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