An βorβ pattern was used where the variable bindings are not consistently bound across patterns.
Erroneous code example:
match x {
Some(y) | None => { /* use y */ } // error: variable `y` from pattern #1 is
// not bound in pattern #2
_ => ()
}
RunHere, y
is bound to the contents of the Some
and can be used within the
block corresponding to the match arm. However, in case x
is None
, we have
not specified what y
is, and the block will use a nonexistent variable.
To fix this error, either split into multiple match arms:
let x = Some(1);
match x {
Some(y) => { /* use y */ }
None => { /* ... */ }
}
Runor, bind the variable to a field of the same type in all sub-patterns of the or pattern:
let x = (0, 2);
match x {
(0, y) | (y, 0) => { /* use y */}
_ => {}
}
RunIn this example, if x
matches the pattern (0, _)
, the second field is set
to y
. If it matches (_, 0)
, the first field is set to y
; so in all
cases y
is set to some value.