Merge pull request #2707 from petrochenkov/ddpat

Mini-RFC: Make `..` a pattern syntactically

Author: Centril

text/2707-dotdot-patterns.md

@@ -0,0 +1,96 @@
+- Feature Name: `dotdot_patterns`
+- Start Date: 2019-06-01
+- RFC PR: [rust-lang/rfcs#2707](https://github.com/rust-lang/rfcs/pull/2707)
+- Rust Issue: [rust-lang/rust#62254](https://github.com/rust-lang/rust/issues/62254)
+
+# Summary
+[summary]: #summary
+
+Make `..` a pattern rather than a syntactic fragment of some other patterns.
+
+# Motivation
+[motivation]: #motivation
+
+The change simplifies pattern grammar and simplifies use of `..` in macros.  
+In particular, the `pat` macro matcher will now accept `..` and `IDENT @ ..`.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+`..` becomes a pattern syntacticaly.
+The notable consequences of this are listed below.
+
+- `pat` macro matcher will now accept `..` and more complex pattern containing `..`,
+for example `ref x @ ..`.
+
+- A trailing comma is accepted after `..` in tuple struct, tuple or slice pattern.
+```rust
+Variant(a, b, ..,) // OK
+```
+
+- Some nonsensical code can now be accepted under `cfg(FALSE)`.
+```rust
+#[cfg(FALSE)]
+Tuple(.., a, ..) // OK
+```
+
+`..` in "inappropriate" positions is still rejected semantically.
+```rust
+let .. = 10; // Semantic error, `..` is not a part of a "list" pattern
+let Option(.., ..) = 11; // Semantic error, multiple `..`s in a single "list" pattern
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+Pattern grammar is extended with a new production
+```
+PAT = ..
+```
+Special productions allowing `..` in tuple struct, tuple and slice patterns are subsumed by this
+new production and removed.
+
+Semantically, the `..` pattern is accepted
+- Immediately inside a tuple struct/variant pattern `Tuple(PAT, .., PAT)`
+- Immediately inside a tuple pattern `(PAT, .., PAT)`
+- Immediately inside a slice pattern `[PAT, .., PAT]`.
+- Immediately inside a binding pattern inside a slice pattern `[PAT, BINDING @ .., PAT]`.
+
+An error is produced if this pattern is used in any other position.
+
+An error is produced if more that one `..` or `BINDING @ ..` pattern is used inside its containing
+tuple struct / tuple / slice pattern.
+
+`(..)` is still a tuple pattern and not a parenthesized `..` pattern for backward compatibility.
+
+Note that `..` in struct patterns
+```rust
+Struct { field1: PAT, field2, .. }
+```
+is still not a pattern, but a fragment of a struct pattern syntax.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+More meaningless code may be accepted under `cfg(FALSE)` where semantic checks are not performed.
+
+# Rationale and alternatives
+[rationale-and-alternatives]: #rationale-and-alternatives
+
+See "Motivation" for the rationale.  
+Status quo is always an alternative.
+
+# Prior art
+[prior-art]: #prior-art
+
+This RFC is a follow up to https://github.com/rust-lang/rfcs/pull/2359.
+
+# Unresolved questions
+[unresolved-questions]: #unresolved-questions
+
+None so far.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+Accept `BINDING @ ..` in tuple patterns, `(head, tail @ ..)`.