Compiling PHP's conditional statements to Rust

programming-languages

Welcome back to the series. It's time to take another step into compilation land and look at compiling PHP's conditional statements (if statements) into Rust code.

The goal for this post will be compiling the following code:

$guess = readline("Guess a number between 1 and 3: ");
$number = rand(1, 3);

if ($guess == $number) {
    echo "You guessed the number correctly, well done!";
} else {
    echo "The correct answer is " . $number . ". Better luck next time!";
}

Before we start writing some Rust, let's analyze the code and look at the things we'll need to implement.

At the very top of the script we've got some variable assignments. This is a type of expression so we'll need to add some new code to the compile_expression() function.

On the right-hand side of those assignments we're calling some native / first-party PHP functions. These don't exist in our runtime at the moment, so we'll need to implement those in Rust land as part of our runtime.rs file.

We then reach the conditional statements. We'll need to handle the compilation of the structure itself, along with the expressions used inside of blocks.

The condition in our if statement uses the == operator which is referred to as an infix operator. The compile_expression() will need to be updated to handle this new type of expression as well. We'll also need to keep in mind that Rust doesn't have any concept of loose or strict comparisons, instead we'll need to implement thing logic ourself.

Let start by supporting the assignment expression and writing our own implementations of readline() and rand().

Assignment expressions

An assignment expression in the Rust code will be represented with a let statement. PHP variables are all mutable but Rust lets us redeclare and rebind a variable after it's original definition with another let statement. Here's an example.

let foo = 1;
let foo = 2;

The second binding will replace the original without needing to make the original assignment mutable. Let's add this code to compile_expression().

fn compile_expression(expression: &Expression) -> Result<String, CompileError> {
    let result = match expression {
        // ...
        Expression::Assign(target, value) => {
            format!("let {} = {};", compile_expression(target)?, compile_expression(value)?)
        },
        _ => todo!(),
    };

    Ok(result)
}

If we try to run this code, there will actually be an unimplemented / todo panic earlier on in the code. Our parser actually stores random expressions like an assignment inside of a statement, so we need to tell compile_statement() to send our expression statements through the compile_expression() function.

fn compile_statement(statement: &Statement, source: &mut String) -> Result<(), CompileError> {
    match statement {
        // ...
        Statement::Expression { expr } => {
            source.push_str(&compile_expression(expr)?);
        },
        _ => todo!(),
    };

    Ok(())
}

The next thing to do is add a new Int type to the PhpValue enumeration and compile integer expressions. This is so we can eventually call the rand() function.

fn compile_expression(expression: &Expression) -> Result<String, CompileError> {
    let result = match expression {
        // ...
        Expression::Int(i) => format!("PhpValue::from({})", i),
        _ => todo!(),
    };

    Ok(result)
}

And updating our PhpValue enumeration to support creation from an i64.

enum PhpValue {
    String(String),
    Int(i64),
}

impl From<i64> for PhpValue {
    fn from(value: i64) -> Self {
        Self::Int(value)
    }
}

Let's write some native PHP functions in Rust. We'll start with readline().

This function accepts an optional string which will be printed before asking for user input. We'll make this argument required for now since our compiler doesn't know how to handle optional arguments just yet.

pub fn readline(prompt: PhpValue) -> PhpValue {
    print!("{}", prompt);

    std::io::stdout().flush().unwrap();

    let mut result = String::new();
    std::io::stdin().lock().read_line(&mut result).unwrap();

    PhpValue::from(result.trim_end())
}

We flush stdout() so that any previous print!() calls reach the terminal before we lock it for input. We then read in a line of text from the terminal and store it inside of the result variable.

Rust's read_line() method will also include the \n character at the end of the string so using a .trim_end() call will tidy that up.

Time for the rand() function. This is going to require a third-party crate since I don't particularly want to write my own PRNG. We'll be using the defacto rand crate.

use rand::Rng;

fn rand(from: PhpValue, to: PhpValue) -> PhpValue {
    let from: i64 = from.into();
    let to: i64 = to.into();

    let mut rng = rand::thread_rng();

    PhpValue::from(rng.gen_range(from..to))
}

For type conversions between native Rust types and PhpValue, we'll start to implement Into<T> traits. Rust will call the appropriate method based on the inferred type or provided type of the target, in this case the from and to values are both i64 so it will call the Into<i64> method.

impl Into<i64> for PhpValue {
    fn into(self) -> i64 {
        match self {
            Self::Int(i) => i,
            _ => todo!(),
        }
    }
}

Compiling if statements

This might sound a complex task but since we're compiling from one language to another, we can actually take advantage of Rust's own conditional statements. We'll just be translating one syntax to another.

Updating compile_statement() to support conditionals is quite simple:

fn compile_statement(statement: &Statement, source: &mut String) -> Result<(), CompileError> {
    match statement {
        // ...
        Statement::If { condition, then, else_ifs, r#else } => {
            source.push_str("if ");
            source.push_str(&compile_expression(condition)?);
            source.push('{');

            for statement in then {
                compile_statement(statement, source)?;
            }

            source.push('}');

            if let Some(r#else) = r#else {
                source.push_str("else {");
                for statement in r#else {
                    compile_statement(statement, source)?;
                }
                source.push('}');
            }
        },
        _ => todo!(),
    };

    Ok(())
}

It doesn't support any elseif conditions at the moment since those don't exist in our sample code. For now it compiles the initial if statement and checks to see if there is a valid else statement at the end. If there is it compiles that too.

We'll also need to support equality checks inside of compile_expression(). There's a couple of ways to do this.

  1. Manually implement PartialEq on the PhpValue enumeration and perform the equality comparisons there.
  2. Write our own .eq() and .identical() methods since PHP has some type juggling rules that would be easier to implement here.

I'm going to go with option 2 here since I think there will be more long-term flexibility when compared to Rust's own PartialEq trait. Right now the compiler only needs to know about loose comparisons so we'll only implement the eq() method.

impl PhpValue {
    pub fn eq(&self, other: Self) -> bool {
        match (self, &other) {
            (Self::Int(a), Self::String(b)) | (Self::String(b), Self::Int(a)) => match b.parse::<i64>() {
                Ok(b) => *a == b,
                _ => false,
            },
            _ => todo!(),
        }
    }
}

The result of readline() should be a string so the compiler only needs to support loose comparisons between Int and String right now. Rust doesn't let you do this natively so the first step is to try and parse an i64 from the given String.

If that is successful, the result of the function will be an equality check between the a and b. If it fails it means the String couldn't be converted into an i64 and it's impossible for the values to be equal.

Now for the expression compilation itself.

fn compile_expression(expression: &Expression) -> Result<String, CompileError> {
    let result = match expression {
        // ...
        Expression::Infix(lhs, op, rhs) => {
            let lhs = compile_expression(lhs)?;
            let rhs = compile_expression(rhs)?;

            match op {
                InfixOp::Equals => format!("{}.eq({})", lhs, rhs),
                _ => todo!(),
            }
        },
        Expression::Variable(var) => var.to_string(),
        _ => todo!(),
    };

    Ok(result)
}

The compiler didn't know how to handle variables either so that has been added too.

The last type of expression the compiler needs to understand is string concatenation. This is another type of infix operation so it's a case of adding another pattern to the match expression.

fn compile_expression(expression: &Expression) -> Result<String, CompileError> {
    let result = match expression {
        // ...
        Expression::Infix(lhs, op, rhs) => {
            let lhs = compile_expression(lhs)?;
            let rhs = compile_expression(rhs)?;

            match op {
                InfixOp::Equals => format!("{}.eq({})", lhs, rhs),
                InfixOp::Concat => format!("_php_concat({}, {})", lhs, rhs),
                _ => todo!(),
            }
        },
        // ...
        _ => todo!(),
    };

    Ok(result)
}

Instead of mutating existing PhpValue values the runtime will create an entirely new one from 2 separate PhpValue arguments. This function needs to be written in the runtime.rs file alongside our _php_echo() function.

fn _php_concat(left: PhpValue, right: PhpValue) -> PhpValue {
    format!("{}{}", left, right).into()
}

In the previous post we implemented the Display trait for PhpValue which allows us to natively use the enumerations inside of Rust's first-party formatting macros such as format!(), print!() and println!().

With all of that done, it's time to compile the file! And it doesn't work.

Using dependencies

If you haven't read the first blog post, the way this compiler works is by essentially concatenating the compiled PHP code with a runtime.rs file which is written inside of the phpc crate. That file is then stored inside of a temporary directory and compiled using rustc directly.

The problem with this approach is that we can't use any external dependencies inside of the runtime.rs file because they're not going to be linked against during compilation.

One potential solution to this problem is generating a static object file for the runtime and linking against that when compiling the PHP code. As the dependency list grows though the number of libraries that would need to be compiled would grow quite quickly.

I'm instead going to go down the route of ditching rustc and using cargo to build the project instead. The benefit here is that we can let cargo do all of the heavy lifting instead and run away from the rustc API.

I won't go over each step individually but will just paste the new main function code here.

The error handling of commands could be a lot tidier if there was a wrapper around the Command API, but this will do for now. The Cargo.toml file for the PHP project is taken from a new phpc_runtime crate and modified slightly.

Dependencies are now compiled into our project correctly, but there's still a problem. The compiled Rust code isn't memory safe!

The problematic code is coming from our equality expression. We're moving the value out of the current block into the .eq() function which means we can no longer reference it in the main flow of execution. A hacky fix is just to clone the value before we send it through, that way the original value isn't actually being used and a freshly allocated one is instead.

fn compile_expression(expression: &Expression) -> Result<String, CompileError> {
    let result = match expression {
        // ...
        Expression::Infix(lhs, op, rhs) => {
            let lhs = compile_expression(lhs)?;
            let rhs = compile_expression(rhs)?;

            match op {
                InfixOp::Equals => format!("{}.eq(({}).clone())", lhs, rhs),
                InfixOp::Concat => format!("_php_concat({}, {})", lhs, rhs),
                _ => todo!(),
            }
        },
        // ...
        _ => todo!(),
    };

    Ok(result)
}

A better way of doing this is probably with a smart pointer or even a garbage collector. Since we're still prototyping this code a .clone() is fine and won't hurt anybody.

The project successfully compiles but the executable hasn't been moved into the current working directory. A bit of extra logic at the end of the main() function should sort this.

fn main() {
    // ...

    let executable_path = format!("{}/target/release/{}", &temp_path, &file_stem);

    match std::fs::copy(executable_path, format!("./{}", &file_stem)) {
        Ok(_) => {
            println!("> Executable copied.");
        },
        Err(e) => {
            eprintln!("Failed to copy executable file. Error: {:?}", e);
            exit(1);
        },
    };
}

Compiling the project now will create a new guess-a-number file in the current directory. Executing that file and trying to guess a number results in this:

$ ~ ./guess-a-number
Guess a number between 1 and 3: 1
You guessed the number correctly, well done!%

Here's a list of the things that accomplished:

  • Compile variable assignments.
  • Write a native PHP function in Rust.
  • Compile if and else statements.
  • Migrate project compilation to Cargo to allow use of external crates.

All in all this was a pretty successful article. Again, if you made it to the end then thank you.

Until next time.

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