`rustup set default-host` does not change the default toolchain

[jyn514]

Problem you are trying to solve

i have a stable-x86_64-pc-windows-gnu toolchain as my default:

PS C:\Users\jyn\src\example> rustup default
stable-x86_64-pc-windows-gnu (default)

i want it to be -msvc instead so i can use windbg (i don't have gdb installed). i ran rustup set default-host x86_64-pc-windows-msvc, which succeeded, but did not install any toolchains or change the default:

PS C:\Users\jyn\src\example> rustup show
Default host: x86_64-pc-windows-msvc
rustup home:  C:\Users\jyn\.rustup

installed toolchains
--------------------

stable-x86_64-pc-windows-gnu (default)
nightly-x86_64-pc-windows-gnu
stage1

active toolchain
----------------

stable-x86_64-pc-windows-gnu (default)
rustc 1.75.0 (82e1608df 2023-12-21)

then i tried rustup update stable, which did install the -msvc toolchain, but did not change the default:

info: syncing channel updates for 'stable-x86_64-pc-windows-msvc'
...
PS C:\Users\jyn\src\example> rustup default
stable-x86_64-pc-windows-gnu (default)

Solution you'd like

rustup set default-host foo should behave the same as rustup set default-host foo && rustup default $channel-foo, where $channel is the channel of the current default toolchain.

ideally, rustup would give a warning if the default toolchain did not match the default host (e.g. rustup default nightly-x86_64-pc-windows-gnu with a default-host of msvc).

Notes

rustup 1.26.0 (5af9b94 2023-04-05)

[rami3l]

Following the long-term goal of eliminating implicit installation (#3635), I think the best thing to do here is to add a warning.

[jyn514]

@rami3l i still think this suggestion from the description is the best approach:

rustup set default-host foo should behave the same as rustup set default-host foo && rustup default $channel-foo, where $channel is the channel of the current default toolchain.

if rustup default nightly for a non-installed toolchain warns instead of installing, that's fine. but i do not think rustup should leave the default toolchain so that it doesn't match the default host.