Suggest annotating traits rather than methods when appropriate

[hkmatsumoto]

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=d2a32fe1f3460b6583534eb53d045165

fn foo() {
    let _ = "foo".into::<Option<_>>();
}

The current output is:

Compiling playground v0.0.1 (/playground)
error[E0107]: this associated function takes 0 generic arguments but 1 generic argument was supplied
   --> src/lib.rs:2:19
    |
2   |     let _ = "foo".into::<Option<_>>();
    |                   ^^^^------------- help: remove these generics
    |                   |
    |                   expected 0 generic arguments
    |
note: associated function defined here, with 0 generic parameters

For more information about this error, try `rustc --explain E0107`.
error: could not compile `playground` due to previous error

Ideally the output should look like:

error[E0107]: this associated function takes 0 generic arguments but 1 generic argument was supplied
   --> src/lib.rs:2:19
    |
2   |     let _ = "foo".into::<Option<_>>();
    |                   ^^^^------------- help: remove these generics
    |                   |
    |                   expected 0 generic arguments
    |
note: associated function defined here, with 0 generic parameters
help: `Into` trait takes 1 generic argument, try annotating that instead: `Into::<Option<_>>::into("foo")

When a user called into() with an annotation, it's helpful to suggest T::from(_) too.

[chansuke]

@hkmatsumoto If it's ok, I would like to give a shot. If you plan to work on it, please go ahead.

[hkmatsumoto]

It's yours :)

[chansuke]

@rustbot claim

[chansuke]

Hi, I would like to ask a question...
I'm reading the rustc_typecheck code now and am thinking of extending the error messages by adding fn diagnostic_extended to wrong_number_of_generics_args.rs. My plan is that start adding some fields (like ty::TraitRef) to pub struct WrrongnarofGenericArgs<'a, 'tcx> {} and dbg! the output to get the value of trait information like Into for a new error message.
Is my approach out of line or is there a better approach for that.

[hkmatsumoto]

My plan is that start adding some fields (like ty::TraitRef) to pub struct WrrongnarofGenericArgs<'a, 'tcx> {} and dbg! the output to get the value of trait information like Into for a new error message.

You don't have to hold information of TraitRef in WrongNumberOfGenericArgs because we can get the info on-demand by the combination of TyCtxt::trait_of_item and TyCtxt::generics_of.
Say you'd add a method of WrongNumberOfGenericArgs, the code will be something like this:

let trait_: Option<DefId> = self.tcx.trait_of_item(self.def_id);
let generics: &Generics = self.tcx.generics_of(trait_.unwrap());

For getting the name of the trait in question, TyCtxt::item_name or TyCtxt::def_path_str will be handy for you.

[chansuke]

I appreciate your answer.

[vincenzopalazzo]

@chansuke are you still working on this? :)