gdb/rust: fix crash for expression debug with strings

While working on another patch I did this:

  (gdb) set debug expression 1
  (gdb) set language rust
  (gdb) p "foo"
  Operation: OP_AGGREGATE
   Type: &str

  Fatal signal: Segmentation fault
  ... etc ...

The problem is that the second field of the rust_aggregate_operation
is created as a nullptr, this can be seen in rust-parse.c. in the
function rust_parser::parse_string().

However, in expop.h, in the function dump_for_expression, we make the
assumption that the expressions will never be nullptr.

I did consider moving the nullptr handling into a new function
rust_aggregate_operation::dump, however, as the expression debug
dumping code is not exercised as much as it might be, I would rather
that this code be hardened and able to handle a nullptr without
crashing, so I propose that we add nullptr handling into the general
dump_for_expression function.  The behaviour is now:

  (gdb) set debug expression 1
  (gdb) set language rust
  (gdb) p "foo"
  Operation: OP_AGGREGATE
   Type: &str
   nullptr
   Vector:
    String: data_ptr
    Operation: UNOP_ADDR
     Operation: OP_STRING
      String: foo
    String: length
    Operation: OP_LONG
     Type: usize
     Constant: 3
  evaluation of this expression requires the target program to be active
  (gdb)

There's a new test to check for this case.

Reviewed-By: Tom Tromey <tom@tromey.com>

1 files changed, 21 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.rust/expr.exp b/gdb/testsuite/gdb.rust/expr.exp
index 908d1a3..ce2cce2 100644
--- a/gdb/testsuite/gdb.rust/expr.exp
+++ b/gdb/testsuite/gdb.rust/expr.exp
@@ -148,3 +148,24 @@ gdb_test "print r#" "No symbol 'r' in current context"
 gdb_test "printf \"%d %d\\n\", 23+1, 23-1" "24 22"
 
 gdb_test "print 5," "Syntax error near ','"
+
+# Check expression debug works for strings.
+gdb_test "with debug expression 1 -- print \"foo\"" \
+    [multi_line \
+	 "Operation: OP_AGGREGATE" \
+	 " Type: &str" \
+	 " nullptr" \
+	 " Vector:" \
+	 "  String: data_ptr" \
+	 "  Operation: UNOP_ADDR" \
+	 "   Operation: OP_STRING" \
+	 "    String: foo" \
+	 "  String: length" \
+	 "  Operation: OP_LONG" \
+	 "   Type: usize" \
+	 "   Constant: 3" \
+	 "Operation: OP_LONG" \
+	 " Type: i32" \
+	 " Constant: 0" \
+	 "evaluation of this expression requires the target program to be active"] \
+    "print a string with expression debug turned on"