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+This error code indicates a mismatch between the lifetimes appearing in the
+function signature (i.e., the parameter types and the return type) and the
+data-flow found in the function body.
+
+Erroneous code example:
+
+```compile_fail,E0621
+fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 { // error: explicit lifetime
+                                             //        required in the type of
+                                             //        `y`
+    if x > y { x } else { y }
+}
+```
+
+In the code above, the function is returning data borrowed from either `x` or
+`y`, but the `'a` annotation indicates that it is returning data only from `x`.
+To fix the error, the signature and the body must be made to match. Typically,
+this is done by updating the function signature. So, in this case, we change
+the type of `y` to `&'a i32`, like so:
+
+```
+fn foo<'a>(x: &'a i32, y: &'a i32) -> &'a i32 {
+    if x > y { x } else { y }
+}
+```
+
+Now the signature indicates that the function data borrowed from either `x` or
+`y`. Alternatively, you could change the body to not return data from `y`:
+
+```
+fn foo<'a>(x: &'a i32, y: &i32) -> &'a i32 {
+    x
+}
+```