Very bad error message for missing trait

[jesse99]

The following code is missing a cmp::Eq type constraint:

// rustc --lib equals.rs
type IMap<K: Copy, V: Copy> = ~[(K, V)];

trait ImmutableMap<K: Copy, V: Copy>
{
    pure fn contains_key(key: K) -> bool;
}

impl<K: Copy, V: Copy> IMap<K, V> : ImmutableMap<K, V>
{
    pure fn contains_key(key: K) -> bool
    {
        vec::find(self, |e| {e.first() == key}).is_some()
    }
}

But the error is extremely confusing (and doesn't get any better if you split the code apart into multiple lines and add type annotations):

equals.rs:13:36: 13:39 error: mismatched types: expected `&const <V41>` but found `'a` (expected &-ptr but found type parameter)
equals.rs:13        vec::find(self, |e| {e.first() == key}).is_some()
                                                 ^~~
equals.rs:13:23: 13:39 error: cannot determine a type for this bounded type parameter: unconstrained type
equals.rs:13        vec::find(self, |e| {e.first() == key}).is_some()
                                    ^~~~~~~~~~~~~~~~

[catamorphism]

This is slightly better ( with d2ad028 ) but I still don't understand why this is the error:

../src/test/run-pass/issue-3480.rs:13:29: 13:45 error: binary operation == cannot be applied to type `'a`
../src/test/run-pass/issue-3480.rs:13         vec::find(self, |e| {e.first() == key}).is_some()
                                                                   ^~~~~~~~~~~~~~~~
../src/test/run-pass/issue-3480.rs:13:28: 13:46 error: mismatched types: expected `bool` but found `'a` (expected bool but found type parameter)
../src/test/run-pass/issue-3480.rs:13         vec::find(self, |e| {e.first() == key}).is_some()

[jesse99]

That's quite a bit better: at least it points you to the operator having problems. My understanding is that it is an error because arbitrary generic types cannot be equated. You need to constrain them with the Eq trait which the code above does not do.

[pcwalton]

Closing.