Undocumented environment variables?

Problem you are trying to solve

https://rust-lang.github.io/rustup/environment-variables.html
https://rust-lang.github.io/rustup/dev-guide/tips-and-tricks.html

After reading the rustup code a little (https://github.com/rust-lang/rust-forge/issues/760), I found out that not all the environment variables it supports are documented. A quick search using grep -rniI -e "var(" -e "env\!(" -e "env(" -e "var_os(" revealed at least these:

https://github.com/rust-lang/rustup/blob/edf5c22d478ecce60ae35e3d3a1ae5ff04c6bedb/src/cli/self_update/unix.rs#L21

https://github.com/rust-lang/rustup/blob/edf5c22d478ecce60ae35e3d3a1ae5ff04c6bedb/src/cli/self_update/windows.rs#L186

https://github.com/rust-lang/rustup/blob/edf5c22d478ecce60ae35e3d3a1ae5ff04c6bedb/src/cli/self_update.rs#L512

https://github.com/rust-lang/rustup/blob/edf5c22d478ecce60ae35e3d3a1ae5ff04c6bedb/src/cli/self_update.rs#L627

https://github.com/rust-lang/rustup/blob/edf5c22d478ecce60ae35e3d3a1ae5ff04c6bedb/src/dist/mod.rs#L368

https://github.com/rust-lang/rustup/blob/edf5c22d478ecce60ae35e3d3a1ae5ff04c6bedb/src/dist/mod.rs#L534

https://github.com/rust-lang/rustup/blob/edf5c22d478ecce60ae35e3d3a1ae5ff04c6bedb/src/dist/mod.rs#L660

• Particularly important because current doc directly advises using RUSTUP_DIST_SERVER for this, contrary to rust-lang/rustup@f08c4fe8026cb48582b01a27d4ff72cc51b5299d:
  https://github.com/rust-lang/rustup/blob/edf5c22d478ecce60ae35e3d3a1ae5ff04c6bedb/doc/user-guide/src/environment-variables.md?plain=1#L17-L20

https://github.com/rust-lang/rustup/blob/edf5c22d478ecce60ae35e3d3a1ae5ff04c6bedb/src/utils/utils.rs#L248

https://github.com/rust-lang/rustup/blob/edf5c22d478ecce60ae35e3d3a1ae5ff04c6bedb/src/config.rs#L274

https://github.com/rust-lang/rustup/blob/edf5c22d478ecce60ae35e3d3a1ae5ff04c6bedb/src/config.rs#L400

https://github.com/rust-lang/rustup/blob/edf5c22d478ecce60ae35e3d3a1ae5ff04c6bedb/src/toolchain.rs#L247

Solution you'd like

Describe all this somewhere and/or make a list of variables for internal use so as not to confuse people.

Start with doc/user-guide/src/environment-variables.md and compare its list with the environment-variable reads linked in src/cli/self_update/unix.rs, src/cli/self_update/windows.rs, src/cli/self_update.rs, src/dist/mod.rs, src/utils/utils.rs, src/config.rs, and src/toolchain.rs. Identify which variables should be documented or marked internal, resolve the RUSTUP_DIST_SERVER guidance, and ensure the documentation covers the supported variables.