Add 'ast::PatKind::Rest'.

Author: Centril

src/librustc/hir/lowering.rs

@@ -4265,6 +4265,10 @@ impl<'a> LoweringContext<'a> {
                 slice.as_ref().map(|x| self.lower_pat(x)),
                 after.iter().map(|x| self.lower_pat(x)).collect(),
             ),
+            PatKind::Rest => {
+                // If we reach here the `..` pattern is not semantically allowed.
+                self.ban_illegal_rest_pat(p.span)
+            }
             PatKind::Paren(ref inner) => return self.lower_pat(inner),
             PatKind::Mac(_) => panic!("Shouldn't exist here"),
         };
@@ -4276,6 +4280,19 @@ impl<'a> LoweringContext<'a> {
         })
     }
 
+    /// Used to ban the `..` pattern in places it shouldn't be semantically.
+    fn ban_illegal_rest_pat(&self, sp: Span) -> hir::PatKind {
+        self.diagnostic()
+            .struct_span_err(sp, "`..` patterns are not allowed here")
+            .note("only allowed in tuple, tuple struct, and slice patterns")
+            .emit();
+
+        // We're not in a list context so `..` can be reasonably treated
+        // as `_` because it should always be valid and roughly matches the
+        // intent of `..` (notice that the rest of a single slot is that slot).
+        hir::PatKind::Wild
+    }
+
     fn lower_range_end(&mut self, e: &RangeEnd) -> hir::RangeEnd {
         match *e {
             RangeEnd::Included(_) => hir::RangeEnd::Included,

src/libsyntax/ast.rs

@@ -572,6 +572,7 @@ impl Pat {
                     && after.iter().all(|p| p.walk(it))
             }
             PatKind::Wild
+            | PatKind::Rest
             | PatKind::Lit(_)
             | PatKind::Range(..)
             | PatKind::Ident(..)
@@ -661,6 +662,20 @@ pub enum PatKind {
     ///     `PatKind::Slice(box [a, b], Some(i), box [y, z])`
     Slice(Vec<P<Pat>>, Option<P<Pat>>, Vec<P<Pat>>),
 
+    /// A rest pattern `..`.
+    ///
+    /// Syntactically it is valid anywhere.
+    ///
+    /// Semantically however, it only has meaning immediately inside:
+    /// - a slice pattern: `[a, .., b]`,
+    /// - a binding pattern immediately inside a slice pattern: `[a, r @ ..]`,
+    /// - a tuple pattern: `(a, .., b)`,
+    /// - a tuple struct/variant pattern: `$path(a, .., b)`.
+    ///
+    /// In all of these cases, an additional restriction applies,
+    /// only one rest pattern may occur in the pattern sequences.
+    Rest,
+
     /// Parentheses in patterns used for grouping (i.e., `(PAT)`).
     Paren(P<Pat>),
 

src/libsyntax/mut_visit.rs

@@ -1020,7 +1020,7 @@ pub fn noop_visit_pat<T: MutVisitor>(pat: &mut P<Pat>, vis: &mut T) {
     let Pat { id, node, span } = pat.deref_mut();
     vis.visit_id(id);
     match node {
-        PatKind::Wild => {}
+        PatKind::Wild | PatKind::Rest => {}
         PatKind::Ident(_binding_mode, ident, sub) => {
             vis.visit_ident(ident);
             visit_opt(sub, |sub| vis.visit_pat(sub));

src/libsyntax/print/pprust.rs

@@ -2478,6 +2478,7 @@ impl<'a> State<'a> {
                                    |s, p| s.print_pat(p));
                 self.s.word("]");
             }
+            PatKind::Rest => self.s.word(".."),
             PatKind::Paren(ref inner) => {
                 self.popen();
                 self.print_pat(inner);

src/libsyntax/visit.rs

@@ -463,7 +463,7 @@ pub fn walk_pat<'a, V: Visitor<'a>>(visitor: &mut V, pattern: &'a Pat) {
             visitor.visit_expr(lower_bound);
             visitor.visit_expr(upper_bound);
         }
-        PatKind::Wild => (),
+        PatKind::Wild | PatKind::Rest => {},
         PatKind::Slice(ref prepatterns, ref slice_pattern, ref postpatterns) => {
             walk_list!(visitor, visit_pat, prepatterns);
             walk_list!(visitor, visit_pat, slice_pattern);