Inconsistent "pattern doesn't bind x" messages in or patterns

[osa1]

Compiling this using rustc nightly (tried with 29dece1 2017-02-08)

enum T {
    T1(i32),
    T2(i32),
}

fn main() {
    match T::T1(123) {
        T::T1(a) | T::T2(b) => { println!("{:?}", a); }
    }
}

prints

error[E0408]: variable `a` from pattern #1 is not bound in pattern #2
 --> pat.rs:8:20
  |
8 |         T::T1(a) | T::T2(b) => { println!("{:?}", a); }
  |                    ^^^^^^^^ pattern doesn't bind `a`

error[E0408]: variable `b` from pattern #2 is not bound in pattern #1
 --> pat.rs:8:26
  |
8 |         T::T1(a) | T::T2(b) => { println!("{:?}", a); }
  |                          ^ pattern doesn't bind `b`

This is confusing, ideally I think second error message would be

error[E0408]: variable `b` from pattern #2 is not bound in pattern #1
 --> pat.rs:8:26
  |
8 |         T::T1(a) | T::T2(b) => { println!("{:?}", a); }
  |         ^^^^^^^^ pattern doesn't bind `b`

At least not pointing at b while saying the pattern doesn't bind b would be better.

[estebank]

There're two error messages because they are pointing out two different problems:

• the second pattern doesn't bound a variable from the first pattern
• the second pattern bounds a variable that isn't present in the first pattern

If your code where

enum T {
    T1(i32),
    T2(),
}

fn main() {
    match T::T1(123) {
        T::T1(a) | T::T2() => { println!("{:?}", a); }
    }
}

the output would be

error[E0408]: variable `a` from pattern #1 is not bound in pattern #2
 --> pat.rs:8:20
  |
8 |         T::T1(a) | T::T2() => { println!("{:?}", a); }
  |                    ^^^^^^^ pattern doesn't bind `a`

---

That being said, I think the output should be changed to something along the lines of

error[E0408]: variable `a` from pattern #1 is not bound in pattern #2
 --> file.rs:8:20
  |
8 |         T::T1(a) | T::T2(b) => { println!("{:?}", a); }
  |               -    ^^^^^^^^ pattern doesn't bind `a`
  |               |
  |               unbound variable

error[E0408]: variable `b` from pattern #2 is not bound in pattern #1
 --> file.rs:8:26
  |
8 |         T::T1(a) | T::T2(b) => { println!("{:?}", a); }
  |         --------         ^ unbound variable
  |         |
  |         pattern doesn't bind `b`

It can probably be improved further for cases with more patterns. The current output is

error[E0408]: variable `a` from pattern #1 is not bound in pattern #2
  --> file.rs:10:20
   |
10 |         T::T1(a) | T::T2(b) | T::T3(c) | T::T4(a)=> { println!("{:?}", a); }
   |                    ^^^^^^^^ pattern doesn't bind `a`

error[E0408]: variable `b` from pattern #2 is not bound in pattern #1
  --> file.rs:10:26
   |
10 |         T::T1(a) | T::T2(b) | T::T3(c) | T::T4(a)=> { println!("{:?}", a); }
   |                          ^ pattern doesn't bind `b`

error[E0408]: variable `a` from pattern #1 is not bound in pattern #3
  --> file.rs:10:31
   |
10 |         T::T1(a) | T::T2(b) | T::T3(c) | T::T4(a)=> { println!("{:?}", a); }
   |                               ^^^^^^^^ pattern doesn't bind `a`

error[E0408]: variable `c` from pattern #3 is not bound in pattern #1
  --> file.rs:10:37
   |
10 |         T::T1(a) | T::T2(b) | T::T3(c) | T::T4(a)=> { println!("{:?}", a); }
   |                                     ^ pattern doesn't bind `c`

and I feel it should be closer to

error[E0408]: variable `a` from pattern #1 is not bound in patterns #2 and #3
  --> file.rs:10:20
   |
10 |         T::T1(a) | T::T2(b) | T::T3(c) | T::T4(a)=> { println!("{:?}", a); }
   |               -    ^^^^^^^^   ^^^^^^^^ pattern doesn't bind `a`
   |               |    |
   |               |    pattern doesn't bind `a`
   |               missing variable

error[E0408]: variable `b` from pattern #2 and variable `c` from pattern #3 are not bound in pattern #1
  --> file.rs:10:26
   |
10 |         T::T1(a) | T::T2(b) | T::T3(c) | T::T4(a)=> { println!("{:?}", a); }
   |         --------         ^          ^ unused variable
   |         |                |
   |         |                unused variable
   |         pattern doesn't bind `b` or `c`

cc @jonathandturner thoughts?

[estebank]

@osa1, could you check out #39713 and tell me if that wording makes more sense to you?

Colored output for that PR:

[sophiajt]

I think I like the general idea, but I'm not sure about the specific approach. For example, in this one:

error[E0408]: variable `a` from pattern #1 is not bound in patterns #2 and #3
  --> file.rs:10:20
   |
10 |         T::T1(a) | T::T2(b) | T::T3(c) | T::T4(a)=> { println!("{:?}", a); }
   |               -    ^^^^^^^^   ^^^^^^^^ pattern doesn't bind `a`
   |               |    |
   |               |    pattern doesn't bind `a`
   |               missing variable

You put missing variable on places where that variable isn't missing, which is a bit confusing. Maybe we can work say something like variable not in all patterns?

Also, does it really say this?

error[E0408]: variable `b` from pattern #2 is not bound in pattern #1
  --> file.rs:10:26
   |
10 |         T::T1(a) | T::T2(b) | T::T3(c) | T::T4(a)=> { println!("{:?}", a); }
   |                          ^ pattern doesn't bind `b`

If so, that sounds like a bug unto itself.

[estebank]

You put missing variable on places where that variable isn't missing, which is a bit confusing. Maybe we can work say something like variable not in all patterns?

I'm ok with changing the wording to that. Do you want me to set you as reviewer?

Also, does it really say this? If so, that sounds like a bug unto itself.

Yeah, and I agree. That's the reason I actually started the PR (the grouping was merely gold plating).

[estebank]

Another option is to do

error[E0408]: multiple variables not present in all patterns
  --> $DIR/issue-39698.rs:20:37
   |
20 |         T::T1(a, d) | T::T2(d, b) | T::T3(c) | T::T4(a) => { println!("{:?}", a); }
   |         ------^--^-   ---------^-   ------^-
   |               |  |             |          |
   |               |  |             |          variable not in all patterns
   |               |  |             variable not in all patterns
   |               |  variable not in all patterns
   |               variable not in all patterns

error: aborting due to 1 previous errors

which has less vertical clutter, but loses clarity on which variable is missing on which patterns, as well as a more generic error message.

[osa1]

I added a comment in #39713.

I should add, maybe I'm unfamiliar with Rust's error messages and this is actually consistent in Rust's other messages. I just realized this while trying some language features and wanted to report as it looked weird. The PR doesn't actually solve the problem I tried to describe, maybe because I did a bad job at that. I tried to explain further in the PR.

[sophiajt]

@estebank - having all of them in the same error seems a bit confusing. I don't think we do that with many errors, so it'd be something users wouldn't expect. Maybe in the future we can work on more compact errors?

For now, I think combining our ideas together, something like this would be a decent solution for the time being:

error[E0408]: variable `a` from pattern #1 is not bound in patterns #2 and #3
  --> file.rs:10:20
   |
10 |         T::T1(a) | T::T2(b) | T::T3(c) | T::T4(a)=> { println!("{:?}", a); }
   |               ^    --------   -------- pattern doesn't bind `a`
   |               |    |
   |               |    pattern doesn't bind `a`
   |               variable not in all patterns

I switched primary and secondary, also, from what you had. I'm not 100% sure this is right, but it looked better this morning.

[estebank]

@osa1 does the new output in #39713 make sense to you?

[osa1]

@estebank this looks amazing, thanks for all the work.