set RUSTUP_UPDATE_ROOT to empty update `failed to parse url`

[A4-Tacks]

Verification

• I searched for recent similar issues at https://github.com/rust-lang/rustup/issues?q=is%3Aissue+is%3Aopen%2Cclosed and found no duplicates.
• I am on the latest version of Rustup according to https://github.com/rust-lang/rustup/tags and am still able to reproduce my issue.

Problem

When RUSTUP_UPDATE_ROOT is empty, the URL will still be concatenated

Steps

1. Run RUSTUP_DIST_SERVER= RUSTUP_UPDATE_ROOT= rustup update nightly

   Output:

   info: syncing channel updates for 'nightly-aarch64-unknown-linux-gnu'
   
     nightly-aarch64-unknown-linux-gnu unchanged - rustc 1.90.0-nightly (9982d6462 2025-07-20)
   
   info: checking for self-update
   error: failed to parse url: /release-stable.toml: relative URL without a base
   

Possible Solution(s)

No response

Notes

No response

Rustup version

rustup 1.28.2 (e4f3ad6f8 2025-04-28)
info: This is the version for the rustup toolchain manager, not the rustc compiler.
info: The currently active `rustc` version is `rustc 1.90.0-nightly (9982d6462 2025-07-20)`

Installed toolchains

Default host: aarch64-unknown-linux-gnu
rustup home:  /home/lrne/.rustup

installed toolchains
--------------------
stable-aarch64-unknown-linux-gnu
nightly-aarch64-unknown-linux-gnu (active, default)

active toolchain
----------------
name: nightly-aarch64-unknown-linux-gnu
active because: it's the default toolchain
installed targets:
  aarch64-unknown-linux-gnu

OS version

ArchLinux

[FranciscoTGouveia]

Hello @A4-Tacks,

Thanks for the minimal repro! I was able to reproduce the issue on my machine.

Just to clarify, what would you expect the correct behavior to be? Should it ignore the empty string and fall back to the default update root?

[A4-Tacks]

Just to clarify, what would you expect the correct behavior to be? Should it ignore the empty string and fall back to the default update root?

I hope it can be the same as (unset RUSTUP_DIST_SERVER RUSTUP_UPDATE_ROOT; rustup update nightly)

[djc]

Submitted a potential fix in #4422.