go/ast: add Slice3 field to SliceExpr

If Slice3 is set, the expression is
a 3-index slice expression (2 colons).
Required for type-checking.

Backward-compatible API extension.

R=r, rsc
CC=golang-dev
https://golang.org/cl/13826050

src/pkg/go/ast/ast.go

@@ -298,6 +298,7 @@ type (
 		Low    Expr      // begin of slice range; or nil
 		High   Expr      // end of slice range; or nil
 		Max    Expr      // maximum capacity of slice; or nil
+		Slice3 bool      // true if 3-index slice (2 colons present)
 		Rbrack token.Pos // position of "]"
 	}
 

src/pkg/go/parser/parser.go

@@ -1187,7 +1187,7 @@ func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
 
 	if ncolons > 0 {
 		// slice expression
-		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Rbrack: rbrack}
+		return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Slice3: ncolons == 2, Rbrack: rbrack}
 	}
 
 	return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}