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.

18 files changed, 142 insertions, 9 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 59fa9510c0..1cc5dc9d59 100644
--- a/gdb/ChangeLog
+++ b/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
diff --git a/gdb/ada-lang.c b/gdb/ada-lang.c
index 58c8a2efc9..25c49c589f 100644
--- a/gdb/ada-lang.c
+++ b/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,
diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
index 9d6a2f4eb8..150b08ca9d 100644
--- a/gdb/breakpoint.c
+++ b/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);
diff --git a/gdb/c-lang.c b/gdb/c-lang.c
index 19a8608a01..a6d533d082 100644
--- a/gdb/c-lang.c
+++ b/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,
diff --git a/gdb/c-lang.h b/gdb/c-lang.h
index 9746238972..5ce64861c4 100644
--- a/gdb/c-lang.h
+++ b/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 *,
diff --git a/gdb/d-lang.c b/gdb/d-lang.c
index 3ffdbdf508..434b2e6ba5 100644
--- a/gdb/d-lang.c
+++ b/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,
diff --git a/gdb/f-lang.c b/gdb/f-lang.c
index 3c30d75627..a9663d415b 100644
--- a/gdb/f-lang.c
+++ b/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,
diff --git a/gdb/go-lang.c b/gdb/go-lang.c
index 88ddfb66be..3b80cec445 100644
--- a/gdb/go-lang.c
+++ b/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,
diff --git a/gdb/language.c b/gdb/language.c
index ee0101db74..e9d9803aee 100644
--- a/gdb/language.c
+++ b/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,
diff --git a/gdb/language.h b/gdb/language.h
index 58bcbe8fae..cc0df8c5ce 100644
--- a/gdb/language.h
+++ b/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.
diff --git a/gdb/m2-lang.c b/gdb/m2-lang.c
index 03b955cb63..35840bf56f 100644
--- a/gdb/m2-lang.c
+++ b/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,
diff --git a/gdb/objc-lang.c b/gdb/objc-lang.c
index e102b800aa..934d9c6d16 100644
--- a/gdb/objc-lang.c
+++ b/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,
diff --git a/gdb/opencl-lang.c b/gdb/opencl-lang.c
index 6e4c185a64..df57b61529 100644
--- a/gdb/opencl-lang.c
+++ b/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,
diff --git a/gdb/p-lang.c b/gdb/p-lang.c
index 1b069ef17f..c4fe2f6c2a 100644
--- a/gdb/p-lang.c
+++ b/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,
diff --git a/gdb/rust-lang.c b/gdb/rust-lang.c
index 898eddaba2..dce184ced0 100644
--- a/gdb/rust-lang.c
+++ b/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,
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index a3a75bcb7f..adbf3989b6 100644
--- a/gdb/testsuite/ChangeLog
+++ b/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.
diff --git a/gdb/testsuite/gdb.rust/watch.exp b/gdb/testsuite/gdb.rust/watch.exp
new file mode 100644
index 0000000000..0a3c055b2a
--- /dev/null
+++ b/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 .*"
diff --git a/gdb/testsuite/gdb.rust/watch.rs b/gdb/testsuite/gdb.rust/watch.rs
new file mode 100644
index 0000000000..4146ecbbbd
--- /dev/null
+++ b/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;
+}