From e2a875143cf1449935b8828189304eaea1708933 Mon Sep 17 00:00:00 2001
From: Kivooeo <Kivooeo123@gmail.com>
Date: Tue, 28 Oct 2025 21:15:59 +0500
Subject: [PATCH] add misc 7

---
 code/examples/misc_7.rs | 19 +++++++++++++++++++
 src/SUMMARY.md          |  1 +
 src/misc/7.md           | 21 ++++++++++++++++++---
 3 files changed, 38 insertions(+), 3 deletions(-)
 create mode 100644 code/examples/misc_7.rs

diff --git a/code/examples/misc_7.rs b/code/examples/misc_7.rs
new file mode 100644
index 0000000..a39ef02
--- /dev/null
+++ b/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?
+}
\ No newline at end of file
diff --git a/src/SUMMARY.md b/src/SUMMARY.md
index 5f791ef..50496d6 100644
--- a/src/SUMMARY.md
+++ b/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)
diff --git a/src/misc/7.md b/src/misc/7.md
index c19adde..b03dc13 100644
--- a/src/misc/7.md
+++ b/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}}
 
--->
\ No newline at end of file
+```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>
\ No newline at end of file