`impl core::fmt::Write` for `rustdoc::html::render::Buffer`

[jyn514]

A further performance improvement here would be to make sure that Buffer impls core::fmt::Write, in which case we could use render_into(buf) rather than render(), avoiding the allocation of a temporary String here. Buffer internally seems to just hold a String and it has write_str() and write_fmt() methods already, so perhaps it would make sense to implement that trait? I'm not sure how big/relevant the performance improvement would be.

Originally posted by @djc in #92526 (comment)

[djc]

Since askama::Template implementers get a std::fmt::Display generated for them by Askama's macro, the simpler way to get the meat of this change would probably be to call buf.write_fmt(format_args!("{}", tmpl));

[mfrw]

@rustbot claim
Let me take a stab at it :)