[−][src]Crate replace_with
Temporarily take ownership of a value at a mutable location, and replace it with a new value based on the old one.
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This crate provides the function replace_with()
, which is like std::mem::replace()
except it allows the replacement value to be mapped from the original value.
See RFC 1736 for a lot of discussion as to its
merits. It was never merged, and the desired ability to temporarily move out of &mut T
doesn't
exist yet, so this crate is my interim solution.
It's very akin to take_mut
, though uses Drop
instead of
std::panic::catch_unwind()
to react to unwinding, which avoids the optimisation barrier of
calling the extern "C" __rust_maybe_catch_panic()
. As such it's up to ∞x faster. The API also
attempts to make slightly more explicit the behavior on panic – replace_with()
accepts two
closures such that aborting in the "standard case" where the mapping closure (FnOnce(T) -> T
)
panics (as take_mut::take()
does) is
avoided. If the second closure (FnOnce() -> T
) panics, however, then it does indeed abort.
The "abort on first panic" behaviour is available with replace_with_or_abort()
.
Example
Consider this motivating example:
enum States { A(String), B(String), } impl States { fn poll(&mut self) { // error[E0507]: cannot move out of borrowed content *self = match *self { // ^^^^^ cannot move out of borrowed content States::A(a) => States::B(a), States::B(a) => States::A(a), }; } }
Depending on context this can be quite tricky to work around. With this crate, however:
enum States { A(String), B(String), } impl States { fn poll(&mut self) { replace_with_or_abort(self, |self_| match self_ { States::A(a) => States::B(a), States::B(a) => States::A(a), }); } }
Huzzah!
Functions
replace_with | Temporarily takes ownership of a value at a mutable location, and replace it with a new value based on the old one. |
replace_with_and_return | Temporarily takes ownership of a value at a mutable location, and replace it with a new value based on the old one. Lets the closure return a custom value as well. |
replace_with_or_abort | Temporarily takes ownership of a value at a mutable location, and replace it with a new value based on the old one. Aborts on panic. |
replace_with_or_abort_and_return | Temporarily takes ownership of a value at a mutable location, and replace it with a new value based on the old one. Aborts on panic. Lets the closure return a custom value as well. |
replace_with_or_default | Temporarily takes ownership of a value at a mutable location, and replace it with a new value
based on the old one. Replaces with |
replace_with_or_default_and_return | Temporarily takes ownership of a value at a mutable location, and replace it with a new value
based on the old one. Replaces with |