Introduce `X..`, `..X`, and `..=X` range patterns Tracking issue: rust-lang#67264 Feature gate: `#![feature(half_open_range_patterns)]` --------------------------- In this PR, we introduce range-from (`X..`), range-to (`..X`), and range-to-inclusive (`..=X`) patterns. These correspond to the `RangeFrom`, `RangeTo`, and `RangeToInclusive` expression forms introduced with the same syntaxes. The correspondence is both syntactic and semantic (in the sense that e.g. a `X..` pattern matching on a scrutinee `s` holds exactly when `(X..).contains(&s)` holds). --------------------------- Noteworthy: - The compiler complexity added with this PR is around 10 lines (discounting new tests, which account for the large PR size). - `...X` is accepted syntactically with the same meaning as `..=X`. This is done primarily to simplify and unify the implementation & spec. If-and-when we decide to make `X...Y` a hard error on a new edition, we can do the same for `...X` patterns as well. - `X...` and `X..=` is rejected syntactically just like it is for the expression equivalents. We should perhaps make these into semantic restrictions (cc @petrochenkov). - In HAIR, these half-open ranges are represented by inserting the max/min values for the approprate types. That is, `X..` where `X: u8` would become `X..=u8::MAX` in HAIR (note the `..=` since `RangeFrom` includes the end). - Exhaustive integer / char matching does not (yet) allow for e.g. exhaustive matching on `0usize..` or `..5usize | 5..` (same idea for `isize`). This would be a substantially more invasive change, and could be added in some other PR. - The issues with slice pattern syntax has been resolved as we decided to use `..` to mean a "rest-pattern" and `[xs @ ..]` to bind the rest to a name in a slice pattern. - Like with rust-lang#35712, which provided `X..Y` range patterns, this is not yet backed up by an RFC. I'm providing this experimental implementation now to have something concrete to discuss. I would be happy to provide an RFC for this PR as well as for rust-lang#35712 to finalize and confirm the ideas with the larger community. Closes rust-lang/rfcs#947. --------------------------- r? @varkor cc @matthewjasper @oli-obk I would recommend reviewing this (in particular HAIR-lowering and pattern parsing changes) with whitespace changes ignored.

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