Reorder error message parts for single source lines

[rpjohnst]

I don't know that this is the level of detail that matters yet, but here goes.

The single-line errors don't follow the narrative structure in the order that they're displayed:

error[E0506]: cannot write to `x` while borrowed
 --> <anon>:4:5
   |
 3 |     x.push(x.pop().unwrap());
   |     - ---- ^^^^^^^^^^^^^^^^
   |     | |    write to `x` occurs here, while borrow is still in active use
   |     | borrow is later used here, during the call
   |     `x` borrowed here

It would be easier to understand if the vertical lines connecting the spans to the messages could be tweaked somehow to allow the messages to be vertically ordered to match the code's execution order.

[nikomatsakis]

This is a good comment. I'm not sure how best to handle the formatting. I think it's mildly orthogonal from the RFC, but still worthy of consideration in general.

@estebank may have thoughts here!

[estebank]

I see a few solutions to this, none of them exactly nice:

error[E0506]: cannot write to `x` while borrowed
 --> <anon>:4:5
   |
 3 |     x.push(x.pop().unwrap());
   |            ^^^^^^^^^^^^^^^^ write to `x` occurs here, while borrow is still in active use
   = note: `x` borrowed here
   |
 3 |     x.push(x.pop().unwrap());
   |     ^
   = note: borrow is later used here, during the call
   |
 3 |     x.push(x.pop().unwrap());
   |       ^^^^ 

error[E0506]: cannot write to `x` while borrowed
 --> <anon>:4:5
   |     `x` borrowed here
   |     | borrow is later used here, during the call
   |     | |
 3 |     x.push(x.pop().unwrap());
   |     - ---- ^^^^^^^^^^^^^^^^ write to `x` occurs here, while borrow is still in active use

error[E0506]: cannot write to `x` while borrowed
 --> <anon>:4:5
   |
 3 |     x.push(x.pop().unwrap());
   |     - ---- ^^^^^^^^^^^^^^^^ write to `x` occurs here, while borrow is still in active use
   |     | |
   |     `x` borrowed here
   |       |
   |       borrow is later used here, during the call    

error[E0506]: cannot write to `x` while borrowed
 --> <anon>:4:5
   |
 3 |     x.push(x.pop().unwrap());
   |     - ---- ^^^^^^^^^^^^^^^^ write to `x` occurs here, while borrow is still in active use
   |     | |________________
   |     `x` borrowed here |
   |                       |
   |                       borrow is later used here, during the call

Of all of these, I'm partial to the first one, as it is the easiest to read. Diagnostics could have a flag stating that the readability is heavily reliant on the order of the spans. If it is set and the span labels would be rendered out of order, use that presentation, so that we get the more compact representation for:

error[E0506]: cannot write to `x` while borrowed
 --> <anon>:4:5
   |
 3 |     this_is_a_long_variable_name.push(x.pop().unwrap());
   |     ---------------------------- ---- ^^^^^^^^^^^^^^^^ write to `x` occurs here, while borrow is still in active use
   |     |                            |
   |      `x` borrowed here           borrow is later used here, during the call

[nikomatsakis]

Of all of these, I'm partial to the first one, as it is the easiest to read.

Funny, this is what we used to do, but we moved away, because it was kind of hard to read -- it felt easier and lighterweight to see the labels "in context".

[Nashenas88]

What about numbering them?

error[E0506]: cannot write to `x` while borrowed
 --> <anon>:4:5
   |
 3 |     x.push(x.pop().unwrap());
   |     - ---- ^^^^^^^^^^^^^^^^
   |     | |    3: write to `x` occurs here, while borrow is still in active use
   |     | 2: borrow is later used here, during the call
   |     1: `x` borrowed here

In that case my vote would be for something like this, where both the numbering and ordering of the text make it very clear (at least to me) of how to read the text:

error[E0506]: cannot write to `x` while borrowed
 --> <anon>:4:5
   |     1: `x` borrowed here
   |     | 2: borrow is later used here, during the call
   |     | |
 3 |     x.push(x.pop().unwrap());
   |     - ---- ^^^^^^^^^^^^^^^^ 3: write to `x` occurs here, while borrow is still in active use

[rpjohnst]

Isn't that the wrong ordering for the narrative structure? It should be:

1. x borrowed here
2. write to x occurs here, while borrow is still in active use
3. borrow is later used here, during the call

[nikomatsakis]

Numbering is an interesting idea, though:

error[E0506]: cannot write to `x` while borrowed
 --> <anon>:4:5
   |
 3 |     x.push(x.pop().unwrap());
   |     - ---- ^^^^^^^^^^^^^^^^
   |     | |    [2] then, write to `x` occurs here, while borrow is still in active use
   |     | [3] later, borrow is used here, during the call
   |     [1] first, `x` is borrowed here

Note that this particular case -- a function call -- is the "worst case" in terms of violating expectations. The others work more smoothly.

[estebank]

Coming back to this, I don't think the following looks too bad:

error[E0506]: cannot write to `x` while borrowed
 --> <anon>:4:5
   |
   |     `x` borrowed here
   |     |
 3 |     x.push(x.pop().unwrap());
   |     - ---- ^^^^^^^^^^^^^^^^ write to `x` occurs here, while borrow is still in active use
   |       |
   |       borrow is later used here, during the call

[rpjohnst]

IMO that one combined with numbering would be the easiest to follow.