Use watchpoint's language when re-parsing expression

PR rust/21484 notes that watch -location does not work with Rust:

    (gdb) watch -location a
    syntax error in expression, near `) 0x00007fffffffe0f4'.

update_watchpoint tries to tell gdb that the new expression it creates
has C syntax:

      /* The above expression is in C.  */
      b->language = language_c;

However, update_watchpoint doesn't actually use this language when
re-parsing the expression.

Originally I was going to fix this by saving and restoring the
language in update_watchpoint, but this regressed
gdb.dlang/watch-loc.exp, because the constructed expression actually
has D syntax (specifically the name is not parseable by C).

Next I looked at directly constructing an expression, and not relying
on the parser at all; but it seemed to me that upon a re-set, we'd
want to reparse the type, and there is no existing API to do this
correctly.

So, in the end I made a hook to let each language choose what
expression to use.  I made all the languages other than Rust use the C
expression, because that is the status quo ante.  However, this is
probably not truly correct.  After this patch, at least, it is easy to
correct by someone who knows the language(s) in question.

Regtested by the buildbot.

ChangeLog
2017-05-19  Tom Tromey  <tom@tromey.com>

	PR rust/21484:
	* rust-lang.c (exp_descriptor_rust): New function.
	(rust_language_defn): Use it.
	* p-lang.c (pascal_language_defn): Update.
	* opencl-lang.c (opencl_language_defn): Update.
	* objc-lang.c (objc_language_defn): Update.
	* m2-lang.c (m2_language_defn): Update.
	* language.h (struct language_defn)
	<la_watch_location_expression>: New member.
	* language.c (unknown_language_defn, auto_language_defn)
	(local_language_defn): Update.
	* go-lang.c (go_language_defn): Update.
	* f-lang.c (f_language_defn): Update.
	* d-lang.c (d_language_defn): Update.
	* c-lang.h (c_watch_location_expression): Declare.
	* c-lang.c (c_watch_location_expression): New function.
	(c_language_defn, cplus_language_defn, asm_language_defn)
	(minimal_language_defn): Use it.
	* breakpoint.c (watch_command_1): Call
	la_watch_location_expression.
	* ada-lang.c (ada_language_defn): Update.

testsuite/ChangeLog
2017-05-19  Tom Tromey  <tom@tromey.com>

	PR rust/21484:
	* gdb.rust/watch.exp: New file.
	* gdb.rust/watch.rs: New file.

gdb/ChangeLog

@@ -1,3 +1,27 @@
+2017-05-19  Tom Tromey  <tom@tromey.com>
+
+	PR rust/21484:
+	* rust-lang.c (exp_descriptor_rust): New function.
+	(rust_language_defn): Use it.
+	* p-lang.c (pascal_language_defn): Update.
+	* opencl-lang.c (opencl_language_defn): Update.
+	* objc-lang.c (objc_language_defn): Update.
+	* m2-lang.c (m2_language_defn): Update.
+	* language.h (struct language_defn)
+	<la_watch_location_expression>: New member.
+	* language.c (unknown_language_defn, auto_language_defn)
+	(local_language_defn): Update.
+	* go-lang.c (go_language_defn): Update.
+	* f-lang.c (f_language_defn): Update.
+	* d-lang.c (d_language_defn): Update.
+	* c-lang.h (c_watch_location_expression): Declare.
+	* c-lang.c (c_watch_location_expression): New function.
+	(c_language_defn, cplus_language_defn, asm_language_defn)
+	(minimal_language_defn): Use it.
+	* breakpoint.c (watch_command_1): Call
+	la_watch_location_expression.
+	* ada-lang.c (ada_language_defn): Update.
+
 2017-05-19  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>
 
 	PR tui/21482

gdb/ada-lang.c

@@ -14047,6 +14047,7 @@ const struct language_defn ada_language_defn = {
   ada_print_array_index,
   default_pass_by_reference,
   c_get_string,
+  c_watch_location_expression,
   ada_get_symbol_name_cmp,	/* la_get_symbol_name_cmp */
   ada_iterate_over_symbols,
   &ada_varobj_ops,

gdb/breakpoint.c

@@ -11256,18 +11256,11 @@ watch_command_1 (const char *arg, int accessflag, int from_tty,
       struct type *t = value_type (val);
       CORE_ADDR addr = value_as_address (val);
 
-      t = check_typedef (TYPE_TARGET_TYPE (check_typedef (t)));
-
-      std::string name = type_to_string (t);
-
-      w->exp_string_reparse = xstrprintf ("* (%s *) %s", name.c_str (),
-					  core_addr_to_string (addr));
+      w->exp_string_reparse
+	= current_language->la_watch_location_expression (t, addr).release ();
 
       w->exp_string = xstrprintf ("-location %.*s",
 				  (int) (exp_end - exp_start), exp_start);
-
-      /* The above expression is in C.  */
-      b->language = language_c;
     }
   else
     w->exp_string = savestring (exp_start, exp_end - exp_start);

gdb/c-lang.c

@@ -712,7 +712,17 @@ evaluate_subexp_c (struct type *expect_type, struct expression *exp,
     }
   return evaluate_subexp_standard (expect_type, exp, pos, noside);
 }
+
+/* la_watch_location_expression for C.  */
 
+gdb::unique_xmalloc_ptr<char>
+c_watch_location_expression (struct type *type, CORE_ADDR addr)
+{
+  type = check_typedef (TYPE_TARGET_TYPE (check_typedef (type)));
+  std::string name = type_to_string (type);
+  return gdb::unique_xmalloc_ptr<char>
+    (xstrprintf ("* (%s *) %s", name.c_str (), core_addr_to_string (addr)));
+}
 
 
 /* Table mapping opcodes into strings for printing operators
@@ -865,6 +875,7 @@ const struct language_defn c_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   c_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &c_varobj_ops,
@@ -1008,6 +1019,7 @@ const struct language_defn cplus_language_defn =
   default_print_array_index,
   cp_pass_by_reference,
   c_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &cplus_varobj_ops,
@@ -1060,6 +1072,7 @@ const struct language_defn asm_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   c_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &default_varobj_ops,
@@ -1112,6 +1125,7 @@ const struct language_defn minimal_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   c_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &default_varobj_ops,

gdb/c-lang.h

@@ -112,6 +112,9 @@ extern void c_emit_char (int c, struct type *type,
 
 extern const struct op_print c_op_print_tab[];
 
+extern gdb::unique_xmalloc_ptr<char> c_watch_location_expression
+     (struct type *type, CORE_ADDR addr);
+
 /* These are in c-typeprint.c: */
 
 extern void c_type_print_base (struct type *, struct ui_file *,

gdb/d-lang.c

@@ -244,6 +244,7 @@ static const struct language_defn d_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   c_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &default_varobj_ops,

gdb/f-lang.c

@@ -287,6 +287,7 @@ const struct language_defn f_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   default_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &default_varobj_ops,

gdb/go-lang.c

@@ -605,6 +605,7 @@ static const struct language_defn go_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   c_get_string,
+  c_watch_location_expression,
   NULL,
   iterate_over_symbols,
   &default_varobj_ops,

gdb/language.c

@@ -43,6 +43,7 @@
 #include "symfile.h"
 #include "cp-support.h"
 #include "frame.h"
+#include "c-lang.h"
 
 extern void _initialize_language (void);
 
@@ -859,6 +860,7 @@ const struct language_defn unknown_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   default_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &default_varobj_ops,
@@ -908,6 +910,7 @@ const struct language_defn auto_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   default_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &default_varobj_ops,
@@ -955,6 +958,7 @@ const struct language_defn local_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   default_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &default_varobj_ops,

gdb/language.h

@@ -358,6 +358,12 @@ struct language_defn
     void (*la_get_string) (struct value *value, gdb_byte **buffer, int *length,
 			   struct type **chartype, const char **charset);
 
+    /* Return an expression that can be used for a location
+       watchpoint.  TYPE is a pointer type that points to the memory
+       to watch, and ADDR is the address of the watched memory.  */
+    gdb::unique_xmalloc_ptr<char> (*la_watch_location_expression)
+         (struct type *type, CORE_ADDR addr);
+
     /* Return a pointer to the function that should be used to match
        a symbol name against LOOKUP_NAME. This is mostly for languages
        such as Ada where the matching algorithm depends on LOOKUP_NAME.

gdb/m2-lang.c

@@ -393,6 +393,7 @@ const struct language_defn m2_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   default_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &default_varobj_ops,

gdb/objc-lang.c

@@ -402,6 +402,7 @@ const struct language_defn objc_language_defn = {
   default_print_array_index,
   default_pass_by_reference,
   default_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &default_varobj_ops,

gdb/opencl-lang.c

@@ -1082,6 +1082,7 @@ const struct language_defn opencl_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   c_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &default_varobj_ops,

gdb/p-lang.c

@@ -30,6 +30,7 @@
 #include "valprint.h"
 #include "value.h"
 #include <ctype.h>
+#include "c-lang.h"
 
 extern void _initialize_pascal_language (void);
 
@@ -455,6 +456,7 @@ const struct language_defn pascal_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   default_get_string,
+  c_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &default_varobj_ops,

gdb/rust-lang.c

@@ -2122,6 +2122,20 @@ rust_sniff_from_mangled_name (const char *mangled, char **demangled)
 
 
 
+/* la_watch_location_expression for Rust.  */
+
+static gdb::unique_xmalloc_ptr<char>
+rust_watch_location_expression (struct type *type, CORE_ADDR addr)
+{
+  type = check_typedef (TYPE_TARGET_TYPE (check_typedef (type)));
+  std::string name = type_to_string (type);
+  return gdb::unique_xmalloc_ptr<char>
+    (xstrprintf ("*(%s as *mut %s)", core_addr_to_string (addr),
+		 name.c_str ()));
+}
+
+
+
 static const struct exp_descriptor exp_descriptor_rust = 
 {
   rust_print_subexp,
@@ -2176,6 +2190,7 @@ static const struct language_defn rust_language_defn =
   default_print_array_index,
   default_pass_by_reference,
   c_get_string,
+  rust_watch_location_expression,
   NULL,				/* la_get_symbol_name_cmp */
   iterate_over_symbols,
   &default_varobj_ops,

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2017-05-19  Tom Tromey  <tom@tromey.com>
+
+	PR rust/21484:
+	* gdb.rust/watch.exp: New file.
+	* gdb.rust/watch.rs: New file.
+
 2017-05-18  Tom Tromey  <tom@tromey.com>
 
 	* gdb.rust/simple.exp: Allow Box to be qualified.

gdb/testsuite/gdb.rust/watch.exp

@@ -0,0 +1,35 @@
+# Copyright (C) 2017 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 watch -location with Rust.
+
+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
+}
+
+# Just setting a watchpoint was enough to trigger the bug.
+gdb_test "watch -location y" ".*watchpoint .* -location .*"

gdb/testsuite/gdb.rust/watch.rs

@@ -0,0 +1,24 @@
+// Copyright (C) 2017 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(dead_code)]
+#![allow(unused_variables)]
+#![allow(unused_assignments)]
+
+fn main() {
+    let mut y = 7;
+    y = y + 1;                  // set breakpoint here
+    y = y + 1;
+}