E0015 "function calls in statics" recommends using constant functions in stable even though constant functions are not stable

[masklinn]

Using rustc 1.3.0 (9a92aaf 2015-09-15)

struct Foo { a: u8 }
fn bar() -> Foo {
    Foo { a: 5 }
}

static foo: Foo = bar();

fn main() {}

> rustc test.rs
test.rs:6:19: 6:24 error: function calls in statics are limited to constant functions, struct and enum constructors [E0015]
test.rs:6 static foo: Foo = bar();
                            ^~~~~
test.rs:6:19: 6:24 help: run `rustc --explain E0015` to see a detailed explanation

> rustc --explain E0015
The only functions that can be called in static or constant expressions are
`const` functions. Rust currently does not support more general compile-time
function execution.

See [RFC 911] for more details on the design of `const fn`s.

[RFC 911]: https://github.com/rust-lang/rfcs/blob/master/text/0911-const-fn.md

Apply suggestion and make bar a const fn:

struct Foo { a: u8 }
const fn bar() -> Foo {
    Foo { a: 5 }
}

static foo: Foo = bar();

fn main() {}

yields

> rustc test.rs
test.rs:2:1: 4:2 error: const fn is unstable
test.rs:2 const fn bar() -> Foo {
test.rs:3     Foo { a: 5 }
test.rs:4 }
error: aborting due to previous error

[alexispurslane]

Should we change the "explain" message?

[Manishearth]

We should make the error message change depending on the compiler version. Perhaps replace it with a note "switch to a nightly compiler to be able to use constant functions". The explain message should just be expanded a bit to mention nightlies.

Of course, we should leave comments there with an issue for fixing these once const fn stabilizes