Avoid needless memcpy for codeblock

[Mark-Simulacrum]

Following up on #85833, but probably not actually directly related.

r? @ghost until we get (potentially) positive perf

[Mark-Simulacrum]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 745ab1a with merge b540e394ce589b5d0dbafd8632c8fa5565f71eb5...

[bors]

☀️ Try build successful - checks-actions
Build commit: b540e394ce589b5d0dbafd8632c8fa5565f71eb5 (b540e394ce589b5d0dbafd8632c8fa5565f71eb5)

[rust-timer]

Queued b540e394ce589b5d0dbafd8632c8fa5565f71eb5 with parent e269e6b, future comparison URL.

[rust-timer]

Finished benchmarking commit (b540e394ce589b5d0dbafd8632c8fa5565f71eb5): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

@bors rollup=never
@rustbot label: +S-waiting-on-review -S-waiting-on-perf -perf-regression