rust/compiler/rustc_data_structures/src/map_in_place.rs

use smallvec::{Array, SmallVec};
use std::ptr;

pub trait MapInPlace<T>: Sized {
    fn map_in_place<F>(&mut self, mut f: F)
    where
        F: FnMut(T) -> T,
    {
        self.flat_map_in_place(|e| Some(f(e)))
    }

    fn flat_map_in_place<F, I>(&mut self, f: F)
    where
        F: FnMut(T) -> I,
        I: IntoIterator<Item = T>;
}

impl<T> MapInPlace<T> for Vec<T> {
    fn flat_map_in_place<F, I>(&mut self, mut f: F)
    where
        F: FnMut(T) -> I,
        I: IntoIterator<Item = T>,
    {
        let mut read_i = 0;
        let mut write_i = 0;
        unsafe {
            let mut old_len = self.len();
            self.set_len(0); // make sure we just leak elements in case of panic

            while read_i < old_len {
                // move the read_i'th item out of the vector and map it
                // to an iterator
                let e = ptr::read(self.get_unchecked(read_i));
                let iter = f(e).into_iter();
                read_i += 1;

                for e in iter {
                    if write_i < read_i {
                        ptr::write(self.get_unchecked_mut(write_i), e);
                        write_i += 1;
                    } else {
                        // If this is reached we ran out of space
                        // in the middle of the vector.
                        // However, the vector is in a valid state here,
                        // so we just do a somewhat inefficient insert.
                        self.set_len(old_len);
                        self.insert(write_i, e);

                        old_len = self.len();
                        self.set_len(0);

                        read_i += 1;
                        write_i += 1;
                    }
                }
            }

            // write_i tracks the number of actually written new items.
            self.set_len(write_i);
        }
    }
}

impl<T, A: Array<Item = T>> MapInPlace<T> for SmallVec<A> {
    fn flat_map_in_place<F, I>(&mut self, mut f: F)
    where
        F: FnMut(T) -> I,
        I: IntoIterator<Item = T>,
    {
        let mut read_i = 0;
        let mut write_i = 0;
        unsafe {
            let mut old_len = self.len();
            self.set_len(0); // make sure we just leak elements in case of panic

            while read_i < old_len {
                // move the read_i'th item out of the vector and map it
                // to an iterator
                let e = ptr::read(self.get_unchecked(read_i));
                let iter = f(e).into_iter();
                read_i += 1;

                for e in iter {
                    if write_i < read_i {
                        ptr::write(self.get_unchecked_mut(write_i), e);
                        write_i += 1;
                    } else {
                        // If this is reached we ran out of space
                        // in the middle of the vector.
                        // However, the vector is in a valid state here,
                        // so we just do a somewhat inefficient insert.
                        self.set_len(old_len);
                        self.insert(write_i, e);

                        old_len = self.len();
                        self.set_len(0);

                        read_i += 1;
                        write_i += 1;
                    }
                }
            }

            // write_i tracks the number of actually written new items.
            self.set_len(write_i);
        }
    }
}