RFC to add retain_mut to Vec and VecDeque

Author: Amanieu

text/0000-vec-retain_mut.md

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+- Feature Name: retain_mut
+- Start Date: 2015-11-6
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Add a `retain_mut` method to `Vec` and `VecDeque` which allows elements to be
+mutated before deciding whether to retain them.
+
+# Motivation
+[motivation]: #motivation
+
+A common pattern with vectors is to look at each element, modify it and
+remove it from the vector if a certain condition is met. The `retain` method
+allows elements to be removed from a vector in-place but does not allow existing
+elements to be modified.
+
+The lack of a `retain_mut` method means that users have to do 2 passes, one with
+`iter_mut` and one with `retain`. The resulting code is harder to read and
+slower. Another, more efficient, way of doing this would be to work with vector
+indices directly and swap elements into place while mutating them, similar to
+what `retain` already does.
+
+# Detailed design
+[design]: #detailed-design
+
+The only difference between `retain_mut` and `retain` is that `retain_mut` takes
+a `FnMut(&mut T)` parameter instead of a `FnMut(&T)`. This allows elements to
+be mutated before deciding whether to retain them.
+
+The new `retain_mut` method will be added to both `Vec` and `VecDeque`.
+
+Here is an example which will decrement each element of a vector and filter out
+elements that have reached a value of zero.
+
+    let mut vec = vec![7, 1, 3, 10];
+    vec.retain_mut(|x| {
+        *x -= 1;
+        *x != 0
+    });
+    assert_eq!(vec, [6, 2, 9]);
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+The `retain` method really should have had a `FnMut(&mut T)` parameter from the
+start, but it is too late to change that. Adding `retain_mut` will result in
+two methods that have almost identical implementations.
+
+# Alternatives
+[alternatives]: #alternatives
+
+Changing the existing `retain` method to take a `FnMut(&mut T)` was considered
+in rust-lang/rust#25477 but this is likely to break a lot of existing code which
+passes a closure defined using `|&x| {...}`.
+
+Another alternative is to not do anything. Users can implement their own version
+of `retain_mut` or they can restructure their code into an `iter_mut` pass on
+the vector followed by a `retain` pass.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None