Print Rust unsized array types a bit more nicely

It's a bit difficult to create an unsized array type in Rust, but if
you do, right now ptype will show something like "[u8; ]".  It really
should print "[u8]", though, which is what this patch implements.

This is part of PR 21466.

Built and regtested on x86-64 Fedora 25.  I'm checking this in.

ChangeLog
2017-05-21  Tom Tromey  <tom@tromey.com>

	PR rust/21466:
	* rust-lang.c (rust_print_type) <TYPE_CODE_ARRAY>: Print unsized
	arrays as "[T]", not "[T; ]".

testsuite/ChangeLog
2017-05-21  Tom Tromey  <tom@tromey.com>

	PR rust/21466:
	* gdb.rust/unsized.exp: New file.
	* gdb.rust/unsized.rs: New file.

2 files changed, 68 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.rust/unsized.exp b/gdb/testsuite/gdb.rust/unsized.exp
new file mode 100644
index 0000000..700b214
--- /dev/null
+++ b/gdb/testsuite/gdb.rust/unsized.exp
@@ -0,0 +1,35 @@
+# 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 expression parsing and evaluation that requires Rust compiler.
+
+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 line [gdb_get_line_number "set breakpoint here"]
+if {![runto ${srcfile}:$line]} {
+    untested "could not run to breakpoint"
+    return -1
+}
+
+gdb_test "ptype *us" \
+    " = struct \[a-z:\]*V<\\\[u8\\\]> {.* data: \\\[u8\\\],.*}"
diff --git a/gdb/testsuite/gdb.rust/unsized.rs b/gdb/testsuite/gdb.rust/unsized.rs
new file mode 100644
index 0000000..7081a11
--- /dev/null
+++ b/gdb/testsuite/gdb.rust/unsized.rs
@@ -0,0 +1,33 @@
+// 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)]
+
+struct V<T: ?Sized> {
+    data: T,
+}
+
+type Unsized = V<[u8]>;
+
+fn ignore<T>(x: T) { }
+
+fn main() {
+    let v: Box<V<[u8; 3]>> = Box::new(V { data: [1, 2, 3] });
+    let us: Box<Unsized> = v;
+
+    ignore(us);     // set breakpoint here
+}