wasmtime_environ/
scopevec.rs

1use crate::prelude::*;
2use core::cell::RefCell;
3
4/// Small data structure to help extend the lifetime of a slice to a higher
5/// scope.
6///
7/// This is currently used during component translation where translation in
8/// general works on a borrowed slice which contains all input modules, but
9/// generated adapter modules for components don't live within the original
10/// slice but the data structures are much easier if the dynamically generated
11/// adapter modules live for the same lifetime as the original input slice. To
12/// solve this problem this `ScopeVec` helper is used to move ownership of a
13/// `Vec<T>` to a higher scope in the program, then borrowing the slice from
14/// that scope.
15pub struct ScopeVec<T> {
16    data: RefCell<Vec<Box<[T]>>>,
17}
18
19impl<T> ScopeVec<T> {
20    /// Creates a new blank scope.
21    pub fn new() -> ScopeVec<T> {
22        ScopeVec {
23            data: Default::default(),
24        }
25    }
26
27    /// Transfers ownership of `data` into this scope and then yields the slice
28    /// back to the caller.
29    ///
30    /// The original data will be deallocated when `self` is dropped.
31    pub fn push(&self, data: Vec<T>) -> &mut [T] {
32        let data: Box<[T]> = data.into();
33        let len = data.len();
34
35        let mut storage = self.data.borrow_mut();
36        storage.push(data);
37        let ptr = storage.last_mut().unwrap().as_mut_ptr();
38
39        // This should be safe for a few reasons:
40        //
41        // * The returned pointer on the heap that `data` owns. Despite moving
42        //   `data` around it doesn't actually move the slice itself around, so
43        //   the pointer returned should be valid (and length).
44        //
45        // * The lifetime of the returned pointer is connected to the lifetime
46        //   of `self`. This reflects how when `self` is destroyed the `data` is
47        //   destroyed as well, or otherwise the returned slice will be valid
48        //   for as long as `self` is valid since `self` owns the original data
49        //   at that point.
50        //
51        // * This function was given ownership of `data` so it should be safe to
52        //   hand back a mutable reference. Once placed within a `ScopeVec` the
53        //   data is never mutated so the caller will enjoy exclusive access to
54        //   the slice of the original vec.
55        //
56        // This all means that it should be safe to return a mutable slice of
57        // all of `data` after the data has been pushed onto our internal list.
58        unsafe { core::slice::from_raw_parts_mut(ptr, len) }
59    }
60}
61
62#[cfg(test)]
63mod tests {
64    use super::ScopeVec;
65    use crate::prelude::*;
66
67    #[test]
68    fn smoke() {
69        let scope = ScopeVec::new();
70        let a = scope.push(Vec::new());
71        let b = scope.push(vec![1, 2, 3]);
72        let c = scope.push(vec![4, 5, 6]);
73        assert_eq!(a.len(), 0);
74        b[0] = 4;
75        c[2] = 5;
76        assert_eq!(a, []);
77        assert_eq!(b, [4, 2, 3]);
78        assert_eq!(c, [4, 5, 5]);
79    }
80}
wasmtime_environ/scopevec.rs

wasmtime_environ/
scopevec.rs
