Add RFC 'contains_method_for_various_collections'

Author: LukasKalbertodt

text/0000-contains-method-for-various-collections.md

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+- Feature Name: contains_method_for_various_collections
+- Start Date: 2016-03-16
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Add a `contains` method to `VecDeque` and `LinkedList` that checks if the
+collection contains a given item.
+
+# Motivation
+[motivation]: #motivation
+
+A `contains` method exists for the slice type `[T]` and for `Vec` through
+`Deref`, but there is no easy way to check if a `VecDeque` or `LinkedList`
+contains a specific item. Currently, the shortest way to do it is something
+like:
+
+```rust
+vec_deque.iter().any(|e| e == item)
+```
+
+While this is not insanely verbose, a `contains` method has the following
+advantages:
+
+- the name `contains` expresses the programmer's intent...
+- ... and thus is more idiomatic
+- it's as short as it can get
+- programmers that are used to call `contains` on a `Vec` are confused by the
+  non-existence of the method for `VecDeque` or `LinkedList`
+
+# Detailed design
+[design]: #detailed-design
+
+Add the following method to `std::collections::VecDeque`:
+
+```rust
+impl<T> VecDeque<T> {
+    /// Returns `true` if the `VecDeque` contains an element equal to the
+    /// given value.
+    pub fn contains(&self, x: &T) -> bool
+        where T: PartialEq<T>
+    {
+        // implementation with a result equivalent to the result
+        // of `self.iter().any(|e| e == x)`
+    }
+}
+```
+
+Add the following method to `std::collections::LinkedList`:
+
+```rust
+impl<T> LinkedList<T> {
+    /// Returns `true` if the `LinkedList` contains an element equal to the
+    /// given value.
+    pub fn contains(&self, x: &T) -> bool
+        where T: PartialEq<T>
+    {
+        // implementation with a result equivalent to the result
+        // of `self.iter().any(|e| e == x)`
+    }
+}
+```
+
+The new methods should probably be marked as unstable initially and be
+stabilized later.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Obviously more methods increase the complexity of the standard library, but in
+case of this RFC the increase is rather tiny.
+
+While `VecDeque::contains` should be (nearly) as fast as `[T]::contains`,
+`LinkedList::contains` will probably be much slower due to the cache
+inefficient nature of a linked list. Offering a method that is short to
+write and convenient to use could lead to excessive use of said method
+without knowing about the problems mentioned above.
+
+# Alternatives
+[alternatives]: #alternatives
+
+There are a few alternatives:
+
+- add `VecDeque::contains` only and do not add `LinkedList::contains`
+- do nothing, because -- technically -- the same functionality is offered
+  through iterators
+- also add `BinaryHeap::contains`, since it could be convenient for some use
+  cases, too
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None so far.