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Inconsistent "pattern doesn't bind x" messages in or patterns #39698
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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?
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 variableYou put
missing variableon places where that variable isn't missing, which is a bit confusing. Maybe we can work say something likevariable 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.
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).
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.
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.
@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 patternsI switched primary and secondary, also, from what you had. I'm not 100% sure this is right, but it looked better this morning.
Compiling this using rustc nightly (tried with 29dece1 2017-02-08)
prints
This is confusing, ideally I think second error message would be
At least not pointing at
bwhile saying the pattern doesn't bindbwould be better.