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+A variable used inside an inner function comes from a dynamic environment.
+
+Erroneous code example:
+
+```compile_fail,E0434
+fn foo() {
+    let y = 5;
+    fn bar() -> u32 {
+        y // error: can't capture dynamic environment in a fn item; use the
+          //        || { ... } closure form instead.
+    }
+}
+```
+
+Inner functions do not have access to their containing environment. To fix this
+error, you can replace the function with a closure:
+
+```
+fn foo() {
+    let y = 5;
+    let bar = || {
+        y
+    };
+}
+```
+
+Or replace the captured variable with a constant or a static item:
+
+```
+fn foo() {
+    static mut X: u32 = 4;
+    const Y: u32 = 5;
+    fn bar() -> u32 {
+        unsafe {
+            X = 3;
+        }
+        Y
+    }
+}
+```