expected &someEnum but found an enum or structure pattern is misleading

[ozten]

I've had a hard time learning match because this error message is misleading.

Steps to reproduce:

1. Compile a small program which uses extra::json with

match value {
    Number(n) => {
        println(fmt!("%?\n", n))
    },
    _ => {
        println(~"Doh")
    }
}

2. Note compiler error

Actual:

json.rs:20:25: 20:34 error: mismatched types: expected `&extra::json::Json` but found an enum or structure pattern
json.rs:20                          Number(n) => {
                                    ^~~~~~~~~
error: aborting due to previous error

Expected:

error: mismatched types: expected `&extra::json::Json` but found enum extra::json::Json

or more generally
expected &someEnum but found enum T

Great work on the rest of the error message, BTW. I like the line number and arrow.

I eventually figured out that Number is an enum value of extra::json::Json and the fix is:

match value {
    & Number(n) => {
        println(fmt!("%?\n", n))
    },
    _ => {
        println(~"Doh")
    }
}

[bblum]

This particular error comes up a lot for me too. It would be cool to have rustc emit a span_note for this special case saying "you probably missed a & in your pattern" or something.

[catamorphism]

(bug triage) I agree that the error message should be improved; not a milestone blocker IMO.

[flaper87]

Triage bump

Here's an test case:

enum A { B, C }

fn main() {
    match (true, false) {
        B => (),
        _ => ()
    }
}

issue-7867.rs:5:9: 5:10 error: mismatched types: expected `(bool,bool)` but found an enum or structure pattern
issue-7867.rs:5         B => (),
                        ^
error: aborting due to previous error

[Valve]

cc me, this error comes up a lot