Maybe tell about rustup show

[bjorn3]

It could be near (or replace):

When playing with different toolchains, it is important to check not only what the default toolchainis, but also if overrides are currently set for some directories.

$ rustup show
Default host: x86_64-unknown-linux-gnu
rustup home:  /home/bjorn/.rustup

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

stable-x86_64-unknown-linux-gnu (default)
nightly-2020-06-23-x86_64-unknown-linux-gnu
nightly-x86_64-unknown-linux-gnu
cg_clif_rustc
rust2-stage1
rust2-stage2

installed targets for active toolchain
--------------------------------------

x86_64-pc-windows-gnu
x86_64-unknown-linux-gnu
x86_64-unknown-linux-musl

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

stable-x86_64-unknown-linux-gnu (default)
rustc 1.44.0 (49cae5576 2020-06-01)

[ad-anssi]

Hello,
Thanks for the good idea. I think we should indeed go for a replacement, since there other ways to override default toolchain that are not yet mentioned in the guide (rust-toolchain file, RUST_TOOLCHAIN env var, etc). It should be also worth to mention the paragraph Override precedence of the cargo README.