Missing comma when defaulting fields on a struct

[gilescope]

Context: You have an existing struct which has some fields defaulted. If you add another field but forget the comma like this:

struct A {
    field1: String,
    field2: String
    ..default()
}

Then currently rustc complains about a range. Here's a snapshot of an actual example:

It could be a legitimate range problem but in that situation it might very well be a missing comma.

[pierwill]

@rustbot claim

[pierwill]

Was able to reproduce this without using default:

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=9af9adb8be6e768517f2893a7cb8dad4

[pierwill]

Looks like maybe a fix could be implemented in parse_pat_fields, which has logic for handling commas and dots (..,...).

[pierwill]

There's also this:

rust/compiler/rustc_typeck/src/check/expr.rs

Lines 1691 to 1694 in d4c3643

	// If the last field is a range literal, but it isn't supposed to be, then they probably
	// meant to use functional update syntax.
	//
	// I don't use 'is_range_literal' because only double-sided, half-open ranges count.

[pierwill]

Using nightly in the playground example above, the slightly confusing error that mentions Range is followed by

error[E0063]: missing fields `n1` and `n2` in initializer of `A`
  --> src/main.rs:11:13
   |
11 |     let x = A {
   |             ^ missing `n1` and `n2`
   |
help: to set the remaining fields from `x`, separate the last named field with a comma
   |
13 |         field2: String::from("yo"),..x,
   |                                   +

This is spot on. See #94939 and #51103.

There is arguably room for improvement here, but if this change from stable to nightly is improvement enough (and I think it is), I'm inclined to have this closed. @gilescope @estebank

[gilescope]

Yeah can’t ask for much more than that. Looking good.

…

On Tue, 10 May 2022 at 22:01, pierwill ***@***.***> wrote: Using nightly in the playground example above, the slightly confusing error that mentions Range is followed by error[[E0063]](https://doc.rust-lang.org/nightly/error-index.html#E0063): missing fields `n1` and `n2` in initializer of `A` [--> src/main.rs:11:13 ](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021#) | 11 | let x = A { | ^ missing `n1` and `n2` | help: to set the remaining fields from `x`, separate the last named field with a comma | 13 | field2: String::from("yo"), | + This is spot on. There is arguably room for improvement here, but if this change from stable to nightly is improvement enough (and I think it is), I'm inclined to have this closed. @gilescope <https://github.com/gilescope> @estebank <https://github.com/estebank> — Reply to this email directly, view it on GitHub <#96871 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAGEJCA673APSHCHNREDVF3VJLFB3ANCNFSM5VOTHOYA> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[estebank]

There is arguably room for improvement here, but if this change from stable to nightly is improvement enough (and I think it is), I'm inclined to have this closed.

I'm ok with closing as is, but would love it if you're still interested in looking into finding a way to improve this if we could silence the type mismatch error when encountering this case and only emit the later one that has the suggestion.

[pierwill]

finding a way to... silence the type mismatch error

I'll give this a look. 👍

[estebank]

Current output:

error[E0308]: mismatched types
  --> src/main.rs:14:11
   |
14 |         ..x
   |           ^ expected `String`, found `A`

error[E0063]: missing fields `n1` and `n2` in initializer of `A`
  --> src/main.rs:11:13
   |
11 |     let x = A {
   |             ^ missing `n1` and `n2`
   |
note: this expression may have been misinterpreted as a `..` range expression
  --> src/main.rs:13:17
   |
13 |         field2: String::from("yo")
   |                 ^^^^^^^^^^^^^^^^^^ this expression does not end in a comma...
14 |         ..x
   |         ^^^ ... so this is interpreted as a `..` range expression, instead of functional record update syntax
help: to set the remaining fields from `x`, separate the last named field with a comma
   |
13 |         field2: String::from("yo"), 
   |                                   +

I'm gonna close as is.