incorrectly ignoring a field with { _ } yields suggestion that assumes _ is a valid field name

[Lotterleben]

hi,
I think I ran into a bit of an error reporting hickup with my erroneous code.

Given the following code: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=cb47de4bf36f023caa0c21fc77ec4271

enum TopException {
    HardFault { stack_overflow: bool },
    Other,
}

fn main() {
    let top_exception = Some(TopException::Other);

    if let Some(TopException::HardFault {_}) = top_exception {
     //                                  ^^ oh no
        todo!();
    }
}

The current output is:

error: expected identifier, found reserved identifier `_`
  --> src/main.rs:10:42
   |
10 |     if let Some(TopException::HardFault {_}) = top_exception {
   |                                          ^ expected identifier, found reserved identifier

error[E0026]: variant `TopException::HardFault` does not have a field named `_`
  --> src/main.rs:10:42
   |
10 |     if let Some(TopException::HardFault {_}) = top_exception {
   |                                          ^ variant `TopException::HardFault` does not have this field

error[E0027]: pattern does not mention field `stack_overflow`
  --> src/main.rs:10:17
   |
10 |     if let Some(TopException::HardFault {_}) = top_exception {
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing field `stack_overflow`
   |
help: include the missing field in the pattern
   |
10 |     if let Some(TopException::HardFault {_, stack_overflow}) = top_exception {
   |                                           ^^^^^^^^^^^^^^^^
help: if you don't care about this missing field, you can explicitly ignore it
   |
10 |     if let Some(TopException::HardFault {_, ..}) = top_exception {
   |                                           ^^^^

error: aborting due to 3 previous errors

Some errors have detailed explanations: E0026, E0027.
For more information about an error, try `rustc --explain E0026`.
error: could not compile `playground`

To learn more, run the command again with --verbose.

Applying this suggestion yields


error: expected identifier, found reserved identifier `_`
  --> src/main.rs:10:42
   |
10 |     if let Some(TopException::HardFault {_, stack_overflow}) = top_exception {
   |                                          ^ expected identifier, found reserved identifier

error[E0026]: variant `TopException::HardFault` does not have a field named `_`
  --> src/main.rs:10:42
   |
10 |     if let Some(TopException::HardFault {_, stack_overflow}) = top_exception {
   |                                          ^ variant `TopException::HardFault` does not have this field

Instead, I would've expected the suggestion to be along the lines of:


    if let Some(TopException::HardFault {stack_overflow: _}) = top_exception {

or at least

    if let Some(TopException::HardFault {stack_overflow}) = top_exception {

@jonas-schievink pointed out to me that this is likely caused by rustc recognizing that _ is not a valid field pattern, trying to recover from it by treating it as something like _: _, which also fails because there's no field named _ – but of course this is not a valid field name at all, which should be recognized as such?

[estebank]

We could at least not emit E0026 in this case.

[Lotterleben]

Hrmmm. I just noticed that I'd totally forgotten to check out what the error looks like on nightly, which imo is already much more helpful because it explicitly states that _ is reserved:

error: expected identifier, found reserved identifier `_`
  --> src/main.rs:10:42
   |
10 |     if let Some(TopException::HardFault {_}) = top_exception {
   |                                          ^ expected identifier, found reserved identifier

error[E0026]: variant `TopException::HardFault` does not have a field named `_`
  --> src/main.rs:10:42
   |
10 |     if let Some(TopException::HardFault {_}) = top_exception {
   |                                          ^ variant `TopException::HardFault` does not have this field

error[E0027]: pattern does not mention field `stack_overflow`
  --> src/main.rs:10:17
   |
10 |     if let Some(TopException::HardFault {_}) = top_exception {
   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^ missing field `stack_overflow`
   |
help: include the missing field in the pattern
   |
10 |     if let Some(TopException::HardFault {_, stack_overflow }) = top_exception {
   |                                           ^^^^^^^^^^^^^^^^^^
help: if you don't care about this missing field, you can explicitly ignore it
   |
10 |     if let Some(TopException::HardFault {_, .. }) = top_exception {

And the E0027 error message adds some extra context. My only remaining nitpick would be the misleading help: suggestion.
(The reason I arrived at this error in the first place at all was because I keep forgetting that I can't use _ to denote "I don't care, throw it away" like that, but need to be explicit about what I want to ignore. So I'm not 100% sure I would've made the connection from the hints as they are now.)

[compiler-errors]

Triage:

error: expected field pattern, found `_`
  --> src/main.rs:10:42
   |
10 |     if let Some(TopException::HardFault {_}) = top_exception {
   |                                          ^
   |
help: to omit remaining fields, use `..`
   |
10 |     if let Some(TopException::HardFault {..}) = top_exception {
   |                                          ~~

I consider this to be sufficiently fixed.