Make RangeInclusive just a two-field struct (amend 1192)

text/1192-inclusive-ranges.md

@@ -27,26 +27,21 @@ more dots means more elements.
 
 ```rust
 pub enum RangeInclusive<T> {
-  Empty {
-    at: T,
-  },
-  NonEmpty {
-    start: T,
-    end: T,
-  }
+  pub start: T,
+  pub end: T,
 }
 
 pub struct RangeToInclusive<T> {
     pub end: T,
 }
 ```
 
-Writing `a...b` in an expression desugars to `std::ops::RangeInclusive::NonEmpty { start: a, end: b }`. Writing `...b` in an
+Writing `a...b` in an expression desugars to
+`std::ops::RangeInclusive { start: a, end: b }`. Writing `...b` in an
 expression desugars to `std::ops::RangeToInclusive { end: b }`.
 
 `RangeInclusive` implements the standard traits (`Clone`, `Debug`
-etc.), and implements `Iterator`. The `Empty` variant is to allow the
-`Iterator` implementation to work without hacks (see Alternatives).
+etc.), and implements `Iterator`.
 
 The use of `...` in a pattern remains as testing for inclusion
 within that range, *not* a struct match.
@@ -57,6 +52,42 @@ now would be `1. ..` i.e. a floating point number on the left,
 however, fortunately, it is actually tokenised like `1 ...`, and is
 hence an error with the current compiler.
 
+This `struct` definition is maximally consistent with the existing `Range`.
+`a..b` and `a...b` are the same size and have the same fields, just with
+the expected difference in semantics.
+
+The range `a...b` contains all `x` where `a <= x && x <= b`.  As such, an
+inclusive range is non-empty _iff_ `a <= b`.  When the range is iterable,
+a non-empty range will produce at least one item when iterated.  Because
+`T::MAX...T::MAX` is a non-empty range, the iteration needs extra handling
+compared to a half-open `Range`.  As such, `.next()` on an empty range
+`y...y` will produce the value `y` and adjust the range such that
+`!(start <= end)`.  Providing such a range is not a burden on the `T` type as
+any such range is acceptable, and only `PartialOrd` is required so
+it can be satisfied with an incomparable value `n` with `!(n <= n)`.
+A caller must not, in general, expect any particular `start` or `end`
+after iterating, and is encouraged to detect empty ranges with
+`ExactSizeIterator::is_empty` instead of by observing fields directly.
+
+Note that because ranges are not required to be well-formed, they have a
+much stronger bound than just needing successor function: they require a
+`b is-reachable-from a` predicate (as `a <= b`). Providing that efficiently
+for a DAG walk, or even a simpler forward list walk, is a substantially
+harder thing to do than providing a pair `(x, y)` such that `!(x <= y)`.
+
+Implementation note: For currently-iterable types, the initial implementation
+of this will have the range become `1...0` after yielding the final value,
+as that can be done using the `replace_one` and `replace_zero` methods on
+the existing (but unstable) [`Step` trait][step_trait].  It's expected,
+however, that the trait will change to allow more type-appropriate `impl`s.
+For example, a `num::BitInt` may rather become empty by incrementing `start`,
+as `Range` does, since it doesn't to need to worry about overflow.  Even for
+primitives, it could be advantageous to choose a different implementation,
+perhaps using `.overflowing_add(1)` and swapping on overflow, or `a...a`
+could become `(a+1)...a` where possible and `a...(a-1)` otherwise.
+
+[step_trait]: https://github.com/rust-lang/rust/issues/27741
+
 # Drawbacks
 
 There's a mismatch between pattern-`...` and expression-`...`, in that
@@ -66,10 +97,9 @@ semantically.)
 
 The `...` vs. `..` distinction is the exact inversion of Ruby's syntax.
 
-Having an extra field in a language-level desugaring, catering to one
-library use-case is a little non-"hygienic". It is especially strange
-that the field isn't consistent across the different `...`
-desugarings.
+This proposal makes the post-iteration values of the `start` and `end` fields
+constant, and thus useless.  Some of the alternatives would expose the
+last value returned from the iteration, through a more complex interface.
 
 # Alternatives
 
@@ -83,28 +113,30 @@ This RFC proposes single-ended syntax with only an end, `...b`, but not
 with only a start (`a...`) or unconstrained `...`. This balance could be
 reevaluated for usefulness and conflicts with other proposed syntax.
 
-The `Empty` variant could be omitted, leaving two options:
-
 - `RangeInclusive` could be a struct including a `finished` field.
+  This makes it easier for the struct to always be iterable, as the extra
+  field is set once the ends match.  But having the extra field in a
+  language-level desugaring, catering to one library use-case is a little
+  non-"hygienic". It is especially strange that the field isn't consistent
+  across the different `...` desugarings.  And the presence of the public
+  field encourages checkinging it, which can be misleading as
+  `r.finished == false` does not guarantee that `r.count() > 0`.
+- `RangeInclusive` could be an enum with `Empty` and `NonEmpty` variants.
+  This is cleaner than the `finished` field, but still has the problem that
+  there's no invariant maintained: while an `Empty` range is definitely empty,
+  a `NonEmpty` range might actually be empty.  And requiring matching on every
+  use of the type is less ergonomic.  For example, the clamp RFC would
+  naturally use a `RangeInclusive` parameter, but because it still needs
+  to `assert!(start <= end)` in the `NonEmpty` arm, the noise of the `Empty`
+  vs `NonEmpty` match provides it no value.
 - `a...b` only implements `IntoIterator`, not `Iterator`, by
   converting to a different type that does have the field. However,
   this means that `a.. .b` behaves differently to `a..b`, so
   `(a...b).map(|x| ...)` doesn't work (the `..` version of that is
   used reasonably often, in the author's experience)
-- `a...b` can implement `Iterator` for types that can be stepped
-  backwards: the only case that is problematic things cases like
-  `x...255u8` where the endpoint is the last value in the type's
-  range. A naive implementation that just steps `x` and compares
-  against the second value will never terminate: it will yield 254
-  (final state: `255...255`), 255 (final state: `0...255`), 0 (final
-  state: `1...255`). I.e. it will wrap around because it has no way to
-  detect whether 255 has been yielded or not. However, implementations
-  of `Iterator` can detect cases like that, and, after yielding `255`,
-  backwards-step the second piece of state to `255...254`.
-
-  This means that `a...b` can only implement `Iterator` for types that
-  can be stepped backwards, which isn't always guaranteed, e.g. types
-  might not have a unique predecessor (walking along a DAG).
+- The name of the `end` field could be different, perhaps `last`, to reflect
+  its different (inclusive) semantics from the `end` (exclusive) field on
+  the other ranges.
 
 # Unresolved questions
 
@@ -114,3 +146,5 @@ None so far.
 
 * In rust-lang/rfcs#1320, this RFC was amended to change the `RangeInclusive`
   type from a struct with a `finished` field to an enum.
+* In rust-lang/rfcs#1980, this RFC was amended to change the `RangeInclusive`
+  type from an enum to a struct with just `start` and `end` fields.