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/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);