Rustc should allow returning reference to return value of const-fn

[Boscop]

It's unintuitive that this doesn't compile:

struct Foo;

const fn bar() -> Foo { Foo }

// doesn't compile
fn foo() -> &'static Foo {
    &bar()
}

// this compiles
fn foo2() -> &'static Foo {
    const FOO: &'static Foo = &bar();
    FOO
}

// this compiles
fn foo3() -> &'static Foo {
    const FOO: Foo = bar();
    &FOO
}

https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=3b50f81085d7e51908ce0378ae865d0f

rustc should just generate a const (or static) behind the scenes and typecheck the code as if the user had written that.

[jonas-schievink]

This is intended behavior according to rust-lang/const-eval#19