Use match to match double letter pattern - rust

Question

I'm iterating over a vector of chars. I would like to detect specific sequences of chars ("ab", "cd", "pq" and "xy"). If I find one of these, I want to return false. However if I find a double letter sequence (e.g: "aa"), I want to return true.
I came up with this:
let chars: Vec<char> = line.chars().collect();
for (idx, c) in chars.iter().enumerate() {
if idx > 0 {
match (chars[idx - 1], c) {
('a', 'b') => return false,
('c', 'd') => return false,
('p', 'q') => return false,
('x', 'y') => return false,
(c, c) => return true,
_ => (),
};
}
However when I run this I get the following error:
36 | (c, c) => return true,
| ^ used in a pattern more than once
and I can't understand why.
running rustc --explain E0416 seems to give a solution:
match (chars[idx - 1], c) {
('a', 'b') => return false,
('c', 'd') => return false,
('p', 'q') => return false,
('x', 'y') => return false,
(prev, curr) if &prev == curr => return true,
_ => (),
};
But I would like to understand what's happening here. I'd have expected the equality check to happen with (c, c).

Answer 1

From the documentation:
To create a match expression that compares the values of the outer x and y, rather than introducing a shadowed variable, we would need to use a match guard conditional
The match pattern needs a literal here, or the declaration of new variables, you can't just put another dynamic value to compare with.
You can use the solution given by the compiler, or the more direct
(a, _) if a==*c => return true,
Note that there are ways to avoid collecting into a vector here. You could for example store the previous value in a mutable variable of type char (by taking the first value before the loop if you know the string isn't empty) or into an Option<char>.