implement the ? operator

The `?` postfix operator is sugar equivalent to the try! macro, but is more amenable to chaining:
`File::open("foo")?.metadata()?.is_dir()`.

`?` is accepted on any *expression* that can return a `Result`, e.g. `x()?`, `y!()?`, `{z}?`,
`(w)?`, etc. And binds more tightly than unary operators, e.g. `!x?` is parsed as `!(x?)`.

cc #31436

src/libsyntax/ast.rs

@@ -1022,6 +1022,9 @@ pub enum ExprKind {
 
     /// No-op: used solely so we can pretty-print faithfully
     Paren(P<Expr>),
+
+    /// `expr?`
+    Try(P<Expr>),
 }
 
 /// The explicit Self type in a "qualified path". The actual

src/libsyntax/feature_gate.rs

@@ -245,6 +245,9 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Option<u32>, Status
 
     // a...b and ...b
     ("inclusive_range_syntax", "1.7.0", Some(28237), Active),
+
+    // `expr?`
+    ("question_mark", "1.9.0", Some(31436), Active)
 ];
 // (changing above list without updating src/doc/reference.md makes @cmr sad)
 
@@ -570,6 +573,7 @@ pub struct Features {
     pub staged_api: bool,
     pub stmt_expr_attributes: bool,
     pub deprecated: bool,
+    pub question_mark: bool,
 }
 
 impl Features {
@@ -603,6 +607,7 @@ impl Features {
             staged_api: false,
             stmt_expr_attributes: false,
             deprecated: false,
+            question_mark: false,
         }
     }
 }
@@ -1001,6 +1006,9 @@ impl<'a, 'v> Visitor<'v> for PostExpansionVisitor<'a> {
                                   e.span,
                                   "inclusive range syntax is experimental");
             }
+            ast::ExprKind::Try(..) => {
+                self.gate_feature("question_mark", e.span, "the `?` operator is not stable");
+            }
             _ => {}
         }
         visit::walk_expr(self, e);
@@ -1203,6 +1211,7 @@ fn check_crate_inner<F>(cm: &CodeMap, span_handler: &Handler,
         staged_api: cx.has_feature("staged_api"),
         stmt_expr_attributes: cx.has_feature("stmt_expr_attributes"),
         deprecated: cx.has_feature("deprecated"),
+        question_mark: cx.has_feature("question_mark"),
     }
 }
 

src/libsyntax/fold.rs

@@ -1332,7 +1332,8 @@ pub fn noop_fold_expr<T: Folder>(Expr {id, node, span, attrs}: Expr, folder: &mu
                         fields.move_map(|x| folder.fold_field(x)),
                         maybe_expr.map(|x| folder.fold_expr(x)))
             },
-            ExprKind::Paren(ex) => ExprKind::Paren(folder.fold_expr(ex))
+            ExprKind::Paren(ex) => ExprKind::Paren(folder.fold_expr(ex)),
+            ExprKind::Try(ex) => ExprKind::Try(folder.fold_expr(ex)),
         },
         span: folder.new_span(span),
         attrs: attrs.map_thin_attrs(|v| fold_attrs(v, folder)),

src/libsyntax/parse/parser.rs

@@ -2534,6 +2534,12 @@ impl<'a> Parser<'a> {
         let mut e = e0;
         let mut hi;
         loop {
+            // expr?
+            while self.eat(&token::Question) {
+                let hi = self.span.hi;
+                e = self.mk_expr(lo, hi, ExprKind::Try(e), None);
+            }
+
             // expr.f
             if self.eat(&token::Dot) {
                 match self.token {
@@ -2907,7 +2913,6 @@ impl<'a> Parser<'a> {
             }
         };
 
-
         if self.expr_is_complete(&lhs) {
             // Semi-statement forms are odd. See https://github.com/rust-lang/rust/issues/29071
             return Ok(lhs);

src/libsyntax/print/pprust.rs

@@ -2277,6 +2277,10 @@ impl<'a> State<'a> {
                 try!(self.print_inner_attributes_inline(attrs));
                 try!(self.print_expr(&e));
                 try!(self.pclose());
+            },
+            ast::ExprKind::Try(ref e) => {
+                try!(self.print_expr(e));
+                try!(word(&mut self.s, "?"))
             }
         }
         try!(self.ann.post(self, NodeExpr(expr)));

src/libsyntax/visit.rs

@@ -793,6 +793,9 @@ pub fn walk_expr<'v, V: Visitor<'v>>(visitor: &mut V, expression: &'v Expr) {
                 visitor.visit_expr(&output.expr)
             }
         }
+        ExprKind::Try(ref subexpression) => {
+            visitor.visit_expr(subexpression)
+        }
     }
 
     visitor.visit_expr_post(expression)