1 This error occurs when there is an unsatisfied outlives bound involving an
2 elided region and a generic type parameter or associated type.
4 Erroneous code example:
7 fn no_restriction<T>(x: &()) -> &() {
8 with_restriction::<T>(x)
11 fn with_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {
16 Why doesn't this code compile? It helps to look at the lifetime bounds that are
17 automatically added by the compiler. For more details see the documentation for
18 [lifetime elision]( https://doc.rust-lang.org/reference/lifetime-elision.html).
20 The compiler elides the lifetime of `x` and the return type to some arbitrary
21 lifetime `'anon` in `no_restriction()`. The only information available to the
22 compiler is that `'anon` is valid for the duration of the function. When
23 calling `with_restriction()`, the compiler requires the completely unrelated
24 type parameter `T` to outlive `'anon` because of the `T: 'a` bound in
25 `with_restriction()`. This causes an error because `T` is not required to
26 outlive `'anon` in `no_restriction()`.
28 If `no_restriction()` were to use `&T` instead of `&()` as an argument, the
29 compiler would have added an implied bound, causing this to compile.
31 This error can be resolved by explicitly naming the elided lifetime for `x` and
32 then explicily requiring that the generic parameter `T` outlives that lifetime:
35 fn no_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {
36 with_restriction::<T>(x)
39 fn with_restriction<'a, T: 'a>(x: &'a ()) -> &'a () {