Handle dereferencing Rust trait objects

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.