Io.Writer fix dangling pointer

While underlying writer is Allocating writer buffer can grow in
vtable.drain call. We should not hold pointer to the buffer before that
call and use it after.
This remembers positions instead of holding reference.

Author: ianic

lib/std/Io/Writer.zig

@@ -317,13 +317,14 @@ test "fixed buffer flush" {
 /// Calls `VTable.drain` but hides the last `preserve_len` bytes from the
 /// implementation, keeping them buffered.
 pub fn drainPreserve(w: *Writer, preserve_len: usize) Error!void {
-    const temp_end = w.end -| preserve_len;
-    const preserved = w.buffer[temp_end..w.end];
-    w.end = temp_end;
-    defer w.end += preserved.len;
+    const preserved_head = w.end -| preserve_len;
+    const preserved_tail = w.end;
+    const preserved_len = preserved_tail - preserved_head;
+    w.end = preserved_head;
+    defer w.end += preserved_len;
     assert(0 == try w.vtable.drain(w, &.{""}, 1));
-    assert(w.end <= temp_end + preserved.len);
-    @memmove(w.buffer[w.end..][0..preserved.len], preserved);
+    assert(w.end <= preserved_head + preserved_len);
+    @memmove(w.buffer[w.end..][0..preserved_len], w.buffer[preserved_head..preserved_tail]);
 }
 
 pub fn unusedCapacitySlice(w: *const Writer) []u8 {