]

hello rusties,

I have a program similar to this:

type Foo = (i32, i32);
static ON: Foo = (1, 1);
static OFF: Foo = (0, 0);

fn main() {
    match (1, 1) {
        ON => { println!("{}" , ON) },
        OFF => { println!("{}" , OFF) }
    }
}

When I try to compile I get the following error:

<anon>:2:18: 2:24 error: unsupported constant expr
<anon>:2 static ON: Foo = (1, 1);
                          ^~~~~~

Is this behavior expected?
http://j.mp/1kjSiMc

[japaric]

I'd expect this to fail with "non-exhaustive pattern".

cc @Kimundi (probably related to RFC #17)

[huonw]

The following all have the same error message/cause but with slightly different types: #6533, #13626 and #13731.

[ghost]

This is interesting as to fix it will probably require rewriting the constant expressions into pattern nodes and inlining them in the pattern matrix.

[ghost]

I started a branch on this but I think it's a bit of a dead-end. What I think would ideally happen is the following:

• Statics get type checked.
• For each PatIdent that references a static, that static gets compile-time evaluated into an equivalent Pat node.
• Patterns get type checked, sanity checked and codegened.

This introduces mutability to the AST, which is a major change to what the current design is. I'm all ears for other solutions. :-) I'd love to fix this as this is just one of several reports on the same issue.