Show other possible implementations in the "trait not implemented" message

[Manishearth]

Currently, if I try to index a [T] by a u8 (buf[1u8], or whatever), I'll get the error

test.rs:6:15: 6:19 error: the trait `core::ops::Index<u8>` is not implemented for the type `[u8]` [E0277]
test.rs:6 println!("{}",y[x])

It would be helpful in such a situation for a note saying that there is an implementation of Index<usize>.

Basically: If a trait isn't found to be implemented, but a version of that trait with different type parameters is, show some help messages containing the candidates. We do something like this already when one tries to call a nonexistent method on a type where bringing some trait into scope might make it work.

In this case, it should show something along the lines of:

test.rs:7:22: 7:22 help: the following implementations were found:
test.rs:7:22: 7:22 help: implementation #1: `Index<usize>`
``

[nikomatsakis]

I'd be willing to mentor this, though I'd not call it easy. Perhaps medium.

[Manishearth]

Agreed. (I can also provide some help on this if someone wants to work on it.)

[leejunseok]

I'd like to work on this, though I might need a lot of hand-holding. Hopefully, the mentor tag means this is acceptable?

[Manishearth]

Sounds good to me.

The relevant bits of code are here, and the trait checks are done here.

You can get a TraitRef out of the predicate in the first link and then compare the self type with the trait, or something.

[nham]

@leejunseok Are you still working on this? If not, I'm interested in giving it a try.

[leejunseok]

nope, feel free

On Mon, Jul 13, 2015, 22:36 Nick Hamann notifications@github.com wrote:

@leejunseok https://github.com/leejunseok Are you still working on
this? If not, I'm interested in giving it a try.

—
Reply to this email directly or view it on GitHub
#21659 (comment).

[fhahn]

@nham are you still working on this? If not, I would like to give it a try.

[nham]

Nope, go for it!