boa-cat 0.6.0

Tree-walking ECMAScript interpreter built on ecma-syntax-cat. v0.6.0 adds `await` evaluator dispatch + async function call wrapping (async track chunk 3 of 5-7): `await Resolved(v)` -> `v`, `await Rejected(v)` -> throw `v`, `await Pending` -> TypeError (no continuation in the synchronous tree-walking model), `await non_promise` -> pass through. Calling an async function (`FunctionDef::is_async()` flag, threaded from `Function::is_async()` / `ArrowFunction::is_async()`) wraps the body's normal completion in a `Resolved` promise and its throw completion in a `Rejected` promise.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52

//! Step-budget newtype.  Every evaluator function spends one unit of fuel
//! per call; when fuel runs out, evaluation aborts with
//! [`Error::FuelExhausted`].
//!
//! [`Error::FuelExhausted`]: crate::error::Error::FuelExhausted

use crate::error::Error;

/// A fuel budget.  Construct with [`Fuel::new`]; spend with [`Fuel::spend`].
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub struct Fuel {
    limit: u64,
    remaining: u64,
}

impl Fuel {
    /// A fresh budget of `limit` steps.
    #[must_use]
    pub fn new(limit: u64) -> Self {
        Self {
            limit,
            remaining: limit,
        }
    }

    /// The original budget.
    #[must_use]
    pub fn limit(&self) -> u64 {
        self.limit
    }

    /// Steps still available.
    #[must_use]
    pub fn remaining(&self) -> u64 {
        self.remaining
    }

    /// Spend one unit; returns [`Error::FuelExhausted`] when exhausted.
    ///
    /// # Errors
    ///
    /// [`Error::FuelExhausted`] when there is no remaining fuel.
    pub fn spend(self) -> Result<Self, Error> {
        match self.remaining {
            0 => Err(Error::FuelExhausted { limit: self.limit }),
            n => Ok(Self {
                limit: self.limit,
                remaining: n - 1,
            }),
        }
    }
}