Introduce a T_err type for type errors

[catamorphism]

fn main() {
  let f = 42;

  let _g = if f < 5 {
      f.honk();
  }
  else {
    12
  };
}

If I compile this code, I get two error messages for (basically) the same span:

/Users/TimChevalier/silly.rs:5:6: 5:12 error: attempted access of field honk on type int, but no method implementation was found
/Users/TimChevalier/silly.rs:5       f.honk();
                                     ^~~~~~
/Users/TimChevalier/silly.rs:5:6: 5:14 error: mismatched types: expected function or native function but found ()
/Users/TimChevalier/silly.rs:5       f.honk();
                                     ^~~~~~~~

This kind of thing gets annoying fast. Would it be reasonable to only print an error message for the smallest sub-span when there are multiple error messages such that one span encloses the other? (In this case, the spans aren't exactly the same, but we should see the first error and realize "oh, we don't need to print out the second error, since its span is a superspan of the other")

[nikomatsakis]

An alternative might be to try and avoid reporting derived errors. In this case, for example, the second error occurs because we assign the type of unit to undefined fields (I think that was my fault, but I'm not sure). It would be perhaps nicer to assign a special type (T_err or some such) indicating a compile-time error and then not report type errors that relate to T_err.

[catamorphism]

That would also be totally plausible in this case! I think a similar situation might come up with user-generated errors and so the approach I suggested would be more general, but I can't think of an example right now.

[marijnh]

Assigning ty_bot should actually also work -- that unifies with everything. (Though it'll probably add spurious unreachable statement warnings that have to somehow be suppressed.)

[marijnh]

#1504 outlines another possible solution.

[catamorphism]

Since no one has jumped on this, I'd like to fix it when I get a chance... but I don't know when that will be.

[catamorphism]

Done, pending review.

[nikomatsakis]

Glad to see progress on this!