Change the Range struct to support inclusive ranges

Author: Stebalien

text/0000-inclusive-ranges.md

@@ -0,0 +1,72 @@
+- Feature Name: inclusive_ranges
+- Start Date: (fill me in with today's date, YYYY-MM-DD)
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+
+Change the Range struct to allow all combinations of inclusive/exclusive ranges.
+
+# Motivation
+
+Regardless of how the range syntax will eventually work, the underlying data
+structure will need to support both inclusive and exclusive ranges. If we don't
+do this now, libraries will define their own way of specifying ranges (as the
+rand crate and the BTreeMap collection have already done) and these custom
+implementations may become entrenched. 
+
+# Detailed design
+
+The design is simply to:
+
+  1. Remove the `Unbounded` variant of the `Bound<Idx>` enum and move it to the
+     ops module.
+  2. Change `Range` (and all variations there of) to use `Bound<Idx>` instead of
+     `Idx` for start and end.
+
+```rust
+pub enum Bound<Idx> {
+    Inclusive(Idx),
+    Exclusive(Idx),
+}
+pub struct Range<Idx> {
+    pub start: Bound<Idx>,
+    pub end: Bound<Idx>,
+}
+pub struct RangeFrom<Idx> {
+    pub start: Bound<Idx>,
+}
+pub struct RangeTo<Idx> {
+    pub end: Bound<Idx>,
+}
+pub struct RangeFull;
+```
+
+# Drawbacks
+
+* The Range struct becomes larger.
+* When checking the bounds, you have to check if they are inclusive/exclusive.
+
+# Alternatives
+
+One obvious alternative is the following:
+
+```rust
+pub enum Bound<Idx> {
+    Inclusive(Idx),
+    Exclusive(Idx),
+    Unbounded,
+}
+pub struct Range<Idx> {
+    pub start: Bound<Idx>,
+    pub end: Bound<Idx>,
+}
+```
+
+However, this would make it impossible to do things like only allowing full
+ranges (see slicing OsString).
+
+# Unresolved questions
+
+We might want some way to extract inclusive bounds from any *integral* range to
+make bounds checking easier.