Fix &str printing in Rust

Printing a string slice ("&str") in Rust would print until the
terminating \0; but that is incorrect because a slice has a length.
This fixes &str printing, and arranges to preserve the type name when
slicing a slice, so that printing a slice of an "&str" works as well.

This is PR rust/22236.

2017-10-02  Tom Tromey  <tom@tromey.com>

	PR rust/22236:
	* rust-lang.c (rust_val_print_str): New function.
	(val_print_struct): Call it.
	(rust_subscript): Preserve name of slice type.

2017-10-02  Tom Tromey  <tom@tromey.com>

	PR rust/22236:
	* gdb.rust/simple.rs (main): New variable "fslice".
	* gdb.rust/simple.exp: Add slice tests.  Update string tests.

1 files changed, 27 insertions, 2 deletions

diff --git a/gdb/rust-lang.c b/gdb/rust-lang.c
index 9b64efa..261ddb1 100644
--- a/gdb/rust-lang.c
+++ b/gdb/rust-lang.c
@@ -467,6 +467,21 @@ rust_printstr (struct ui_file *stream, struct type *type,
 
 
 
+/* Helper function to print a string slice.  */
+
+static void
+rust_val_print_str (struct ui_file *stream, struct value *val,
+		    const struct value_print_options *options)
+{
+  struct value *base = value_struct_elt (&val, NULL, "data_ptr", NULL,
+					 "slice");
+  struct value *len = value_struct_elt (&val, NULL, "length", NULL, "slice");
+
+  val_print_string (TYPE_TARGET_TYPE (value_type (base)), "UTF-8",
+		    value_as_address (base), value_as_long (len), stream,
+		    options);
+}
+
 /* rust_print_type branch for structs and untagged unions.  */
 
 static void
@@ -477,6 +492,13 @@ val_print_struct (struct type *type, int embedded_offset,
 {
   int i;
   int first_field;
+
+  if (rust_slice_type_p (type) && strcmp (TYPE_NAME (type), "&str") == 0)
+    {
+      rust_val_print_str (stream, val, options);
+      return;
+    }
+
   bool is_tuple = rust_tuple_type_p (type);
   bool is_tuple_struct = !is_tuple && rust_tuple_struct_type_p (type);
   struct value_print_options opts;
@@ -1562,8 +1584,11 @@ rust_subscript (struct expression *exp, int *pos, enum noside noside,
 	  usize = language_lookup_primitive_type (exp->language_defn,
 						  exp->gdbarch,
 						  "usize");
-	  slice = rust_slice_type ("&[*gdb*]", value_type (result),
-				   usize);
+	  const char *new_name = ((type != nullptr
+				   && rust_slice_type_p (type))
+				  ? TYPE_NAME (type) : "&[*gdb*]");
+
+	  slice = rust_slice_type (new_name, value_type (result), usize);
 
 	  addrval = value_allocate_space_in_inferior (TYPE_LENGTH (slice));
 	  addr = value_as_long (addrval);