Answer: 1214
Difficulty: 2

# Hint

According to `macro_rules!`, `==` is one token and `=>` is one token.

# Explanation

Adjacent punctuation characters in the input pattern of a `macro_rules!` macro
are grouped according to how those characters are used by native Rust tokens.

[This page][tokens] contains a list of the single-character and multi-character
punctuation tokens involved in the Rust grammar.

[tokens]: https://docs.rs/syn/0.15.22/syn/token/index.html#structs

As one example from that list, `<<=` is a single token because the Rust grammar
uses that sequence of characters to mean [left shift assignment][ShlAssign].
Thus a `macro_rules!` input rule containing `<<=` would only match if all three
characters `<<=` are written consecutively without spaces in the invocation.

[ShlAssign]: https://doc.rust-lang.org/std/ops/trait.ShlAssign.html

But for example `=<<` is not a native token in the Rust grammar. The parser of
`macro_rules!` will decompose this into Rust tokens according to a greedy
process. `=<` is also not a native token, so first we would need to match a `=`
by itself. Then `<<` *is* a native token. Writing `=<<` in a macro rule behaves
exactly the same as writing `= <<`.

Now let's decompose the rules in the quiz code the same way.

- `==>` decomposes as `== >`.
- `= = >` is already decomposed into Rust tokens.
- `== >` is already decomposed.
- `= =>` is already decomposed.

Our macro is the same as if we had written the first rule with a space. The
third rule is unreachable.

```rust
macro_rules! m {
    (== >) => { print!("1"); };
    (= = >) => { print!("2"); };
    (== >) => { print!("3"); };
    (= =>) => { print!("4"); };
}
```

Within `main`, the first and third lines *both* match the first macro rule. The
second line matches the second rule and the fourth line matches the fourth rule.
The output is `1214`.

[Procedural macros][syn] use a more flexible and powerful macro API and can
always distinguish between different spacings of the same characters, such as
`== >` vs `==>`.

[syn]: https://github.com/dtolnay/syn