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+An immutable variable was reassigned.
+
+Erroneous code example:
+
+```compile_fail,E0384
+fn main() {
+    let x = 3;
+    x = 5; // error, reassignment of immutable variable
+}
+```
+
+By default, variables in Rust are immutable. To fix this error, add the keyword
+`mut` after the keyword `let` when declaring the variable. For example:
+
+```
+fn main() {
+    let mut x = 3;
+    x = 5;
+}
+```