gdb: add string_file::release method

A common pattern for string_file is to want to move out the internal
string buffer, because it is the result of the computation that we want
to return.  It is the reason why string_file::string returns a non-const
reference, as explained in the comment.  I think it would make sense to
have a dedicated method for that instead and make string_file::string
return a const reference.

This allows removing the explicit std::move in the typical case.  Note
that compile_program::compute was missing a move, meaning that the
resulting string was copied.  With the new version, it's not possible to
forget to move.

Change-Id: Ieaefa35b73daa7930b2f3a26988b6e3b4121bb79

gdb/ada-lang.c

@@ -6915,7 +6915,7 @@ type_as_string (struct type *type)
 
   type_print (type, "", &tmp_stream, -1);
 
-  return std::move (tmp_stream.string ());
+  return tmp_stream.release ();
 }
 
 /* Given a type TYPE, look up the type of the component of type named NAME.

gdb/ada-valprint.c

@@ -307,7 +307,7 @@ ada_print_floating (const gdb_byte *valaddr, struct type *type,
 
   print_floating (valaddr, type, &tmp_stream);
 
-  std::string &s = tmp_stream.string ();
+  std::string s = tmp_stream.release ();
   size_t skip_count = 0;
 
   /* Modify for Ada rules.  */

gdb/ada-varobj.c

@@ -82,7 +82,7 @@ ada_varobj_scalar_image (struct type *type, LONGEST val)
   string_file buf;
 
   ada_print_scalar (type, val, &buf);
-  return std::move (buf.string ());
+  return buf.release ();
 }
 
 /* Assuming that the (PARENT_VALUE, PARENT_TYPE) pair designates
@@ -817,7 +817,7 @@ ada_varobj_get_value_image (struct value *value,
   string_file buffer;
 
   common_val_print (value, &buffer, 0, opts, current_language);
-  return std::move (buffer.string ());
+  return buffer.release ();
 }
 
 /* Assuming that the (VALUE, TYPE) pair designates an array varobj,

gdb/c-exp.y

@@ -1780,11 +1780,11 @@ oper:	OPERATOR NEW
 	|	OPERATOR OBJC_LBRAC ']'
 			{ $$ = operator_stoken ("[]"); }
 	|	OPERATOR conversion_type_id
-			{ string_file buf;
-
+			{
+			  string_file buf;
 			  c_print_type ($2, NULL, &buf, -1, 0,
 					&type_print_raw_options);
-			  std::string name = std::move (buf.string ());
+			  std::string name = buf.release ();
 
 			  /* This also needs canonicalization.  */
 			  gdb::unique_xmalloc_ptr<char> canon

gdb/cli/cli-setshow.c

@@ -659,7 +659,7 @@ get_setshow_command_value_string (const setting &var)
       gdb_assert_not_reached ("bad var_type");
     }
 
-  return std::move (stb.string ());
+  return stb.release ();
 }
 
 

gdb/compile/compile-c-support.c

@@ -635,7 +635,7 @@ public:
       PopUserExpressionPolicy::pop_user_expression (&buf);
 
     AddCodeFooterPolicy::add_code_footer (m_instance->scope (), &buf);
-    return buf.string ();
+    return buf.release ();
   }
 
 private:

gdb/guile/scm-type.c

@@ -109,7 +109,7 @@ tyscm_type_name (struct type *type)
       string_file stb;
 
       LA_PRINT_TYPE (type, "", &stb, -1, 0, &type_print_raw_options);
-      return std::move (stb.string ());
+      return stb.release ();
     }
   catch (const gdb_exception &except)
     {

gdb/location.c

@@ -447,7 +447,7 @@ explicit_to_string_internal (bool as_linespec,
 		  explicit_loc->line_offset.offset);
     }
 
-  return std::move (buf.string ());
+  return buf.release ();
 }
 
 /* See description in location.h.  */

gdb/maint-test-options.c

@@ -295,8 +295,7 @@ save_completion_result (const test_options_opts &opts, bool res,
 
       stream.puts ("1 ");
       opts.dump (&stream, text);
-      maintenance_test_options_command_completion_text
-	= std::move (stream.string ());
+      maintenance_test_options_command_completion_text = stream.release ();
     }
   else
     {

gdb/remote.c

@@ -9519,7 +9519,7 @@ escape_buffer (const char *buf, int n)
   string_file stb;
 
   stb.putstrn (buf, n, '\\');
-  return std::move (stb.string ());
+  return stb.release ();
 }
 
 int

gdb/tui/tui-disasm.c

@@ -130,14 +130,12 @@ tui_disassemble (struct gdbarch *gdbarch,
 	}
 
       /* Capture the disassembled instruction.  */
-      tal.insn = std::move (gdb_dis_out.string ());
-      gdb_dis_out.clear ();
+      tal.insn = gdb_dis_out.release ();
 
       /* And capture the address the instruction is at.  */
       tal.addr = orig_pc;
       print_address (gdbarch, orig_pc, &gdb_dis_out);
-      tal.addr_string = std::move (gdb_dis_out.string ());
-      gdb_dis_out.clear ();
+      tal.addr_string = std::move (gdb_dis_out.release ());
 
       if (addr_size != nullptr)
 	{

gdb/tui/tui-regs.c

@@ -100,7 +100,7 @@ tui_register_format (struct frame_info *frame, int regnum)
   gdbarch_print_registers_info (gdbarch, &stream, frame, regnum, 1);
 
   /* Remove the possible \n.  */
-  std::string &str = stream.string ();
+  std::string str = stream.release ();
   if (!str.empty () && str.back () == '\n')
     str.resize (str.size () - 1);
 

gdb/tui/tui-stack.c

@@ -181,12 +181,14 @@ tui_locator_window::make_status_line () const
       string.puts (pc_buf);
     }
 
-  if (string.size () < status_size)
-    string.puts (n_spaces (status_size - string.size ()));
-  else if (string.size () > status_size)
-    string.string ().erase (status_size, string.size ());
+  std::string string_val = string.release ();
 
-  return std::move (string.string ());
+  if (string.size () < status_size)
+    string_val.append (status_size - string.size (), ' ');
+  else if (string.size () > status_size)
+    string_val.erase (status_size, string.size ());
+
+  return string_val;
 }
 
 /* Get a printable name for the function at the address.  The symbol

gdb/typeprint.c

@@ -406,7 +406,7 @@ type_to_string (struct type *type)
       string_file stb;
 
       type_print (type, "", &stb, -1);
-      return std::move (stb.string ());
+      return stb.release ();
     }
   catch (const gdb_exception &except)
     {

gdb/ui-file.h

@@ -160,17 +160,19 @@ public:
   /* string_file-specific public API.  */
 
   /* Accesses the std::string containing the entire output collected
-     so far.
+     so far.  */
+  const std::string &string () { return m_string; }
 
-     Returns a non-const reference so that it's easy to move the
-     string contents out of the string_file.  E.g.:
+  /* Return an std::string containing the entire output collected so far.
 
-      string_file buf;
-      buf.printf (....);
-      buf.printf (....);
-      return std::move (buf.string ());
-  */
-  std::string &string () { return m_string; }
+     The internal buffer is cleared, such that it's ready to build a new
+     string.  */
+  std::string release ()
+  {
+    std::string ret = std::move (m_string);
+    m_string.clear ();
+    return ret;
+  }
 
   /* Provide a few convenience methods with the same API as the
      underlying std::string.  */

gdb/utils.c

@@ -1986,7 +1986,7 @@ vfprintf_unfiltered (struct ui_file *stream, const char *format, va_list args)
       /* Print the message.  */
       string_file sfile;
       cli_ui_out (&sfile, 0).vmessage (ui_file_style (), format, args);
-      std::string linebuffer = std::move (sfile.string ());
+      const std::string &linebuffer = sfile.string ();
       fputs_unfiltered (linebuffer.c_str (), stream);
 
       size_t len = linebuffer.length ();

gdb/varobj.c

@@ -2242,7 +2242,7 @@ varobj_value_get_print_value (struct value *value,
     /* All other cases.  */
     common_val_print (value, &stb, 0, &opts, current_language);
 
-  return std::move (stb.string ());
+  return stb.release ();
 }
 
 bool