Add a speaker note about trying to use a variable as a pattern (#2779)

Author: randomPoison

src/pattern-matching/match.md

@@ -47,6 +47,21 @@ Key Points:
   result in other arms of the original `match` expression being considered.
 - The condition defined in the guard applies to every expression in a pattern
   with an `|`.
+- Note that you can't use an existing variable as the condition in a match arm,
+  as it will instead be interpreted as a variable name pattern, which creates a
+  new variable that will shadow the existing one. For example:
+  ```rust
+  let expected = 5;
+  match 123 {
+      expected => println!("Expected value is 5, actual is {expected}"),
+      _ => println!("Value was something else"),
+  }
+  ```
+  Here we're trying to match on the number 123, where we want the first case to
+  check if the value is 5. The naive expectation is that the first case won't
+  match because the value isn't 5, but instead this is interpreted as a variable
+  pattern which always matches, meaning the first branch will always be taken.
+  If a constant is used instead this will then work as expected.
 
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