Handle dereferencing Rust trait objects

In Rust, virtual tables work a bit differently than they do in C++.  In
C++, as you know, they are connected to a particular class hierarchy.
Rust, instead, can generate a virtual table for potentially any type --
in fact, one such virtual table for each trait (a trait is similar to an
abstract class or to a Java interface) that a type implements.

Objects that are referenced via a trait can't currently be inspected by
gdb.  This patch implements the Rust equivalent of "set print object".

gdb relies heavily on the C++ ABI to decode virtual tables; primarily to
make "set print object" work; but also "info vtbl".  However, Rust does
not currently have a specified ABI, so this approach seems unwise to
emulate.

Instead, I've changed the Rust compiler to emit some DWARF that
describes trait objects (previously their internal structure was
opaque), vtables (currently just a size -- but I hope to expand this in
the future), and the concrete type for which a vtable was emitted.

The concrete type is expressed as a DW_AT_containing_type on the
vtable's type.  This is a small extension to DWARF.

This patch adds a new entry to quick_symbol_functions to return the
symtab that holds a data address.  Previously there was no way in gdb to
look up a full (only minimal) non-text symbol by address.  The psymbol
implementation of this method works by lazily filling in a map that is
added to the objfile.  This avoids slowing down psymbol reading for a
feature that is likely to not be used too frequently.

I did not update .gdb_index.  My thinking here is that the DWARF 5
indices will obsolete .gdb_index soon-ish, meaning that adding a new
feature to them is probably wasted work.  If necessary I can update the
DWARF 5 index code when it lands in gdb.

Regression tested on x86-64 Fedora 25.

2017-11-17  Tom Tromey  <tom@tromey.com>

	* symtab.h (struct symbol) <is_rust_vtable>: New member.
	(struct rust_vtable_symbol): New.
	(find_symbol_at_address): Declare.
	* symtab.c (find_symbol_at_address): New function.
	* symfile.h (struct quick_symbol_functions)
	<find_compunit_symtab_by_address>: New member.
	* symfile-debug.c (debug_qf_find_compunit_symtab_by_address): New
	function.
	(debug_sym_quick_functions): Link to
	debug_qf_find_compunit_symtab_by_address.
	* rust-lang.c (rust_get_trait_object_pointer): New function.
	(rust_evaluate_subexp) <case UNOP_IND>: New case.  Call
	rust_get_trait_object_pointer.
	* psymtab.c (psym_relocate): Clear psymbol_map.
	(psym_fill_psymbol_map, psym_find_compunit_symtab_by_address): New
	functions.
	(psym_functions): Link to psym_find_compunit_symtab_by_address.
	* objfiles.h (struct objfile) <psymbol_map>: New member.
	* dwarf2read.c (dwarf2_gdb_index_functions): Update.
	(process_die) <DW_TAG_variable>: New case.  Call read_variable.
	(rust_containing_type, read_variable): New functions.

2017-11-17  Tom Tromey  <tom@tromey.com>

	* gdb.rust/traits.rs: New file.
	* gdb.rust/traits.exp: New file.

1 files changed, 53 insertions, 0 deletions

diff --git a/gdb/rust-lang.c b/gdb/rust-lang.c
index c1b2d6e..832f77f 100644
--- a/gdb/rust-lang.c
+++ b/gdb/rust-lang.c
@@ -29,6 +29,7 @@
 #include "gdbarch.h"
 #include "infcall.h"
 #include "objfiles.h"
+#include "psymtab.h"
 #include "rust-lang.h"
 #include "valprint.h"
 #include "varobj.h"
@@ -396,6 +397,39 @@ rust_chartype_p (struct type *type)
 	  && TYPE_UNSIGNED (type));
 }
 
+/* If VALUE represents a trait object pointer, return the underlying
+   pointer with the correct (i.e., runtime) type.  Otherwise, return
+   NULL.  */
+
+static struct value *
+rust_get_trait_object_pointer (struct value *value)
+{
+  struct type *type = check_typedef (value_type (value));
+
+  if (TYPE_CODE (type) != TYPE_CODE_STRUCT || TYPE_NFIELDS (type) != 2)
+    return NULL;
+
+  /* Try to be a bit resilient if the ABI changes.  */
+  int vtable_field = 0;
+  for (int i = 0; i < 2; ++i)
+    {
+      if (strcmp (TYPE_FIELD_NAME (type, i), "vtable") == 0)
+	vtable_field = i;
+      else if (strcmp (TYPE_FIELD_NAME (type, i), "pointer") != 0)
+	return NULL;
+    }
+
+  CORE_ADDR vtable = value_as_address (value_field (value, vtable_field));
+  struct symbol *symbol = find_symbol_at_address (vtable);
+  if (symbol == NULL || !symbol->is_rust_vtable)
+    return NULL;
+
+  struct rust_vtable_symbol *vtable_sym
+    = static_cast<struct rust_vtable_symbol *> (symbol);
+  struct type *pointer_type = lookup_pointer_type (vtable_sym->concrete_type);
+  return value_cast (pointer_type, value_field (value, 1 - vtable_field));
+}
+
 
 
 /* la_emitchar implementation for Rust.  */
@@ -1617,6 +1651,25 @@ rust_evaluate_subexp (struct type *expect_type, struct expression *exp,
 
   switch (exp->elts[*pos].opcode)
     {
+    case UNOP_IND:
+      {
+	if (noside != EVAL_NORMAL)
+	  result = evaluate_subexp_standard (expect_type, exp, pos, noside);
+	else
+	  {
+	    ++*pos;
+	    struct value *value = evaluate_subexp (expect_type, exp, pos,
+						   noside);
+
+	    struct value *trait_ptr = rust_get_trait_object_pointer (value);
+	    if (trait_ptr != NULL)
+	      value = trait_ptr;
+
+	    result = value_ind (value);
+	  }
+      }
+      break;
+
     case UNOP_COMPLEMENT:
       {
 	struct value *value;