librustc: Change the syntax of subslice matching to use postfix ..

instead of prefix `..`.

This breaks code that looked like:

    match foo {
        [ first, ..middle, last ] => { ... }
    }

Change this code to:

    match foo {
        [ first, middle.., last ] => { ... }
    }

RFC #55.

Closes #16967.

[breaking-change]

Author: pcwalton

src/doc/rust.md

@@ -3300,7 +3300,7 @@ it will bind the corresponding slice to the variable. Example:
 fn is_symmetric(list: &[uint]) -> bool {
     match list {
         [] | [_]                   => true,
-        [x, ..inside, y] if x == y => is_symmetric(inside),
+        [x, inside.., y] if x == y => is_symmetric(inside),
         _                          => false
     }
 }

src/doc/tutorial.md

@@ -1707,7 +1707,7 @@ let score = match numbers {
     [] => 0,
     [a] => a * 10,
     [a, b] => a * 6 + b * 4,
-    [a, b, c, ..rest] => a * 5 + b * 3 + c * 2 + rest.len() as int
+    [a, b, c, rest..] => a * 5 + b * 3 + c * 2 + rest.len() as int
 };
 ~~~~
 

src/librustc/middle/resolve.rs

@@ -1549,18 +1549,13 @@ impl<'a> Resolver<'a> {
                             PathListMod { .. } => Some(item.span),
                             _ => None
                         }).collect::<Vec<Span>>();
-                        match mod_spans.as_slice() {
-                            [first, second, ..other] => {
-                                self.resolve_error(first,
-                                    "`mod` import can only appear once in the list");
-                                self.session.span_note(second,
-                                        "another `mod` import appears here");
-                                for &other_span in other.iter() {
-                                    self.session.span_note(other_span,
-                                        "another `mod` import appears here");
-                                }
-                            },
-                            [_] | [] => ()
+                        if mod_spans.len() > 1 {
+                            self.resolve_error(mod_spans[0],
+                                "`mod` import can only appear once in the list");
+                            for other_span in mod_spans.iter().skip(1) {
+                                self.session.span_note(*other_span,
+                                    "another `mod` import appears here");
+                            }
                         }
 
                         for source_item in source_items.iter() {

src/libsyntax/parse/obsolete.rs

@@ -35,6 +35,7 @@ pub enum ObsoleteSyntax {
     ObsoleteManagedType,
     ObsoleteManagedExpr,
     ObsoleteImportRenaming,
+    ObsoleteSubsliceMatch,
 }
 
 pub trait ParserObsoleteMethods {
@@ -87,6 +88,10 @@ impl<'a> ParserObsoleteMethods for parser::Parser<'a> {
             ObsoleteImportRenaming => (
                 "`use foo = bar` syntax",
                 "write `use bar as foo` instead"
+            ),
+            ObsoleteSubsliceMatch => (
+                "subslice match syntax",
+                "instead of `..xs`, write `xs..` in a pattern"
             )
         };
 

src/libsyntax/parse/parser.rs

@@ -2858,43 +2858,42 @@ impl<'a> Parser<'a> {
         let mut before_slice = true;
 
         while self.token != token::RBRACKET {
-            if first { first = false; }
-            else { self.expect(&token::COMMA); }
+            if first {
+                first = false;
+            } else {
+                self.expect(&token::COMMA);
+            }
 
-            let mut is_slice = false;
             if before_slice {
                 if self.token == token::DOTDOT {
                     self.bump();
-                    is_slice = true;
-                    before_slice = false;
-                }
-            }
 
-            if is_slice {
-                if self.token == token::COMMA || self.token == token::RBRACKET {
-                    slice = Some(box(GC) ast::Pat {
-                        id: ast::DUMMY_NODE_ID,
-                        node: PatWild(PatWildMulti),
-                        span: self.span,
-                    })
-                } else {
-                    let subpat = self.parse_pat();
-                    match *subpat {
-                        ast::Pat { node: PatIdent(_, _, _), .. } => {
-                            slice = Some(subpat);
-                        }
-                        ast::Pat { span, .. } => self.span_fatal(
-                            span, "expected an identifier or nothing"
-                        )
+                    if self.token == token::COMMA ||
+                            self.token == token::RBRACKET {
+                        slice = Some(box(GC) ast::Pat {
+                            id: ast::DUMMY_NODE_ID,
+                            node: PatWild(PatWildMulti),
+                            span: self.span,
+                        });
+                        before_slice = false;
+                    } else {
+                        let _ = self.parse_pat();
+                        let span = self.span;
+                        self.obsolete(span, ObsoleteSubsliceMatch);
                     }
+                    continue
                 }
+            }
+
+            let subpat = self.parse_pat();
+            if before_slice && self.token == token::DOTDOT {
+                self.bump();
+                slice = Some(subpat);
+                before_slice = false;
+            } else if before_slice {
+                before.push(subpat);
             } else {
-                let subpat = self.parse_pat();
-                if before_slice {
-                    before.push(subpat);
-                } else {
-                    after.push(subpat);
-                }
+                after.push(subpat);
             }
         }
 
@@ -3065,7 +3064,11 @@ impl<'a> Parser<'a> {
             // These expressions are limited to literals (possibly
             // preceded by unary-minus) or identifiers.
             let val = self.parse_literal_maybe_minus();
-            if self.eat(&token::DOTDOT) {
+            if self.token == token::DOTDOT &&
+                    self.look_ahead(1, |t| {
+                        *t != token::COMMA && *t != token::RBRACKET
+                    }) {
+                self.bump();
                 let end = if is_ident_or_path(&self.token) {
                     let path = self.parse_path(LifetimeAndTypesWithColons)
                                    .path;
@@ -3106,7 +3109,10 @@ impl<'a> Parser<'a> {
                 }
             });
 
-            if self.look_ahead(1, |t| *t == token::DOTDOT) {
+            if self.look_ahead(1, |t| *t == token::DOTDOT) &&
+                    self.look_ahead(2, |t| {
+                        *t != token::COMMA && *t != token::RBRACKET
+                    }) {
                 let start = self.parse_expr_res(RESTRICT_NO_BAR_OP);
                 self.eat(&token::DOTDOT);
                 let end = self.parse_expr_res(RESTRICT_NO_BAR_OP);

src/libsyntax/print/pprust.rs

@@ -1912,13 +1912,13 @@ impl<'a> State<'a> {
                                    |s, p| s.print_pat(&**p)));
                 for p in slice.iter() {
                     if !before.is_empty() { try!(self.word_space(",")); }
+                    try!(self.print_pat(&**p));
                     match **p {
                         ast::Pat { node: ast::PatWild(ast::PatWildMulti), .. } => {
                             // this case is handled by print_pat
                         }
                         _ => try!(word(&mut self.s, "..")),
                     }
-                    try!(self.print_pat(&**p));
                     if !after.is_empty() { try!(self.word_space(",")); }
                 }
                 try!(self.commasep(Inconsistent,

src/test/compile-fail/borrowck-move-out-of-vec-tail.rs

@@ -25,7 +25,7 @@ pub fn main() {
     );
     let x: &[Foo] = x.as_slice();
     match x {
-        [_, ..tail] => {
+        [_, tail..] => {
             match tail {
                 [Foo { string: a }, //~ ERROR cannot move out of dereference of `&`-pointer
                  Foo { string: b }] => {

src/test/compile-fail/borrowck-vec-pattern-element-loan.rs

@@ -12,7 +12,7 @@ fn a<'a>() -> &'a [int] {
     let vec = vec!(1, 2, 3, 4);
     let vec: &[int] = vec.as_slice(); //~ ERROR does not live long enough
     let tail = match vec {
-        [_, ..tail] => tail,
+        [_, tail..] => tail,
         _ => fail!("a")
     };
     tail
@@ -22,7 +22,7 @@ fn b<'a>() -> &'a [int] {
     let vec = vec!(1, 2, 3, 4);
     let vec: &[int] = vec.as_slice(); //~ ERROR does not live long enough
     let init = match vec {
-        [..init, _] => init,
+        [init.., _] => init,
         _ => fail!("b")
     };
     init
@@ -32,7 +32,7 @@ fn c<'a>() -> &'a [int] {
     let vec = vec!(1, 2, 3, 4);
     let vec: &[int] = vec.as_slice(); //~ ERROR does not live long enough
     let slice = match vec {
-        [_, ..slice, _] => slice,
+        [_, slice.., _] => slice,
         _ => fail!("c")
     };
     slice

src/test/compile-fail/borrowck-vec-pattern-loan-from-mut.rs

@@ -12,7 +12,7 @@ fn a() {
     let mut v = vec!(1, 2, 3);
     let vb: &mut [int] = v.as_mut_slice();
     match vb {
-        [_a, ..tail] => {
+        [_a, tail..] => {
             v.push(tail[0] + tail[1]); //~ ERROR cannot borrow
         }
         _ => {}

src/test/compile-fail/borrowck-vec-pattern-move-tail.rs

@@ -11,7 +11,7 @@
 fn main() {
     let mut a = [1i, 2, 3, 4];
     let t = match a {
-        [1, 2, ..tail] => tail,
+        [1, 2, tail..] => tail,
         _ => unreachable!()
     };
     a[0] = 0; //~ ERROR cannot assign to `a[..]` because it is borrowed