[WIP]: Move std::io into alloc and core

[bushrat011899]

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ACP: rust-lang/libs-team#755
Tracking issue: #154046
Blocked on: #155625
Blocked on: #155849

Description

Moves as much of std::io into core::io and alloc::io as practically possible. Work In Progress until the prerequisite PRs are merged. Likely has documentation issues that will need to be resolved. Will almost certainly be split into smaller PRs as +/- 5000 LoC is a bit much for a single PR IMO.

---

Notes

• No AI tooling of any kind was used during the creation of this PR.

[rust-log-analyzer]

The job aarch64-gnu-llvm-21-2 failed! Check out the build log: (web) (plain enhanced) (plain)

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

##[endgroup]
[TIMING:end] tool::ToolBuild { build_compiler: Compiler { stage: 0, host: aarch64-unknown-linux-gnu, forced_compiler: false }, target: aarch64-unknown-linux-gnu, tool: "linkchecker", path: "src/tools/linkchecker", mode: ToolBootstrap, source_type: InTree, extra_features: [], allow_features: "", cargo_args: [], artifact_kind: Binary } -- 0.144
[TIMING:end] tool::Linkchecker { compiler: Compiler { stage: 0, host: aarch64-unknown-linux-gnu, forced_compiler: false }, target: aarch64-unknown-linux-gnu } -- 0.000
##[group]Testing stage1 Linkcheck (aarch64-unknown-linux-gnu)
std/io/prelude/trait.Read.html:41: broken link - `std/std/io/trait.BufRead.html`
std/io/prelude/trait.Read.html:42: broken link - `std/std/io/struct.BufReader.html`
std/io/prelude/trait.Read.html:216: broken link - `std/std/io/struct.BufReader.html`
std/io/prelude/trait.Read.html:247: broken link - `std/std/io/struct.BufReader.html`
std/io/prelude/trait.Read.html:333: broken link - `std/std/io/struct.BufReader.html`
checked links in: 26.0s
number of HTML files scanned: 62227
number of HTML redirects found: 22172
number of links checked: 2929131
number of links ignored due to external: 180388
number of links ignored due to exceptions: 31
number of intra doc links ignored: 9
errors found: 5
found some broken links
NOTE: if you are adding or renaming a markdown file in a mdBook, don't forget to register the page in SUMMARY.md
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage1-tools-bin/linkchecker /checkout/obj/build/aarch64-unknown-linux-gnu/doc` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1618:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:191:77

--- BACKTRACE vvv
   0: std::backtrace_rs::backtrace::libunwind::trace
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/../../backtrace/src/backtrace/libunwind.rs:117:9
   1: std::backtrace_rs::backtrace::trace_unsynchronized::<<std::backtrace::Backtrace>::create::{closure#0}>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/../../backtrace/src/backtrace/mod.rs:66:14
   2: <std::backtrace::Backtrace>::create
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/backtrace.rs:331:13
   3: <bootstrap::utils::exec::DeferredCommand>::finish_process
             at /checkout/src/bootstrap/src/utils/exec.rs:939:17
   4: <bootstrap::utils::exec::DeferredCommand>::wait_for_output::<&bootstrap::utils::exec::ExecutionContext>
             at /checkout/src/bootstrap/src/utils/exec.rs:831:21
   5: <bootstrap::utils::exec::ExecutionContext>::run
             at /checkout/src/bootstrap/src/utils/exec.rs:741:45
   6: <bootstrap::utils::exec::BootstrapCommand>::run::<&bootstrap::core::builder::Builder>
             at /checkout/src/bootstrap/src/utils/exec.rs:339:27
   7: <bootstrap::core::build_steps::test::Linkcheck as bootstrap::core::builder::Step>::run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:191:77
   8: <bootstrap::core::builder::Builder>::ensure::<bootstrap::core::build_steps::test::Linkcheck>
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1595:36
   9: <bootstrap::core::build_steps::test::Linkcheck as bootstrap::core::builder::Step>::make_run
             at /checkout/src/bootstrap/src/core/build_steps/test.rs:137:21
  10: <bootstrap::core::builder::StepDescription>::maybe_run
             at /checkout/src/bootstrap/src/core/builder/mod.rs:476:13
  11: bootstrap::core::builder::cli_paths::match_paths_to_steps_and_run
             at /checkout/src/bootstrap/src/core/builder/cli_paths.rs:141:22
  12: <bootstrap::core::builder::Builder>::run_step_descriptions
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1138:9
  13: <bootstrap::core::builder::Builder>::execute_cli
             at /checkout/src/bootstrap/src/core/builder/mod.rs:1117:14
  14: <bootstrap::Build>::build
             at /checkout/src/bootstrap/src/lib.rs:803:25
  15: bootstrap::main
             at /checkout/src/bootstrap/src/bin/main.rs:130:11
  16: <fn() as core::ops::function::FnOnce<()>>::call_once
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/core/src/ops/function.rs:250:5
  17: std::sys::backtrace::__rust_begin_short_backtrace::<fn(), ()>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/sys/backtrace.rs:166:18
  18: std::rt::lang_start::<()>::{closure#0}
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/rt.rs:206:18
  19: <&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync as core::ops::function::FnOnce<()>>::call_once
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/core/src/ops/function.rs:287:21
  20: std::panicking::catch_unwind::do_call::<&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync, i32>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panicking.rs:581:40
  21: std::panicking::catch_unwind::<i32, &dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panicking.rs:544:19
  22: std::panic::catch_unwind::<&dyn core::ops::function::Fn<(), Output = i32> + core::panic::unwind_safe::RefUnwindSafe + core::marker::Sync, i32>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panic.rs:359:14
  23: std::rt::lang_start_internal::{closure#0}
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/rt.rs:175:24
  24: std::panicking::catch_unwind::do_call::<std::rt::lang_start_internal::{closure#0}, isize>
             at /rustc/ef0fb8a2563200e322fa4419f09f65a63742038c/library/std/src/panicking.rs:581:40
---
  31: __libc_start_main
  32: _start


Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip tidy --skip src/tools/rust-analyzer --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 0:33:53
  local time: Tue May 19 12:37:44 UTC 2026
  network time: Tue, 19 May 2026 12:37:44 GMT
##[error]Process completed with exit code 1.