More stream-2025-04-16.md.

Author: dkorolev

stream-2025-04-16.md

@@ -110,9 +110,13 @@ fn main() {
 }
 ```
 
-Polymorphic type and `Into<T>`: [link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=70746dda53fe77e47ea1c8f68e6ef169).
+Polymorphic type and `Into<T>`: [link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=6e83d0668f7734f455e5b90980d791f9).
 
 ```rust
+#![allow(unused_variables)]
+#![allow(unused_assignments)]
+
+#[derive(Debug)]
 struct DontPanic;
 
 fn do_not_panic() -> DontPanic { DontPanic }
@@ -136,7 +140,22 @@ fn print_int<T: Into<i32>>(val: T) {
 }
 
 fn main() {
+    // This works.
     print_string(do_not_panic());
     print_int(do_not_panic());
+    
+    // Can do this btw!
+    let mut v = Vec::new();
+    v.push(do_not_panic().into());
+    v.push(do_not_panic().into());
+    v.push(do_not_panic());  // But at least one must specify the actual type!
+    println!("{:?}", v);
+    
+    // Same with Option: either at least one proper assignment, or explicit type spec.
+    let mut o: Option<_> = None;
+    o = None;
+    o = None;
+    o = Some(true);  // Need either this assignment or an explicit type annotation.
+    o = None;
 }
 ```