Improve warning message for UnusedVariable

crates/aiken-lang/src/tipo/error.rs

@@ -1382,9 +1382,18 @@ pub enum Warning {
     },
 
     #[error("I came across an unused variable.\n")]
-    #[diagnostic(help(
-        "No big deal, but you might want to remove it to get rid of that warning."
-    ))]
+    #[diagnostic(help("{}", formatdoc! {
+        r#"No big deal, but you might want to remove it to get rid of that warning.
+
+           You should also know that, unlike in typical imperative languages, unused let-bindings are {fully_ignored} in Aiken.
+           They will not produce any side-effect (such as error calls). Programs with or without unused variables are semantically equivalent.
+
+           If you do want to enforce some side-effects, use {keyword_expect} instead of {keyword_let}.
+        "#,
+        fully_ignored = "fully_ignored".if_supports_color(Stderr, |s| s.bold()),
+        keyword_expect = "expect".if_supports_color(Stderr, |s| s.yellow()),
+        keyword_let = "let".if_supports_color(Stderr, |s| s.yellow()),
+    }))]
     #[diagnostic(code("unused::variable"))]
     UnusedVariable {
         #[label("unused")]