Fix Bugzilla 20870 - std.outbuffer.printf is trusted (dlang#9037)

std/outbuffer.d

@@ -22,9 +22,10 @@ import std.traits : isSomeString;
  * OutBuffer's byte order is the format native to the computer.
  * To control the byte order (endianness), use a class derived
  * from OutBuffer.
+ *
  * OutBuffer's internal buffer is allocated with the GC. Pointers
  * stored into the buffer are scanned by the GC, but you have to
- * ensure proper alignment, e.g. by using alignSize((void*).sizeof).
+ * ensure proper alignment, e.g. by using `alignSize((void*).sizeof)`.
  */
 
 class OutBuffer
@@ -297,7 +298,7 @@ class OutBuffer
  * Append output of C's vprintf() to internal buffer.
  */
 
- void vprintf(scope string format, va_list args) @trusted nothrow
+ void vprintf(scope string format, va_list args) @system nothrow
  {
  import core.stdc.stdio : vsnprintf;
  import core.stdc.stdlib : alloca;
@@ -342,7 +343,7 @@ class OutBuffer
  * Append output of C's printf() to internal buffer.
  */
 
- void printf(scope string format, ...) @trusted
+ void printf(scope string format, ...) @system
  {
  va_list ap;
  va_start(ap, format);
@@ -475,7 +476,7 @@ class OutBuffer
  buf.write("hello");
  buf.write(cast(byte) 0x20);
  buf.write("world");
- buf.printf(" %d", 62665);
+ buf.writef(" %d", 62665);
  assert(cmp(buf.toString(), "hello world 62665") == 0);
 
  buf.clear();