Support ptype/o in Rust

This adds support for ptype/o to the Rust language code.

By default, the Rust compiler reorders fields to reduce padding.  So,
the Rust language code sorts the fields by offset before printing.
This may yield somewhat odd-looking results, but it is faithful to
"what really happens", and might be useful when doing lower-level
debugging.

The reordering can be disabled using #[repr(c)]; ptype/o might be more
useful in this case.

gdb/ChangeLog
2018-06-26  Tom Tromey  <tom@tromey.com>

	PR rust/22574:
	* typeprint.c (whatis_exp): Allow ptype/o for Rust.
	* rust-lang.c (rust_print_struct_def): Add podata parameter.
	Update.
	(rust_internal_print_type): Add podata parameter.
	(rust_print_type): Update.

gdb/testsuite/ChangeLog
2018-06-26  Tom Tromey  <tom@tromey.com>

	PR rust/22574:
	* gdb.rust/simple.exp (test_one_slice): Add ptype/o tests.
	* gdb.rust/simple.rs (struct SimpleLayout): New.

1 files changed, 63 insertions, 19 deletions

diff --git a/gdb/rust-lang.c b/gdb/rust-lang.c
index d9807d0ac1a..5f4aceb86ba 100644
--- a/gdb/rust-lang.c
+++ b/gdb/rust-lang.c
@@ -31,8 +31,10 @@
 #include "objfiles.h"
 #include "psymtab.h"
 #include "rust-lang.h"
+#include "typeprint.h"
 #include "valprint.h"
 #include "varobj.h"
+#include <algorithm>
 #include <string>
 #include <vector>
 
@@ -616,14 +618,14 @@ static void
 rust_internal_print_type (struct type *type, const char *varstring,
 			  struct ui_file *stream, int show, int level,
 			  const struct type_print_options *flags,
-			  bool for_rust_enum);
+			  bool for_rust_enum, print_offset_data *podata);
 
 /* Print a struct or union typedef.  */
 static void
 rust_print_struct_def (struct type *type, const char *varstring,
 		       struct ui_file *stream, int show, int level,
 		       const struct type_print_options *flags,
-		       bool for_rust_enum)
+		       bool for_rust_enum, print_offset_data *podata)
 {
   /* Print a tuple type simply.  */
   if (rust_tuple_type_p (type))
@@ -636,6 +638,13 @@ rust_print_struct_def (struct type *type, const char *varstring,
   if (TYPE_N_BASECLASSES (type) > 0)
     c_print_type (type, varstring, stream, show, level, flags);
 
+  if (flags->print_offsets)
+    {
+      /* Temporarily bump the level so that the output lines up
+	 correctly.  */
+      level += 2;
+    }
+
   /* Compute properties of TYPE here because, in the enum case, the
      rest of the code ends up looking only at the variant part.  */
   const char *tagname = TYPE_NAME (type);
@@ -674,16 +683,41 @@ rust_print_struct_def (struct type *type, const char *varstring,
 
   if (TYPE_NFIELDS (type) == 0 && !is_tuple)
     return;
-  if (for_rust_enum)
+  if (for_rust_enum && !flags->print_offsets)
     fputs_filtered (is_tuple_struct ? "(" : "{", stream);
   else
     fputs_filtered (is_tuple_struct ? " (\n" : " {\n", stream);
 
+  /* When printing offsets, we rearrange the fields into storage
+     order.  This lets us show holes more clearly.  We work using
+     field indices here because it simplifies calls to
+     print_offset_data::update below.  */
+  std::vector<int> fields;
   for (int i = 0; i < TYPE_NFIELDS (type); ++i)
     {
-      QUIT;
       if (field_is_static (&TYPE_FIELD (type, i)))
 	continue;
+      if (is_enum && i == enum_discriminant_index)
+	continue;
+      fields.push_back (i);
+    }
+  if (flags->print_offsets)
+    std::sort (fields.begin (), fields.end (),
+	       [&] (int a, int b)
+	       {
+		 return (TYPE_FIELD_BITPOS (type, a)
+			 < TYPE_FIELD_BITPOS (type, b));
+	       });
+
+  for (int i : fields)
+    {
+      QUIT;
+
+      gdb_assert (!field_is_static (&TYPE_FIELD (type, i)));
+      gdb_assert (! (is_enum && i == enum_discriminant_index));
+
+      if (flags->print_offsets)
+	podata->update (type, i, stream);
 
       /* We'd like to print "pub" here as needed, but rustc
 	 doesn't emit the debuginfo, and our types don't have
@@ -691,27 +725,35 @@ rust_print_struct_def (struct type *type, const char *varstring,
 
       /* For a tuple struct we print the type but nothing
 	 else.  */
-      if (!for_rust_enum)
+      if (!for_rust_enum || flags->print_offsets)
 	print_spaces_filtered (level + 2, stream);
       if (is_enum)
-	{
-	  if (i == enum_discriminant_index)
-	    continue;
-	  fputs_filtered (TYPE_FIELD_NAME (type, i), stream);
-	}
+	fputs_filtered (TYPE_FIELD_NAME (type, i), stream);
       else if (!is_tuple_struct)
 	fprintf_filtered (stream, "%s: ", TYPE_FIELD_NAME (type, i));
 
       rust_internal_print_type (TYPE_FIELD_TYPE (type, i), NULL,
 				stream, (is_enum ? show : show - 1),
-				level + 2, flags, is_enum);
-      if (!for_rust_enum)
+				level + 2, flags, is_enum, podata);
+      if (!for_rust_enum || flags->print_offsets)
 	fputs_filtered (",\n", stream);
+      /* Note that this check of "I" is ok because we only sorted the
+	 fields by offset when print_offsets was set, so we won't take
+	 this branch in that case.  */
       else if (i + 1 < TYPE_NFIELDS (type))
 	fputs_filtered (", ", stream);
     }
 
-  if (!for_rust_enum)
+  if (flags->print_offsets)
+    {
+      /* Undo the temporary level increase we did above.  */
+      level -= 2;
+      podata->finish (type, level, stream);
+      print_spaces_filtered (print_offset_data::indentation, stream);
+      if (level == 0)
+	print_spaces_filtered (2, stream);
+    }
+  if (!for_rust_enum || flags->print_offsets)
     print_spaces_filtered (level, stream);
   fputs_filtered (is_tuple_struct ? ")" : "}", stream);
 }
@@ -735,7 +777,7 @@ static void
 rust_internal_print_type (struct type *type, const char *varstring,
 			  struct ui_file *stream, int show, int level,
 			  const struct type_print_options *flags,
-			  bool for_rust_enum)
+			  bool for_rust_enum, print_offset_data *podata)
 {
   int i;
 
@@ -778,7 +820,7 @@ rust_internal_print_type (struct type *type, const char *varstring,
 	  if (i > 0)
 	    fputs_filtered (", ", stream);
 	  rust_internal_print_type (TYPE_FIELD_TYPE (type, i), "", stream,
-				    -1, 0, flags, false);
+				    -1, 0, flags, false, podata);
 	}
       fputs_filtered (")", stream);
       /* If it returns unit, we can omit the return type.  */
@@ -786,7 +828,7 @@ rust_internal_print_type (struct type *type, const char *varstring,
         {
           fputs_filtered (" -> ", stream);
           rust_internal_print_type (TYPE_TARGET_TYPE (type), "", stream,
-				    -1, 0, flags, false);
+				    -1, 0, flags, false, podata);
         }
       break;
 
@@ -796,7 +838,8 @@ rust_internal_print_type (struct type *type, const char *varstring,
 
 	fputs_filtered ("[", stream);
 	rust_internal_print_type (TYPE_TARGET_TYPE (type), NULL,
-				  stream, show - 1, level, flags, false);
+				  stream, show - 1, level, flags, false,
+				  podata);
 
 	if (TYPE_HIGH_BOUND_KIND (TYPE_INDEX_TYPE (type)) == PROP_LOCEXPR
 	    || TYPE_HIGH_BOUND_KIND (TYPE_INDEX_TYPE (type)) == PROP_LOCLIST)
@@ -811,7 +854,7 @@ rust_internal_print_type (struct type *type, const char *varstring,
     case TYPE_CODE_UNION:
     case TYPE_CODE_STRUCT:
       rust_print_struct_def (type, varstring, stream, show, level, flags,
-			     for_rust_enum);
+			     for_rust_enum, podata);
       break;
 
     case TYPE_CODE_ENUM:
@@ -856,8 +899,9 @@ rust_print_type (struct type *type, const char *varstring,
 		 struct ui_file *stream, int show, int level,
 		 const struct type_print_options *flags)
 {
+  print_offset_data podata;
   rust_internal_print_type (type, varstring, stream, show, level,
-			    flags, false);
+			    flags, false, &podata);
 }