GDB/MI: Add new "--language LANG" command option.

Frontend sometimes need to evaluate expressions that are
language-specific. For instance, Eclipse uses the following
expression to determine the size of an address on the target:

    -data-evaluate-expression "sizeof (void*)"

Unfortunately, if the main of the program being debugged is not C,
this may not work. For instance, if the main is in Ada, you get...

    -data-evaluate-expression "sizeof (void*)"
    ^error,msg="No definition of \"sizeof\" in current context."

... and apparently decides to stop the debugging session as a result.
The  recommendation sent was to specifically set the language to C
before trying to evaluate the expression.  Something such as:

    1. save current language
    2. set language c
    3. -data-evaluate-expression "sizeof (void*)"
    4. Restore language

This has the same disadvantages as the ones outlined in the "Context
Management" section of the GDB/MI documentation regarding setting
the current thread or the current frame, thus recommending the use of
general command-line switches such as --frame, or --thread instead.

This patch follows the same steps for the language, adding a similar
new command option: --language LANG. Example of use:

    -data-evaluate-expression --language c "sizeof (void*)"
    ^done,value="4"

gdb/ChangeLog:

        * mi/mi-parse.h (struct mi_parse) <language>: New field.
        * mi/mi-main.c (mi_cmd_execute): Temporarily set language to
        PARSE->LANGUAGE during command execution, if set.
        * mi/mi-parse.c: Add "language.h" #include.
        (mi_parse): Add parsing of "--language" command option.

        * NEWS: Add entry mentioning the new "--language" command option.

gdb/testsuite/ChangeLog:

        * gdb.mi/mi-language.exp: New file.

gdb/doc/ChangeLog:

        * gdb.texinfo (Show): Add xref anchor for "show language" command.
        (Context management): Place current subsection text into its own
        subsubsection.  Add new subsubsection describing the "--language"
        command option.

3 files changed, 35 insertions, 1 deletions

diff --git a/gdb/mi/mi-main.c b/gdb/mi/mi-main.c
index c3f7221..bbf944a 100644
--- a/gdb/mi/mi-main.c
+++ b/gdb/mi/mi-main.c
@@ -2122,6 +2122,7 @@ static void
 mi_cmd_execute (struct mi_parse *parse)
 {
   struct cleanup *cleanup;
+  enum language saved_language;
 
   cleanup = prepare_execute_command ();
 
@@ -2183,6 +2184,12 @@ mi_cmd_execute (struct mi_parse *parse)
 	error (_("Invalid frame id: %d"), frame);
     }
 
+  if (parse->language != language_unknown)
+    {
+      make_cleanup_restore_current_language ();
+      set_language (parse->language);
+    }
+
   current_context = parse;
 
   if (parse->cmd->suppress_notification != NULL)
diff --git a/gdb/mi/mi-parse.c b/gdb/mi/mi-parse.c
index a3c1849..9994307 100644
--- a/gdb/mi/mi-parse.c
+++ b/gdb/mi/mi-parse.c
@@ -27,6 +27,7 @@
 #include <ctype.h>
 #include "gdb_string.h"
 #include "cli/cli-utils.h"
+#include "language.h"
 
 static const char mi_no_values[] = "--no-values";
 static const char mi_simple_values[] = "--simple-values";
@@ -244,6 +245,7 @@ mi_parse (const char *cmd, char **token)
   parse->thread_group = -1;
   parse->thread = -1;
   parse->frame = -1;
+  parse->language = language_unknown;
 
   cleanup = make_cleanup (mi_parse_cleanup, parse);
 
@@ -292,7 +294,10 @@ mi_parse (const char *cmd, char **token)
      some important commands, like '-break-*' are implemented by
      forwarding to the CLI layer directly.  We want to parse --thread
      and --frame here, so as not to leave those option in the string
-     that will be passed to CLI.  */
+     that will be passed to CLI.
+
+     Same for the --language option.  */
+
   for (;;)
     {
       const char *option;
@@ -300,6 +305,7 @@ mi_parse (const char *cmd, char **token)
       size_t tgs = sizeof ("--thread-group ") - 1;
       size_t ts = sizeof ("--thread ") - 1;
       size_t fs = sizeof ("--frame ") - 1;
+      size_t ls = sizeof ("--language ") - 1;
 
       if (strncmp (chp, "--all ", as) == 0)
 	{
@@ -348,6 +354,23 @@ mi_parse (const char *cmd, char **token)
 	  parse->frame = strtol (chp, &endp, 10);
 	  chp = endp;
 	}
+      else if (strncmp (chp, "--language ", ls) == 0)
+	{
+	  char *lang_name;
+	  struct cleanup *old_chain;
+
+	  option = "--language";
+	  chp += ls;
+	  lang_name = extract_arg_const (&chp);
+	  old_chain = make_cleanup (xfree, lang_name);
+
+	  parse->language = language_enum (lang_name);
+	  if (parse->language == language_unknown
+	      || parse->language == language_auto)
+	    error (_("Invalid --language argument: %s"), lang_name);
+
+	  do_cleanups (old_chain);
+	}
       else
 	break;
 
diff --git a/gdb/mi/mi-parse.h b/gdb/mi/mi-parse.h
index b20a389..325c1e1 100644
--- a/gdb/mi/mi-parse.h
+++ b/gdb/mi/mi-parse.h
@@ -50,6 +50,10 @@ struct mi_parse
     int thread_group; /* At present, the same as inferior number.  */
     int thread;
     int frame;
+
+    /* The language that should be used to evaluate the MI command.
+       Ignored if set to language_unknown.  */
+    enum language language;
   };
 
 /* Attempts to parse CMD returning a ``struct mi_parse''.  If CMD is