Better error message for trait bounds on structs.

[smosher]

I was looking at this issue on SO:

https://stackoverflow.com/questions/24363369/polymorphism-in-rust

The reported error doesn't help much for someone who doesn't already know better:

error: trait bounds are not allowed in structure definitions
test.rs:20:1: 22:2 error: trait bounds are not allowed in structure definitions
test.rs:20 struct Board<R: Rules> {
test.rs:21     rules: R
test.rs:22 }
error: aborting due to previous error

The explanation in the first response is straight-forward: You need to refine that in the trait implementation, not the struct definition.

There's enough information present when someone tries to apply trait bounds in a struct to spit out the impl declaration line by way of example, so in this case (note R, Rules, and Board are all present in intelligible positions in the error report above):

test.rs:20:1: 22:2 error: trait bounds are not allowed in structure definitions. 
[ ... error context as above ... ]
Use a trait implementation instead, eg.:
    impl<R: Rules> Board<R> { ... }

[nikomatsakis]

Note that this will not be an error much longer (trait bounds in struct declarations).

[smosher]

Ah, in that case it's probably not worth going the distance to producing an example impl declaration.

[Valve]

@nikomatsakis Niko, why will it not be an error much longer?

[nikomatsakis]

@Valve https://github.com/rust-lang/rfcs/blob/master/active/0011-bounded-type-parameters.md

[ghost]

This is now implemented, closing.