In your example, [1, 2, 3]
is not treated as local variable, but as static one!
Let’s take a look at this code:
fn foo() -> &'static [i32] {
&[1, 2, 3]
}
This works!
Some time ago, RFC 1414: Rvalue Static Promotion was merged: “Promote constexpr rvalues to values in static memory instead of stack slots”. This means that basically all literals you write can live forever. Thus, things like let _: &'static i32 = &42;
also work!
If we avoid using a literal array, we can see the expected error:
fn bar() -> impl Iterator<Item = i32> {
vec![1, 2, 3].iter().map(|&i| i)
}
Here we get the “v
does not live long enough” error.
This isn’t limited to integers or arrays; it applies broadly to any literal that is composed solely of literals:
fn promote_integer() -> &'static i32 {
&42
}
fn promote_float() -> &'static f64 {
&42.42
}
fn promote_str() -> &'static str {
"Hello World!"
}
struct Foo(char);
fn promote_struct() -> &'static Foo {
&Foo('x')
}
Beyond literals, this also works for a tiny number of functions in the standard library, but these were likely a mistake. Deciding on if the result of arbitrary const
functions can be automatically promoted to static
is still an open topic.