[RFA v2 1/3] Handle dereferencing Rust trait objects

Tom Tromey
2017-11-17T19:26:12Z

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.
---
gdb/ChangeLog | 24 ++++++++++++
gdb/dwarf2read.c | 58 +++++++++++++++++++++++++++
gdb/objfiles.h | 5 +++
gdb/psymtab.c | 82 +++++++++++++++++++++++++++++++++++++++
gdb/rust-lang.c | 53 +++++++++++++++++++++++++
gdb/symfile-debug.c | 28 +++++++++++++
gdb/symfile.h | 8 ++++
gdb/symtab.c | 39 +++++++++++++++++++
gdb/symtab.h | 19 +++++++++
gdb/testsuite/ChangeLog | 5 +++
gdb/testsuite/gdb.rust/traits.exp | 47 ++++++++++++++++++++++
gdb/testsuite/gdb.rust/traits.rs | 37 ++++++++++++++++++
12 files changed, 405 insertions(+)
create mode 100644 gdb/testsuite/gdb.rust/traits.exp
create mode 100644 gdb/testsuite/gdb.rust/traits.rs

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 9b1c335..c4d0457 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,27 @@
+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.
+
2017-11-15 Simon Marchi <simon.marchi@ericsson.com>

* tui/tui-win.c (window_name_completer): Replace VEC with
diff --git a/gdb/dwarf2read.c b/gdb/dwarf2read.c
index b6bd291..8283345 100644
--- a/gdb/dwarf2read.c
+++ b/gdb/dwarf2read.c
@@ -1797,6 +1797,8 @@ static void read_lexical_block_scope (struct die_info *, struct dwarf2_cu *);

static void read_call_site_scope (struct die_info *die, struct dwarf2_cu *cu);

+static void read_variable (struct die_info *die, struct dwarf2_cu *cu);
+
static int dwarf2_ranges_read (unsigned, CORE_ADDR *, CORE_ADDR *,
struct dwarf2_cu *, struct partial_symtab *);

@@ -4995,6 +4997,7 @@ const struct quick_symbol_functions dwarf2_gdb_index_functions =
dw2_map_matching_symbols,
dw2_expand_symtabs_matching,
dw2_find_pc_sect_compunit_symtab,
+ NULL,
dw2_map_symbol_filenames
};

@@ -9230,6 +9233,10 @@ process_die (struct die_info *die, struct dwarf2_cu *cu)
process_imported_unit_die (die, cu);
break;

+ case DW_TAG_variable:
+ read_variable (die, cu);
+ break;
+
default:
new_symbol (die, NULL, cu);
break;
@@ -12770,6 +12777,57 @@ read_call_site_scope (struct die_info *die, struct dwarf2_cu *cu)
}
}

+/* Helper function for read_variable. If DIE represents a virtual
+ table, then return the type of the concrete object that is
+ associated with the virtual table. Otherwise, return NULL. */
+
+static struct type *
+rust_containing_type (struct die_info *die, struct dwarf2_cu *cu)
+{
+ struct attribute *attr = dwarf2_attr (die, DW_AT_type, cu);
+ if (attr == NULL)
+ return NULL;
+
+ /* Find the type DIE. */
+ struct die_info *type_die = NULL;
+ struct dwarf2_cu *type_cu = cu;
+
+ if (attr_form_is_ref (attr))
+ type_die = follow_die_ref (die, attr, &type_cu);
+ if (type_die == NULL)
+ return NULL;
+
+ if (dwarf2_attr (type_die, DW_AT_containing_type, type_cu) == NULL)
+ return NULL;
+ return die_containing_type (type_die, type_cu);
+}
+
+/* Read a variable (DW_TAG_variable) DIE and create a new symbol. */
+
+static void
+read_variable (struct die_info *die, struct dwarf2_cu *cu)
+{
+ struct rust_vtable_symbol *storage = NULL;
+
+ if (cu->language == language_rust)
+ {
+ struct type *containing_type = rust_containing_type (die, cu);
+
+ if (containing_type != NULL)
+ {
+ struct objfile *objfile = cu->objfile;
+
+ storage = OBSTACK_ZALLOC (&objfile->objfile_obstack,
+ struct rust_vtable_symbol);
+ initialize_objfile_symbol (storage);
+ storage->concrete_type = containing_type;
+ storage->is_rust_vtable = 1;
+ }
+ }
+
+ new_symbol_full (die, NULL, cu, storage);
+}
+
/* Call CALLBACK from DW_AT_ranges attribute value OFFSET
reading .debug_rnglists.
Callback's type should be:
diff --git a/gdb/objfiles.h b/gdb/objfiles.h
index 12ef5ec..62bd454 100644
--- a/gdb/objfiles.h
+++ b/gdb/objfiles.h
@@ -361,6 +361,11 @@ struct objfile

struct psymbol_bcache *psymbol_cache;

+ /* Map symbol addresses to the partial symtab that defines the
+ object at that address. */
+
+ std::vector<std::pair<CORE_ADDR, partial_symtab *>> psymbol_map;
+
/* Vectors of all partial symbols read in from file. The actual data
is stored in the objfile_obstack. */

diff --git a/gdb/psymtab.c b/gdb/psymtab.c
index 06a65bf..c74ea21 100644
--- a/gdb/psymtab.c
+++ b/gdb/psymtab.c
@@ -36,6 +36,7 @@
#include "cp-support.h"
#include "gdbcmd.h"
#include <algorithm>
+#include <set>

struct psymbol_bcache
{
@@ -823,6 +824,8 @@ psym_relocate (struct objfile *objfile,
if (SYMBOL_SECTION (psym) >= 0)
SYMBOL_VALUE_ADDRESS (psym) += ANOFFSET (delta, SYMBOL_SECTION (psym));
}
+
+ objfile->psymbol_map.clear ();
}

/* Psymtab version of find_last_source_symtab. See its definition in
@@ -1458,6 +1461,84 @@ psym_has_symbols (struct objfile *objfile)
return objfile->psymtabs != NULL;
}

+/* Helper function for psym_find_compunit_symtab_by_address that fills
+ in psymbol_map for a given range of psymbols. */
+
+static void
+psym_fill_psymbol_map (struct objfile *objfile,
+ struct partial_symtab *psymtab,
+ std::set<CORE_ADDR> *seen_addrs,
+ const std::vector<partial_symbol *> &symbols,
+ int start,
+ int length)
+{
+ for (int i = 0; i < length; ++i)
+ {
+ struct partial_symbol *psym = symbols[start + i];
+
+ if (PSYMBOL_CLASS (psym) == LOC_STATIC)
+ {
+ CORE_ADDR addr = SYMBOL_VALUE_ADDRESS (psym);
+ if (seen_addrs->find (addr) == seen_addrs->end ())
+ {
+ seen_addrs->insert (addr);
+ objfile->psymbol_map.emplace_back (addr, psymtab);
+ }
+ }
+ }
+}
+
+/* See find_compunit_symtab_by_address in quick_symbol_functions, in
+ symfile.h. */
+
+static compunit_symtab *
+psym_find_compunit_symtab_by_address (struct objfile *objfile,
+ CORE_ADDR address)
+{
+ if (objfile->psymbol_map.empty ())
+ {
+ struct partial_symtab *pst;
+
+ std::set<CORE_ADDR> seen_addrs;
+
+ ALL_OBJFILE_PSYMTABS_REQUIRED (objfile, pst)
+ {
+ psym_fill_psymbol_map (objfile, pst,
+ &seen_addrs,
+ objfile->global_psymbols,
+ pst->globals_offset,
+ pst->n_global_syms);
+ psym_fill_psymbol_map (objfile, pst,
+ &seen_addrs,
+ objfile->static_psymbols,
+ pst->statics_offset,
+ pst->n_static_syms);
+ }
+
+ objfile->psymbol_map.shrink_to_fit ();
+
+ std::sort (objfile->psymbol_map.begin (), objfile->psymbol_map.end (),
+ [] (const std::pair<CORE_ADDR, partial_symtab *> &a,
+ const std::pair<CORE_ADDR, partial_symtab *> &b)
+ {
+ return a.first < b.first;
+ });
+ }
+
+ auto iter = std::lower_bound
+ (objfile->psymbol_map.begin (), objfile->psymbol_map.end (), address,
+ [] (const std::pair<CORE_ADDR, partial_symtab *> &a,
+ CORE_ADDR b)
+ {
+ return a.first < b;
+ });
+
+ if (iter == objfile->psymbol_map.end () || iter->first != address)
+ return NULL;
+
+ return psymtab_to_symtab (objfile, iter->second);
+}
+
const struct quick_symbol_functions psym_functions =
{
psym_has_symbols,
@@ -1474,6 +1555,7 @@ const struct quick_symbol_functions psym_functions =
psym_map_matching_symbols,
psym_expand_symtabs_matching,
psym_find_pc_sect_compunit_symtab,
+ psym_find_compunit_symtab_by_address,
psym_map_symbol_filenames
};

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));
+}
+
\f

/* 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;
diff --git a/gdb/symfile-debug.c b/gdb/symfile-debug.c
index 49c472e0..1754754 100644
--- a/gdb/symfile-debug.c
+++ b/gdb/symfile-debug.c
@@ -365,6 +365,33 @@ debug_qf_map_symbol_filenames (struct objfile *objfile,
need_fullname);
}

+static struct compunit_symtab *
+debug_qf_find_compunit_symtab_by_address (struct objfile *objfile,
+ CORE_ADDR address)
+{
+ const struct debug_sym_fns_data *debug_data
+ = ((const struct debug_sym_fns_data *)
+ objfile_data (objfile, symfile_debug_objfile_data_key));
+ fprintf_filtered (gdb_stdlog,
+ "qf->find_compunit_symtab_by_address (%s, %s)\n",
+ objfile_debug_name (objfile),
+ hex_string (address));
+
+ struct compunit_symtab *result = NULL;
+ if (debug_data->real_sf->qf->map_symbol_filenames != NULL)
+ result
+ = debug_data->real_sf->qf->find_compunit_symtab_by_address (objfile,
+ address);
+
+ fprintf_filtered (gdb_stdlog,
+ "qf->find_compunit_symtab_by_address (...) = %s\n",
+ result
+ ? debug_symtab_name (compunit_primary_filetab (result))
+ : "NULL");
+
+ return result;
+}
+
static const struct quick_symbol_functions debug_sym_quick_functions =
{
debug_qf_has_symbols,
@@ -381,6 +408,7 @@ static const struct quick_symbol_functions debug_sym_quick_functions =
debug_qf_map_matching_symbols,
debug_qf_expand_symtabs_matching,
debug_qf_find_pc_sect_compunit_symtab,
+ debug_qf_find_compunit_symtab_by_address,
debug_qf_map_symbol_filenames
};
\f
diff --git a/gdb/symfile.h b/gdb/symfile.h
index 3472aa0..e742984 100644
--- a/gdb/symfile.h
+++ b/gdb/symfile.h
@@ -270,6 +270,14 @@ struct quick_symbol_functions
(struct objfile *objfile, struct bound_minimal_symbol msymbol,
CORE_ADDR pc, struct obj_section *section, int warn_if_readin);

+ /* Return the comp unit from OBJFILE that contains a symbol at
+ ADDRESS. Return NULL if there is no such comp unit. Unlike
+ find_pc_sect_compunit_symtab, any sort of symbol (not just text
+ symbols) can be considered, and only exact address matches are
+ considered. This pointer may be NULL. */
+ struct compunit_symtab *(*find_compunit_symtab_by_address)
+ (struct objfile *objfile, CORE_ADDR address);
+
/* Call a callback for every file defined in OBJFILE whose symtab is
not already read in. FUN is the callback. It is passed the file's
FILENAME, the file's FULLNAME (if need_fullname is non-zero), and
diff --git a/gdb/symtab.c b/gdb/symtab.c
index ebb7fbe..38bc713 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -2964,6 +2964,45 @@ find_pc_compunit_symtab (CORE_ADDR pc)
{
return find_pc_sect_compunit_symtab (pc, find_pc_mapped_section (pc));
}
+
+/* See symtab.h. */
+
+struct symbol *
+find_symbol_at_address (CORE_ADDR address)
+{
+ struct objfile *objfile;
+
+ ALL_OBJFILES (objfile)
+ {
+ if (objfile->sf == NULL
+ || objfile->sf->qf->find_compunit_symtab_by_address == NULL)
+ continue;
+
+ struct compunit_symtab *symtab
+ = objfile->sf->qf->find_compunit_symtab_by_address (objfile, address);
+ if (symtab != NULL)
+ {
+ const struct blockvector *bv = COMPUNIT_BLOCKVECTOR (symtab);
+
+ for (int i = GLOBAL_BLOCK; i <= STATIC_BLOCK; ++i)
+ {
+ struct block *b = BLOCKVECTOR_BLOCK (bv, i);
+ struct block_iterator iter;
+ struct symbol *sym;
+
+ ALL_BLOCK_SYMBOLS (b, iter, sym)
+ {
+ if (SYMBOL_CLASS (sym) == LOC_STATIC
+ && SYMBOL_VALUE_ADDRESS (sym) == address)
+ return sym;
+ }
+ }
+ }
+ }
+
+ return NULL;
+}
+
\f

/* Find the source file and line number for a given PC value and SECTION.
diff --git a/gdb/symtab.h b/gdb/symtab.h
index 7b8b5cc..0845ae8 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -1061,6 +1061,11 @@ struct symbol
In this case the symbol is really a "struct template_symbol". */
unsigned is_cplus_template_function : 1;

+ /* True if this is a Rust virtual table. In this case, the symbol
+ can be downcast to "struct rust_vtable_symbol". */
+
+ unsigned is_rust_vtable : 1;
+
/* Line number of this symbol's definition, except for inlined
functions. For an inlined function (class LOC_BLOCK and
SYMBOL_INLINED set) this is the line number of the function's call
@@ -1179,6 +1184,15 @@ struct template_symbol
struct symbol **template_arguments;
};

+/* A symbol that represents a Rust virtual table object. */
+
+struct rust_vtable_symbol : public symbol
+{
+ /* The concrete type for which this vtable was created; that is, in
+ "impl Trait for Type", this is "Type". */
+ struct type *concrete_type;
+};
+
\f
/* Each item represents a line-->pc (or the reverse) mapping. This is
somewhat more wasteful of space than one might wish, but since only
@@ -1605,6 +1619,11 @@ extern struct symbol *find_pc_function (CORE_ADDR);

extern struct symbol *find_pc_sect_function (CORE_ADDR, struct obj_section *);

+/* Find the symbol at the given address. Returns NULL if no symbol
+ found. Only exact matches for ADDRESS are considered. */
+
+extern struct symbol *find_symbol_at_address (CORE_ADDR);
+
extern int find_pc_partial_function_gnu_ifunc (CORE_ADDR pc, const char **name,
CORE_ADDR *address,
CORE_ADDR *endaddr,
diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index bb8dd79..a9e2344 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2017-11-17 Tom Tromey <tom@tromey.com>
+
+ * gdb.rust/traits.rs: New file.
+ * gdb.rust/traits.exp: New file.
+
2017-11-15 Simon Marchi <simon.marchi@ericsson.com>

* gdb.tui/completionn.exp (test_tab_completion): Add space in
diff --git a/gdb/testsuite/gdb.rust/traits.exp b/gdb/testsuite/gdb.rust/traits.exp
new file mode 100644
index 0000000..26686ef
--- /dev/null
+++ b/gdb/testsuite/gdb.rust/traits.exp
@@ -0,0 +1,47 @@
+# Copyright (C) 2017 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+# Test trait object printing.
+
+load_lib rust-support.exp
+if {[skip_rust_tests]} {
+ continue
+}
+
+standard_testfile .rs
+if {[prepare_for_testing "failed to prepare" $testfile $srcfile {debug rust}]} {
+ return -1
+}
+
+set readelf_program [gdb_find_readelf]
+set result [catch "exec $readelf_program --debug-dump=info $binfile" output]
+if {$result != 0} {
+ untested "could not read ${binfile} with readelf"
+ return
+}
+
+if {![regexp DW_AT_containing_type $output]} {
+ untested "Rust compiler did not emit DW_AT_containing_type"
+ return
+}
+
+set line [gdb_get_line_number "set breakpoint here"]
+if {![runto ${srcfile}:$line]} {
+ untested "could not run to breakpoint"
+ return -1
+}
+
+gdb_test "print *td" " = 23.5"
+gdb_test "print *tu" " = 23"
diff --git a/gdb/testsuite/gdb.rust/traits.rs b/gdb/testsuite/gdb.rust/traits.rs
new file mode 100644
index 0000000..98f841b
--- /dev/null
+++ b/gdb/testsuite/gdb.rust/traits.rs
@@ -0,0 +1,37 @@
+// Copyright (C) 2017 Free Software Foundation, Inc.
+
+// This program is free software; you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation; either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program. If not, see <http://www.gnu.org/licenses/>.
+
+#![allow(dead_code)]
+#![allow(unused_variables)]
+#![allow(unused_assignments)]
+
+pub trait T {
+}
+
+impl T for f64 {
+}
+
+impl T for u8 {
+}
+
+pub fn main() {
+ let d = 23.5f64;
+ let u = 23u8;
+
+ let td = &d as &T;
+ let tu = &u as &T;
+
+ println!(""); // set breakpoint here
+}
--
2.9.5