Address most of comments

Author: petrochenkov

text/0000-subslice-pattern-syntax.md

@@ -41,33 +41,67 @@ Sub-slices and sub-arrays can be matched using `..` and `<IDENT> @ ..` can be us
 these sub-slices and sub-arrays to an identifier.
 
 ```rust
-// Matching slices using `ref` patterns:
-let v = vec![1, 2, 3];
+// 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),
-    [5, ref subslice @ ..] => assert_eq!(subslice.len(), 2),
-    [ref subslice @ .., 6] => assert_eq!(subslice.len(), 2),
+    [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 v = vec![1, 2, 3];
+let mut v = vec![1, 2, 3];
 match &v[..] {
-    [1, subslice @ .., 4] => assert_eq!(subslice.len(), 1),
-    [5, subslice @ ..] => assert_eq!(subslice.len(), 2),
-    [subslice @ .., 6] => assert_eq!(subslice.len(), 2),
+    [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 arrays by-value:
-let v = [1, 2, 3];
+// 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]),
-  [5, subarray @ ..] => has_type::<[i32; 2]>(subarray),
-  [subarray @ .., 6] => has_type::<[i32, 2]>(subarray),
-  [x, .., y] => has_type::<[i32, 1]>(x),
+  [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, 1]>(x), // typeof(subarray) == [i32; 1]
+  [..] => {},
+}
+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, 1]>(x), // typeof(subarray) == &[i32; 1]
+  [..] => {},
+}
+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, 1]>(x), // typeof(subarray) == &mut [i32; 1]
   [..] => {},
 }
 ```
@@ -82,9 +116,8 @@ reference or mutable reference to a slice or array.
 
 `@` can be used to bind the result of `..` to an identifier.
 
- When used to match against a non-reference slice (`[u8]`), `x @ ..` would attempt to bind
-by-value, which would fail in the case that users haven't enabled `feature(unsized_locals)`
-(since otherwise it's not possible to bind `[u8]` to a variable directly).
+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]`.
 
 `..`/`IDENT @ ..` 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.
@@ -103,10 +136,10 @@ ambiguity with ranges, for example
 [`.. @ 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 incredibly contrived cases using a
+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 not keeping consistency with `..`, for example
+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..`
@@ -118,30 +151,32 @@ The two simplest variations are `..PAT` and `PAT..`.
 
 #### Ambiguity
 
-The issue is that these syntaxes are ambiguous with half-bounded ranges `..END` and `BEGIN..`.  
+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 overwhelmingly more likely, so we
+In case of slice patterns the subslice interpretation seems more likely, so we
 can take it as a default.  
-There was no visible demand for implementing half-bounded ranges in patterns so far, but if they
+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
-will require explicit grouping with recently implemented
+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'll need
-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.
+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 very unlikely that the `..` syntax
+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 `_`.
 
@@ -153,15 +188,15 @@ That RFC received almost no discussion before it got merged and its motivation i
 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:
+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 2525 we have such ability and it's reasonable to expect it using syntax `..remaining_fields`
+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.
@@ -176,7 +211,7 @@ avoid if possible.
 
 This RFC no longer includes the addition of `..PAT` or `PAT..`, but merely `..` as it results in
 a smaller starting surface-area for the feature which can be expanded in the future if necessary.
-The currently-proposed change is an extremely minimal addition to patterns (`..` for slices) which
+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);`.