RFC: Finalize syntax for slice patterns with subslices

text/2359-subslice-pattern-syntax.md

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+- Feature Name: `slice_patterns`
+- Start Date: 2018-03-08
+- RFC PR: [rust-lang/rfcs#2359](https://github.com/rust-lang/rfcs/pull/2359)
+- Rust Issue: [rust-lang/rust#62254](https://github.com/rust-lang/rust/issues/62254)
+
+# Summary
+[summary]: #summary
+
+Permit matching sub-slices and sub-arrays with the syntax `..`.  
+Binding a variable to the expression matched by a subslice pattern can be done
+using syntax `<IDENT> @ ..` similar to the existing `<IDENT> @ <PAT>` syntax, for example:
+
+```rust
+// Binding a sub-array:
+let [x, y @ .., z] = [1, 2, 3, 4]; // `y: [i32, 2] = [2, 3]`
+
+// Binding a sub-slice:
+let [x, y @ .., z]: &[u8] = &[1, 2, 3, 4]; // `y: &[i32] = &[2, 3]`
+```
+
+# Motivation
+[motivation]: #motivation
+
+## General motivation
+Stabilization of slice pattern with subslices is currently blocked on finalizing syntax for
+these subslices.  
+This RFC proposes a syntax for stabilization.
+
+## Motivation for the specific syntax
+
+### The shortcut form: `..`
+
+This form is already used in the meaning "rest of the list" in struct patterns, tuple struct
+patterns and tuple patterns so it would be logical to use it for slice patterns as well.  
+And indeed, in unstable Rust `..` is used in this meaning since long before 1.0.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Sub-slices and sub-arrays can be matched using `..` and `<IDENT> @ ..` can be used to bind
+these sub-slices and sub-arrays to an identifier.
+
+```rust
+// Matching slices using `ref` and `ref mut`patterns:
+let mut v = vec![1, 2, 3];
+match v[..] {
+    [1, ref subslice @ .., 4] => assert_eq!(subslice.len(), 1), // typeof(subslice) == &[i32]
+    [5, ref subslice @ ..] => assert_eq!(subslice.len(), 2), // typeof(subslice) == &[i32]
+    [ref subslice @ .., 6] => assert_eq!(subslice.len(), 2), // typeof(subslice) == &[i32]
+    [x, .., y] => assert!(v.len() >= 2),
+    [..] => {} // Always matches
+}
+match v[..] {
+    [1, ref mut subslice @ .., 4] => assert_eq!(subslice.len(), 1), // typeof(subslice) == &mut [i32]
+    [5, ref mut subslice @ ..] => assert_eq!(subslice.len(), 2), // typeof(subslice) == &mut [i32]
+    [ref mut subslice @ .., 6] => assert_eq!(subslice.len(), 2), // typeof(subslice) == &mut [i32]
+    [x, .., y] => assert!(v.len() >= 2),
+    [..] => {} // Always matches
+}
+
+// Matching slices using default-binding-modes:
+let mut v = vec![1, 2, 3];
+match &v[..] {
+    [1, subslice @ .., 4] => assert_eq!(subslice.len(), 1), // typeof(subslice) == &[i32]
+    [5, subslice @ ..] => assert_eq!(subslice.len(), 2), // typeof(subslice) == &[i32]
+    [subslice @ .., 6] => assert_eq!(subslice.len(), 2), // typeof(subslice) == &[i32]
+    [x, .., y] => assert!(v.len() >= 2),
+    [..] => {} // Always matches
+}
+match &mut v[..] {
+    [1, subslice @ .., 4] => assert_eq!(subslice.len(), 1), // typeof(subslice) == &mut [i32]
+    [5, subslice @ ..] => assert_eq!(subslice.len(), 2), // typeof(subslice) == &mut [i32]
+    [subslice @ .., 6] => assert_eq!(subslice.len(), 2), // typeof(subslice) == &mut [i32]
+    [x, .., y] => assert!(v.len() >= 2),
+    [..] => {} // Always matches
+}
+
+// Matching slices by value (error):
+let mut v = vec![1, 2, 3];
+match v[..] {
+    [x @ ..] => {} // ERROR cannot move out of type `[i32]`, a non-copy slice
+}
+
+// Matching arrays by-value and by reference (explicitly or using default-binding-modes):
+let mut v = [1, 2, 3];
+match v {
+  [1, subarray @ .., 3] => assert_eq!(subarray, [2]), // typeof(subarray) == [i32; 1]
+  [5, subarray @ ..] => has_type::<[i32; 2]>(subarray), // typeof(subarray) == [i32; 2]
+  [subarray @ .., 6] => has_type::<[i32, 2]>(subarray), // typeof(subarray) == [i32; 2]
+  [x, .., y] => has_type::<i32>(x),
+  [..] => {},
+}
+match v {
+  [1, ref subarray @ .., 3] => assert_eq!(subarray, [2]), // typeof(subarray) == &[i32; 1]
+  [5, ref subarray @ ..] => has_type::<&[i32; 2]>(subarray), // typeof(subarray) == &[i32; 2]
+  [ref subarray @ .., 6] => has_type::<&[i32, 2]>(subarray), // typeof(subarray) == &[i32; 2]
+  [x, .., y] => has_type::<&i32>(x),
+  [..] => {},
+}
+match &mut v {
+  [1, subarray @ .., 3] => assert_eq!(subarray, [2]), // typeof(subarray) == &mut [i32; 1]
+  [5, subarray @ ..] => has_type::<&mut [i32; 2]>(subarray), // typeof(subarray) == &mut [i32; 2]
+  [subarray @ .., 6] => has_type::<&mut [i32, 2]>(subarray), // typeof(subarray) == &mut [i32; 2]
+  [x, .., y] => has_type::<&mut i32>(x),
+  [..] => {},
+}
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+`..` can be used as a pattern fragment for matching sub-slices and sub-arrays.
+
+The fragment's syntax is:
+```
+SUBSLICE = .. | BINDING @ ..
+BINDING = ref? mut? IDENT
+```
+
+The subslice fragment incorporates into the full subslice syntax in the same way as the `..`
+fragment incorporates into the stable tuple pattern syntax (with regards to allowed number of
+subslices, trailing commas, etc).
+
+`@` can be used to bind the result of `..` to an identifier.
+
+`..` is treated as a "non-reference-pattern" for the purpose of determining default-binding-modes,
+and so shifts the binding mode to by-`ref` or by-`ref mut` when used to match a subsection of a
+reference or mutable reference to a slice or array.
+
+When used to match against a non-reference slice (`[u8]`), `x @ ..` would attempt to bind
+by-value, which would fail due a move from a non-copy type `[u8]`.
+
+`..` is not a full pattern syntax, but rather a part of slice, tuple and tuple
+struct pattern syntaxes. In particular, `..` is not accepted by the `pat` macro matcher.
+`BINDING @ ..` is also not a full pattern syntax, but rather a part of slice pattern syntax, so
+it is not accepted by the `pat` macro matcher either.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+None known.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+More complex syntaxes derived from `..` are possible, they use additional tokens to avoid the
+ambiguity with ranges, for example
+[`..PAT..`](https://github.com/rust-lang/rust/issues/23121#issuecomment-301485132), or
+[`.. @ PAT`](https://github.com/rust-lang/rust/issues/23121#issuecomment-280920062) or
+[`PAT @ ..`](https://github.com/rust-lang/rust/issues/23121#issuecomment-280906823), or other
+similar alternatives.  
+We reject these syntaxes because they only bring benefits in contrived cases using a
+feature that doesn't even exist yet, but normally they only add symbolic noise.
+
+More radical syntax changes do not keep consistency with `..`, for example
+[`[1, 2, 3, 4] ++ ref v`](https://github.com/rust-lang/rust/issues/23121#issuecomment-289220169).
+
+### `..PAT` or `PAT..`
+
+If `..` is used in the meaning "match the subslice (`>=0` elements) and ignore it", then it's
+reasonable to expect that syntax for "match the subslice to a pattern" should be some variation
+on `..`.  
+The two simplest variations are `..PAT` and `PAT..`.
+
+#### Ambiguity
+
+The issue is that these syntaxes are ambiguous with half-bounded ranges `..END` and `BEGIN..`,
+and the full range `..`.  
+To be precise, such ranges are not currently supported in patterns, but they may be supported in
+the future.
+
+Syntactic ambiguity is not inherently bad. We see it every day in expressions like
+`a + b * c`. What is important is to disambiguate it reasonably by default and have a way to
+group operands in the alternative way when default disambiguation turns out to be incorrect.  
+In case of slice patterns the subslice interpretation seems more likely, so we
+can take it as a default.  
+There was very little demand for implementing half-bounded ranges in patterns so far
+(see https://github.com/rust-lang/rfcs/issues/947), but if they
+are implemented in the future they will be able to be used in slice patterns as well, but they
+could require explicit grouping with recently implemented
+[parentheses in patterns](https://github.com/rust-lang/rust/pull/48500) (`[a, (..end)]`) or an
+explicitly written start boundary (`[a, 0 .. end]`).  
+We can also make *some* disambiguation effort and, for example, interpret `..LITERAL` as a
+range because `LITERAL` can never match a subslice. Time will show if such an effort is necessary
+or not.
+
+If/when half-bounded ranges are supported in patterns, for better future compatibility we could
+decide to reserve `..PAT` as "rest of the list" in tuples and tuple structs as well, and avoid
+interpreting it as a range pattern in those positions.
+
+Note that ambiguity with unbounded ranges as they are used in expressions (`..`) already exists in
+variant `Variant(..)` and tuple `(a, b, ..)` patterns, but it's unlikely that the `..` syntax
+will ever be used in patterns in the range meaning because it duplicates functionality of the
+wildcard pattern `_`.
+
+#### `..PAT` vs `PAT..`
+
+Originally Rust used syntax `..PAT` for subslice patterns.  
+In 2014 the syntax was changed to `PAT..` by [RFC 202](https://github.com/rust-lang/rfcs/pull/202).
+That RFC received almost no discussion before it got merged and its motivation is no longer
+relevant because arrays now use syntax `[T; N]` instead of `[T, ..N]` used in old Rust.
+
+This RFC originally proposed to switch back to `..PAT`.
+Some reasons to switch were:
+- Symmetry with expressions.  
+One of the general ideas behind patterns is that destructuring with
+patterns has the same syntax as construction with expressions, if possible.  
+In expressions we already have something with the meaning "rest of the list" - functional record
+update in struct expressions `S { field1, field2, ..remaining_fields }`.
+Right now we can use `S { field1, field1, .. }` in a pattern, but can't bind the remaining fields
+as a whole (by creating a new struct type on the fly, for example). It's not inconceivable that
+in Rust 2030 we have such ability and it's reasonable to expect it using syntax `..remaining_fields`
+symmetric to expressions. It would be good for slice patterns to be consistent with it.  
+Without speculations, even if `..remaining_fields` in struct expressions and `..subslice` in slice
+patterns are not entirely the same thing, they are similar enough to keep them symmetric already.
+- Simple disambiguation.  
+When we are parsing a slice pattern and see `..` we immediately know it's
+a subslice and can parse following tokens as a pattern (unless they are `,` or `]`, then it's just
+`..`, without an attached pattern).  
+With `PAT..` we need to consume the pattern first, but that pattern may be a... `RANGE_BEGIN..`
+range pattern, then it means that we consumed too much and need to reinterpret the parsed tokens
+somehow. It's probably possible to make this work, but it's some headache that we would like to
+avoid if possible.
+
+This RFC no longer includes the addition of `..PAT` or `PAT..`.
+The currently-proposed change is a minimal addition to patterns (`..` for slices) which
+already exists in other forms (e.g. tuples) and generalizes well to pattern-matching out sub-tuples,
+e.g. `let (a, b @ .., c) = (1, 2, 3, 4);`.
+
+Additionally, `@` is more consistent with the types of patterns that would be allowable for matching
+slices (only identifiers), whereas `PAT..`/`..PAT` suggest the ability to write e.g. `..(1, x)` or
+`..SomeStruct { x }` sub-patterns, which wouldn't be possible since the resulting bound variables
+don't form a slice (since they're spread out in memory).
+
+# Prior art
+[prior-art]: #prior-art
+
+Some other languages like Haskell (`first_elem : rest_of_the_list`),
+Scala, or F# (`first_elem :: rest_of_the_list`) has list/array patterns, but their
+syntactic choices are quite different from Rust's general style.
+
+"Rest of the list" in patterns was previously discussed in
+[RFC 1492](https://github.com/rust-lang/rfcs/pull/1492)
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None known.
+
+# Future possibilities
+[future-possibilities]: #future-possibilities
+
+Turn `..` into a full pattern syntactically accepted in any pattern position,
+(including `pat` matchers in macros), but rejected semantically outside of slice and tuple patterns.