Warnings being printed after input control is returned to `cmd.exe`

Although this bug has been spotted on outdated versions and a platform that is no longer supported, I still decided to write about it, because it may well still exist and indicate certain problems with the async code. Someone else needs to check it, since I don't have the opportunity to do it yet.

Problem

Assume having a Rust environment with tool management conflict(s) between Cargo and Rustup, which results in the well-known warning: tool is already installed. So, when a version specified in ${RUSTUP_UPDATE_ROOT}/rustup/release-stable.toml is distinct and I run rustup self update, then after downloading and installing it these warnings will be output literally into the command prompt. Moreover, this also sometimes happens not immediately, but literally a few seconds later. To me, this pretty suggests some kind of multithreading issue.

And once I noticed the same bug on another command: rustup toolchain install 1.77-gnu 1.77-i686 1.77-i686-gnu:

Configuration

Rustup versions tested: 1.27.1, 1.27.1+381 (03b7c56bf 2024-07-30)
Operating system: Windows 7 Ultimate (Service Pack 1) x64

Additional notes

1. Setting the environment variable RUSTUP_IO_THREADS=1 doesn't affect the observed behavior.
2. I tested version 1.27.1+381 because 03b7c56bf0b4228977257810218b963b7246cf62 is the last revision that can be built with Rust 1.77.2 and run on Windows 7 - I deliberately ran git bisect to find this out. It's worth noting that the bug is also present there, but in addition the text can no longer be colored in cmd.exe starting with commit 509714deb4b8bc5be7e38e2f3779ed4e2b704177, which is shown on the first screenshot as well.

Reproduce the delayed warnings with rustup self update and rustup toolchain install 1.77-gnu 1.77-i686 1.77-i686-gnu on the Windows configuration described. Compare behavior with RUSTUP_IO_THREADS=1 and trace the asynchronous output paths; done means identifying the cause and preventing warnings from appearing after control returns to cmd.exe.