Fix read_buf uses in std

[a1phyr]

Following lib-team decision here: #78485 (comment)

[rustbot]

r? @workingjubilee

rustbot has assigned @workingjubilee.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[rust-log-analyzer]

The job x86_64-gnu-tools failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Caused by:
  process didn't exit successfully: `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-rustc/release/build/lock_api-7d87a5aef10d3105/build-script-build` (exit status: 101)
  --- stderr
  thread 'main' panicked at /cargo/registry/src/index.crates.io-6f17d22bba15001f/autocfg-1.3.0/src/lib.rs:132:20:
  called `Result::unwrap()` on an `Err` value: Error { kind: Other("could not find rustc release") }
warning: build failed, waiting for other jobs to finish...
Build completed unsuccessfully in 0:05:05
+ set -e
+ cat /tmp/toolstate/toolstates.json
+ cat /tmp/toolstate/toolstates.json
{"lld-wrapper":"test-fail"}
+ python3 ../x.py test --stage 2 check-tools
##[group]Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
##[endgroup]
ERROR: Tool `book` was not recorded in tool state.
ERROR: Tool `nomicon` was not recorded in tool state.
ERROR: Tool `reference` was not recorded in tool state.
ERROR: Tool `rust-by-example` was not recorded in tool state.
ERROR: Tool `edition-guide` was not recorded in tool state.
ERROR: Tool `embedded-book` was not recorded in tool state.
  local time: Wed May 22 16:05:46 UTC 2024
  network time: Wed, 22 May 2024 16:05:46 GMT
##[error]Process completed with exit code 1.
Post job cleanup.

[workingjubilee]

Waiting to see how a bit more of the discussion plays out given #78485 (comment)

[briansmith]

It seems like there should be tests for this?