'Contains' method for ranges

text/0000-contains-method-for-ranges.md

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+- Feature Name: contains_method
+- Start Date: 2015-12-28
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Implement a method, `contains()`, for `Range`, `RangeFrom`, and `RangeTo`, checking if a number is in the range.
+
+Note that the alternatives are just as important as the main proposal.
+
+# Motivation
+[motivation]: #motivation
+
+The motivation behind this is simple: To be able to write simpler and more expressive code. This RFC introduces a "syntactic sugar" without doing so.
+
+# Detailed design
+[design]: #detailed-design
+
+Implement a method, `contains()`, for `Range`, `RangeFrom`, and `RangeTo`. This method will check if a number is bound by the range. It will yield a boolean based on the condition defined by the range.
+
+The implementation is as follows (placed in libcore, and reexported by libstd):
+
+```rust
+use core::ops::{Range, RangeTo, RangeFrom};
+
+impl<Idx> Range<Idx> where Idx: PartialOrd<Idx> {
+    fn contains(&self, item: Idx) -> bool {
+        self.start <= item && self.end > item
+    }
+}
+
+impl<Idx> RangeTo<Idx> where Idx: PartialOrd<Idx> {
+    fn contains(&self, item: Idx) -> bool {
+        self.end > item
+    }
+}
+
+impl<Idx> RangeFrom<Idx> where Idx: PartialOrd<Idx> {
+    fn contains(&self, item: Idx) -> bool {
+        self.start <= item
+    }
+}
+
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Lacks of generics (see Alternatives).
+
+# Alternatives
+[alternatives]: #alternatives
+
+## Add a `Contains` trait
+
+This trait provides the method `.contains()` and implements it for all the Range types.
+
+## Add a `.contains<I: PartialEq<Self::Item>>(i: I)` iterator method
+
+This method returns a boolean, telling if the iterator contains the item given as parameter. Using method specialization, this can achieve the same performance as the method suggested in this RFC.
+
+This is more flexible, and provide better performance (due to specialization) than just passing a closure comparing the items to a `any()` method.
+
+## Make `.any()` generic over a new trait
+
+Call this trait, `ItemPattern<Item>`. This trait is implemented for `Item` and `FnMut(Item) -> bool`. This is, in a sense, similar to `std::str::pattern::Pattern`.
+
+Then let `.any()` generic over this trait (`T: ItemPattern<Self::Item>`) to allow `any()` taking `Self::Item` searching through the iterator for this particular value.
+
+This will not achieve the same performance as the other proposals.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.