Values created by const fns aren't rvalue static promoted

[sfackler]

This function compiles due to rvalue static promotion:

fn bar() -> &'static i32 {
    &1
}

This, however, does not:

struct Foo(i32);

impl Foo {
    const fn new(n: i32) -> Foo {
        Foo(n)
    }
}

fn foo() -> &'static Foo {
    &Foo::new(1)
}

error[E0515]: cannot return reference to temporary value
  --> src/lib.rs:10:5
   |
10 |     &Foo::new(1)
   |     ^-----------
   |     ||
   |     |temporary value created here
   |     returns a reference to data owned by the current function

error: aborting due to previous error

Explicitly creating a static does work, however:

struct Foo(i32);

impl Foo {
    const fn new(n: i32) -> Foo {
        Foo(n)
    }
}

fn foo() -> &'static Foo {
    static FOO: Foo = Foo::new(1);
    &FOO
}

[hanna-kruppe]

This is intentional. A blanket "any const fn call is promotable" is problematic and more restricted versions are, of course, more controversial to design. See #53851 for the PR that implemented the current restriction and rust-lang/const-eval#19 for further discussion.

[Centril]

I think however that the diagnostic message could be clearer about this being intentional; cc @oli-obk @estebank

[sfackler]

Oh, interesting. Yeah, it does seem like it'd be nice if the error message would be a bit more explicit.