Fix Rust parser bug with function fields

In Rust, 'obj.f()' is a method call -- but '(obj.f)()' is a call of a
function-valued field 'f' in 'obj'.  The Rust parser in gdb currently
gets this wrong.  This is PR rust/24082.

The expression and Rust parser rewrites made this simple to fix --
simply wrapping a parenthesized expression in a new operation handles
it.  This patch has a slight hack because I didn't want to introduce a
new exp_opcode enumeration constant just for this.  IMO this doesn't
matter, since we should work toward removing dependencies on these
opcodes anyway; but let me know what you think of this.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=24082

gdb/rust-exp.h

@@ -197,6 +197,33 @@ class rust_aggregate_operation
   { return OP_AGGREGATE; }
 };
 
+/* Rust parenthesized operation.  This is needed to distinguish
+   between 'obj.f()', which is a method call, and '(obj.f)()', which
+   is a call of a function-valued field 'f'.  */
+class rust_parenthesized_operation
+  : public tuple_holding_operation<operation_up>
+{
+public:
+
+  explicit rust_parenthesized_operation (operation_up op)
+    : tuple_holding_operation (std::move (op))
+  {
+  }
+
+  value *evaluate (struct type *expect_type,
+		   struct expression *exp,
+		   enum noside noside) override
+  {
+    return std::get<0> (m_storage)->evaluate (expect_type, exp, noside);
+  }
+
+  enum exp_opcode opcode () const override
+  {
+    /* A lie but this isn't worth introducing a new opcode for.  */
+    return UNOP_PLUS;
+  }
+};
+
 } /* namespace expr */
 
 #endif /* RUST_EXP_H */

gdb/rust-parse.c

@@ -1105,7 +1105,7 @@ rust_parser::parse_tuple ()
     {
       /* Parenthesized expression.  */
       lex ();
-      return expr;
+      return make_operation<rust_parenthesized_operation> (std::move (expr));
     }
 
   std::vector<operation_up> ops;

gdb/testsuite/gdb.rust/fnfield.exp

@@ -0,0 +1,38 @@
+# Copyright (C) 2022 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test trait object printing.
+
+load_lib rust-support.exp
+if {[skip_rust_tests]} {
+    continue
+}
+
+standard_testfile .rs
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug rust}]} {
+    return -1
+}
+
+set line [gdb_get_line_number "set breakpoint here"]
+if {![runto ${srcfile}:$line]} {
+    untested "could not run to breakpoint"
+    return -1
+}
+
+gdb_test "print foo.f()" " = 6" "call impl function"
+gdb_test "print (foo.f)()" " = 5" "call function field"
+gdb_test "print foo.g()" " = 7" "call impl function g"
+gdb_test "print (foo.g)()" "There is no member named g." \
+    "cannot call g with parens"

gdb/testsuite/gdb.rust/fnfield.rs

@@ -0,0 +1,39 @@
+// Copyright (C) 2022 Free Software Foundation, Inc.
+
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+#![allow(warnings)]
+
+fn five() -> i32 { 5 }
+
+fn main() {
+    let foo = Foo {x: 5, f: five};
+    foo.print();  // set breakpoint here
+    println!("Hello, world! {}, {}, {}", foo.f(), (foo.f)(),
+             foo.g ());
+}
+
+struct Foo {
+    x :i32,
+    f: fn () -> i32,
+}
+
+impl Foo {
+    fn print(&self) {
+        println!("hello {}", self.x)
+    }
+
+    fn f(&self) -> i32 { 6 }
+    fn g(&self) -> i32 { 7 }
+}