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```
``````use smallvec::{Array, SmallVec};
use std::ptr;
use thin_vec::ThinVec;

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

// The implementation of this method is syntactically identical for all the
// different vector types.
macro_rules! flat_map_in_place {
() => {
fn flat_map_in_place<F, I>(&mut self, mut f: F)
where
F: FnMut(T) -> I,
I: IntoIterator<Item = T>,
{
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

// move the read_i'th item out of the vector and map it
// to an iterator
let iter = f(e).into_iter();

for e in iter {
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);

write_i += 1;
}
}
}

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

impl<T> FlatMapInPlace<T> for Vec<T> {
flat_map_in_place!();
}

impl<T, A: Array<Item = T>> FlatMapInPlace<T> for SmallVec<A> {
flat_map_in_place!();
}

impl<T> FlatMapInPlace<T> for ThinVec<T> {
flat_map_in_place!();
}
``````