Stageleft

Example

Stageleft makes it easy to write type-safe code generators. For example, consider a function that raises a number to a power, but the power is known at compile time. Then, we can compile away the power into repeatedly squaring the base. We can implement a staged program for this:

use stageleft::{q, BorrowBounds, IntoQuotedOnce, Quoted, RuntimeData};

#[stageleft::entry]
fn raise_to_power(_ctx: BorrowBounds<'_>, value: RuntimeData<i32>, power: u32) -> impl Quoted<i32> {
    if power == 1 {
        q!(value).boxed()
    } else if power % 2 == 0 {
        let half_result = raise_to_power(_ctx, value, power / 2);
        q!({
            let v = half_result;
            v * v
        })
        .boxed()
    } else {
        let half_result = raise_to_power(_ctx, value, power / 2);
        q!({
            let v = half_result;
            (v * v) * value
        })
        .boxed()
    }
}

The q!(...) macro quotes code, which means that it will be spliced into the final generated code. We can take in the unknown base as a runtime parameter (RuntimeData<i32>), but the power is known at compile time so we take it as a u32. The _ctx parameter is unused in this case, because we are returning any borrowed data (see stageleft::entry for more details). The .boxed() API allows us to return different pieces of spliced code from the same function, and the impl Quoted<i32> return type tells the compiler that the function will return a piece of code that evaluates to an i32. We can invoke this staged function just like a regular Rust macro:

let result = raise_to_power!(2, 5);
assert_eq!(result, 1024);

But if we expand the macro, we can see that the code has been optimized (simplified for brevity):

{
    fn expand_staged(value: i32) -> i32 {
        let v = {
            let v = {
                let v = value;
                v * v  // 2^2
            };
            (v * v) * value // 2^5
        };
        v * v // 2^10
    }
    expand_staged(2)
}

Setup

Stageleft requires a particular workspace setup, as any crate that uses Stageleft must have an supporting macro crate (whose contents will be automatically generated). For a crate named foo, you will also need a helper crate foo_macro.

The main crate foo will need the following Cargo.toml:

[package]
// ...

[dependencies]
stageleft = "0.1.0"
foo_macro = { path = "../foo_macro" }

[build-dependencies]
stageleft_tool = "0.1.0"

The helper crate should have the following Cargo.toml:

[package]
name = "foo_macro"
// ...

[lib]
proc-macro = true
path = "../foo/src/lib.rs"

[features]
default = ["macro"]
macro = []

[dependencies]
// all dependencies of foo

[build-dependencies]
stageleft_tool = "0.1.0"

Next, you will need to set up build.rs scripts for both of your crates.

In foo:

fn main() {
    stageleft_tool::gen_final!();
}

and in foo_macro:

use std::path::Path;

fn main() {
    stageleft_tool::gen_macro(Path::new("../foo"), "foo");
}