Temporary stop using std.conv.to, text, and std.string.format at several critical points + issue 10254
[appender] Reuse ensureAddable for reserve implementation

AFAICS, they have mostly same code.
[appender] Accepting qualified buffer is not safe behavior

To reduce existing code breaking, mark such construction as @system.
[appender] Determine @trusted operations
[parse, str.to!x] Determine @trusted operations
[formatValue] (bool, null, integrals, floatings, chars)

Floating point is blocked by sprintf
[non to!string] because they are unrelated to formatValue
[format] Add @safe/pure ability to writeUpToNextSpec, fillUp, and formattedWrite
Infer attributes in retro range primitives
[formatValue] pointer
[to!string] severals
Enable using std.string.format()