inform user where to give a type annotation after a call to collect

[nikomatsakis]

UPDATE: Mentoring instructions below.

---

We are not giving a very good error message in this example:

fn main() {
    let mut dirty_list = (0..5).collect();
    dirty_list.pop();
}

We say:

error[E0619]: the type of this value must be known in this context
 --> src/main.rs:3:5
  |
3 |     dirty_list.pop();
  |     ^^^^^^^^^^^^^^^^

It'd be nice if we at least informed the user that they ought to annotate the type of dirty_list. (Indeed, I know we used to have a bug on this, and I thought we were doing so...?)

It'd be nicer still if we recognized that there is a call to collect and were able to say something like "you need to specify the kind of collection".

cc @estebank

[estebank]

The only thing I'd add is that there are two cases, one where this error happens with a let binding, like in this case, where suggesting giving a type to the binding might be less surprising, and every other case, from having multiple collect to being a return, where the only place to add the type is in the method call itself.

[nikomatsakis]

Those two cases are already handled by the code that seeks to give hints. That code lives in this unfortunately uncommented method:

rust/src/librustc/infer/error_reporting/need_type_info.rs

Line 89 in 70f7d58

pub fn need_type_info(&self, body_id: Option<hir::BodyId>, span: Span, ty: Ty<'tcx>) {

I believe that the idea is that it reports an error that the type ty must be resolved; the error occurs at the given span, and the body_id should be the id of the enclosing function body.

Probably all we have to do is to call this function from here:

rust/src/librustc_typeck/check/mod.rs

Lines 4969 to 4971 in 70f7d58

	type_error_struct!(self.tcx.sess, sp, ty, E0619,
	"the type of this value must be known in this context")
	.emit();

As a bonus, we can remove E0619, which would be subsumed into the existing error code. The call would look something like:

self.need_type_info(Some(self.body_id), sp, ty);

[csmoe]

I’ll take this.

[cengiz-io]

@csmoe hi! don't hesitate to ask anything related to need_type_info to me. I've spent some time around that function 🤣