E4044

Record fields are missing. Use .. to ignore them in patterns.

Erroneous Example

struct S { a : Int; b: Int }
let a : Int = match S::{ a: 2, b: 3 } {
  { a: 2 } => 4
  _ => 6
}

The example above tries to match a record with a missing field b, giving the following error on line 3:

Record fields b are unmatched, use `..` to ignore them.

Suggestion

Make sure to provide all fields in the pattern, or simply ignore the missing fields using ..:

struct S { a : Int; b: Int }
let a : Int = match S::{ a: 2, b: 3 } {
  { a: 2, b: 3 } => 4
  { a: 2, .. } => 5
  _ => 6
}