

There was a recent langdev Stack Exchange question about this very topic. It’s a bit trickier to design than it might seem at first.
Suppose we require a keyword – say var
– before all binding patterns. This results in having to write things like
for (&(var x1, var y1, var z1), &(var x2, var y2, var z2)) in points.iter().tuple_windows() {}
,
which is quite a bit more verbose than the current
for (&(x1, y1, z1), &(x2, y2, z2)) in points.iter().tuple_windows() {}
.
Not to mention you’ll have to write let var x = 0;
just to declare a variable, unless you redesign the language to allow you to just write var x = 0
(and if you do that, you’ll also have to somehow support a coherent way to express if let Some(x) = arr.pop() {}
and let Some(x) = arr.pop() else {todo!()}
).
Suppose we require a keyword – say const
– before all value-matching patterns that look like variables. Then, what’s currently
match (left.next(), right.next()) {
(Some(l), Some(r)) => {}
(Some(l), None) => {}
(None, Some(r)) => {}
(None, None) => {}
}
turns into either the inconsistently ugly
match (left.next(), right.next()) {
(Some(l), Some(r)) => {}
(Some(l), const None) => {}
(const None, Some(r)) => {}
(const None, const None) => {}
}
or the even more verbose
match (left.next(), right.next()) {
(const Some(l), const Some(r)) => {}
(const Some(l), const None) => {}
(const None, const Some(r)) => {}
(const None, const None) => {}
}
and you always run the risk of forgetting a const
and accidentally binding a new match-all variable named None
– the main footgun that syntactically distinguishing binding and value-matching patterns was meant to avoid in the first place.
Suppose we require a sigil such as before one type of pattern. Probably the best solution in my opinion, but that’s one symbol that can no longer be used for other things in a pattern context. Also, if you’re already using sigils before variable names for other purposes (I’ve been sketching out a language where a pointer variable
$x
can be auto-dereferenced by writing x
), doubling up is really unpleasant.
…So I can understand why Rust chose to give the same, most concise possible syntax for both binding and value-matching patterns. At least compiler warnings (unused, non-snake-case variables) are there to provide some protection from accidentally turning one into the other.
The fact that
x += y
modifieslist
s in place might be surprising if you’re expecting it to be exactly equivalent tox = x + y
.