Rustup installs required toolchain when cargo-completion called

Verification

• I searched for recent similar issues at https://github.com/rust-lang/rustup/issues?q=is%3Aissue 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

• Cargo has dynamic completions for shells.
• Rustup respects rust-toolchain(.toml).
• ☝🏻this is conflict

rustup default nightly; rustup uninstall stable; 
echo "toolchain.channel = \"stable\"" > rust-toolchain.toml
CARGO_COMPLETE=fish cargo -
# then rustup installs stable toolchain :(
# my shell (fish) completion function: `source (CARGO_COMPLETE=fish cargo | psub)`

Steps

1. rustup default nightly
2. echo "toolchain.channel = \"not-yet-installed-toolchain\"" > rust-toolchain.toml
3. CARGO_COMPLETE=fish cargo -

Possible Solution(s)

My workaround:

source (CARGO_COMPLETE=fish RUSTUP_TOOLCHAIN="nightly" cargo +nightly | psub)

Notes

I suppose rustup should not respects overrides (rust-toolchain(.toml)) if called with non-empty env CARGO_COMPLETE.

Update (thanks, @djc): source (RUSTUP_AUTO_INSTALL=0 CARGO_COMPLETE=fish cargo | psub)

Rustup version

rustup 1.29.0 (28d1352db 2026-03-05)

Installed toolchains

nightly only

OS version

macOs 15.7.7

Reproduce the issue with rust-toolchain.toml, an uninstalled toolchain, and CARGO_COMPLETE=fish cargo -. Start by tracing how rustup handles toolchain overrides during cargo completion, including the RUSTUP_AUTO_INSTALL workaround. Done means completion works without installing the toolchain selected by rust-toolchain.toml.