add misc 7

Author: Kivooeo

code/examples/misc_7.rs

@@ -0,0 +1,19 @@
+macro_rules! capture {
+    ($e:expr) => {
+        transform!($e)
+    };
+}
+
+macro_rules! transform {
+    ($a:tt + $b:tt) => {
+        $a * $b
+    };
+    ($other:expr) => {
+        $other
+    };
+}
+
+fn main() {
+    let x = capture!(2 + 3);
+    println!("{}", x); // what this will print?
+}

src/SUMMARY.md

@@ -16,6 +16,7 @@
 - [Miscellaneous 4](./misc/4.md)
 - [Miscellaneous 5](./misc/5.md)
 - [Miscellaneous 6](./misc/6.md)
+- [Miscellaneous 7](./misc/7.md)
 - [Miscellaneous 8](./misc/8.md)
 - [Miscellaneous 9](./misc/9.md)
 - [Miscellaneous 10](./misc/10.md)

src/misc/7.md

@@ -1,5 +1,20 @@
-<!--
+# Misc 7 @Kivooeo
 
-try to destructure a captured metavariable into its token parts in a macro rules matcher, which does not work
+{{#include ../include/quiz-is-wip.md}}
 
--->
+```rust
+{{#include ../../code/examples/misc_7.rs}}
+```
+
+<details>
+<summary>Solution</summary>
+
+It will prints `5`.
+
+Once a token is captured as an expression (`:expr`), it becomes opaque to macro pattern matching and cannot be destructured into its component tokens. 
+
+When `capture!(2 + 3)` is invoked, the `$e:expr` matcher captures `2 + 3` as a single expression token tree. This captured expression is then passed to `transform!($e)`, which attempts to match it against the pattern `$a:tt + $b:tt`.
+
+However, because `$e` was already captured as an `:expr`, it cannot be pattern-matched as separate tokens anymore. The first arm `$a:tt + $b:tt` fails to match, so the second arm `$other:expr` matches instead, and the expression `2 + 3` is returned as-is, evaluating to `5`.
+
+</details>