References to Literals in Rust?!

One day messing around with Rust, I found that the following code is valid:

fn main() {
    let x = &0;
}

That's assigning a variable to a reference to the literal 0 - how?! Why?! This absolutely shocked me. Just try doing this in C++ and you'll see why:

error: non-const lvalue reference to type 'int' cannot bind to a temporary of type 'int'
    int &r = 0;
         ^   ~

The literal is a temporary - you can't have a reference to that! String literals are lvalues in C++, but that's a weird special case. That's why you can assign it to a pointer like const char *, but can't get a const int * from an integer literal.

Why is this shocking?

This may not seem that shocking to some. Literals are generally temporary and don't really live anywhere in memory - they're essentially hard coded constants in the program. A reference points to some place in memory. How do we point to something that doesn't live in memory? Well, we can't, and we don't!

Rvalue Static Promotion

This concept in Rust is called rvalue static promotion. We can look at each part to see what that means:

Rvalue: Something that can only be on the right hand side of an assignment. For example, you can't do 1 = x because the literal 1 is an rvalue.

Static: Something that is valid for the whole lifetime of the program.

So we promote the rvalue to a static value in order to take a reference to it. Looking at the program earlier, we can see this in action in Rust's playground. We can see the MIR (one of the intermediate representations of Rust) is:

fn main() -> () {
    let mut _0: ();                      // return place in scope 0 at src/main.rs:1:11: 1:11
    let _1: &i32;                        // in scope 0 at src/main.rs:2:9: 2:10
    let mut _2: &i32;                    // in scope 0 at src/main.rs:2:13: 2:15
    scope 1 {
        debug x => _1;                   // in scope 1 at src/main.rs:2:9: 2:10
    }

    bb0: {
        _2 = const main::promoted[0];    // scope 0 at src/main.rs:2:13: 2:15
                                         // ...
        _1 = _2;                         // scope 0 at src/main.rs:2:13: 2:15
        return;                          // scope 0 at src/main.rs:3:2: 3:2
    }
}

promoted[0] in main: &i32 = {
    let mut _0: &i32;                    // return place in scope 0 at src/main.rs:2:13: 2:15
    let mut _1: i32;                     // in scope 0 at src/main.rs:2:14: 2:15

    bb0: {
        _1 = const 0_i32;                // scope 0 at src/main.rs:2:14: 2:15
        _0 = &_1;                        // scope 0 at src/main.rs:2:13: 2:15
        return;                          // scope 0 at src/main.rs:2:13: 2:15
    }
}

This is a little weird to look at if you've never seen MIR before, but the important part is the line promoted[0] in main: &i32 - that's where we see the promoted variable! Then in the main program we assign with _2 = const main::promoted[0];. So we lift the literal out to a static lifetime in order to return a reference, pretty neat.

Why did they do this?

I find this the interesting part. We can see a lot of the motivation for this in the feature:

The necessary changes in the compiler did already get implemented as part of codegen optimizations (emitting references-to or memcopies-from values in static memory instead of embedding them in the code).

It seems like it was just an easy thing to implement, so they did it. Their drawback is pretty interesting:

One more feature with seemingly ad-hoc rules to complicate the language...

I found this funny. Seems like they just thought "it's easy enough, could be useful, why not?" So, they added a new feature to the Rust language. So many languages get by without this, but the Rust devs said, why not?

It's useful!

You can see this exact thing in action in Rust's source code! At the time of writing, you can see this here:

    dump_mir(infcx.tcx, None, "renumber", &0, body, |_, _| Ok(()));

The fourth parameter is a reference to the literal 0!

Well, I don't know how useful you'd say it is. But, it's an interesting thing in a common compiler that not many languages have.

Comments

[Chayim Friedman]

Note this is not the only promotion in Rust: even code like let x = &"abc".to_owned(); works, even though "abc".to_owned() is not a constant expression and cannot be promoted into a constant. Or let x = &mut 0;, since constant promotion does not work with mutable references (rightfully). What takes place here is temporary lifetime extension, and the compiler extends the lifetime of the temporary to the whole block, like you wrote:

let __temp = "abc".to_owned();
let x = &__temp;
dbg!(x);

Another static promotion, even preceding the static promotion RFC, is empty array static promotion: you can do &mut [], even though you cannot do that for any other type, including ZSTs (let x: &'static mut () = &mut (); is an error, but let x: &'static mut [i32; 0] = &mut [] compiles).