Error for `rustup run +stable cargo check` is misleading

[epage]

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

As a user used to +toolchain, it can be easy to accidentally do that with cargo run but the error you get doesn't communicate that problem:

$ rustup run +stable cargo check
error: toolchain '+stable' is not installed

Steps

1. Run rustup run +stable cargo -h

Possible Solution(s)

If + is reserved from being a leading character in toolchains: focus the error around the + prefix syntax being unsupported and provide a help message saying to drop it

If + is allowed as a leading character in toolchains: Check for a + prefixand add a note saying that the+` prefix syntax isn't used here and it can be dropped if thats the users intent

Notes

No response

Rustup version

rustup 1.28.2 (e4f3ad6f8 2025-04-28)

Installed toolchains

Default host: x86_64-unknown-linux-gnu
rustup home:  /home/epage/.rustup

installed toolchains
--------------------
stable-x86_64-unknown-linux-gnu (active, default)
beta-x86_64-unknown-linux-gnu
nightly-x86_64-unknown-linux-gnu
nightly-2023-11-13-x86_64-unknown-linux-gnu
nightly-2024-06-03-x86_64-unknown-linux-gnu
nightly-2024-09-10-x86_64-unknown-linux-gnu
nightly-2024-10-19-x86_64-unknown-linux-gnu
nightly-2025-01-27-x86_64-unknown-linux-gnu
nightly-2025-04-02-x86_64-unknown-linux-gnu
nightly-2025-06-26-x86_64-unknown-linux-gnu
1.0.0-x86_64-unknown-linux-gnu
1.48-x86_64-unknown-linux-gnu
1.48.0-x86_64-unknown-linux-gnu
1.56.0-x86_64-unknown-linux-gnu
1.57.0-x86_64-unknown-linux-gnu
1.58.0-x86_64-unknown-linux-gnu
1.60.0-x86_64-unknown-linux-gnu
1.64-x86_64-unknown-linux-gnu
1.64.0-x86_64-unknown-linux-gnu
1.65-x86_64-unknown-linux-gnu
1.65.0-x86_64-unknown-linux-gnu
1.66-x86_64-unknown-linux-gnu
1.66.0-x86_64-unknown-linux-gnu
1.70-x86_64-unknown-linux-gnu
1.70.0-x86_64-unknown-linux-gnu
1.71-x86_64-unknown-linux-gnu
1.71.0-x86_64-unknown-linux-gnu
1.71.1-x86_64-unknown-linux-gnu
1.72-x86_64-unknown-linux-gnu
1.74-x86_64-unknown-linux-gnu
1.75-x86_64-unknown-linux-gnu
1.76-x86_64-unknown-linux-gnu
1.80-x86_64-unknown-linux-gnu
1.80.1-x86_64-unknown-linux-gnu
1.81-x86_64-unknown-linux-gnu
1.83-x86_64-unknown-linux-gnu
1.83.0-x86_64-unknown-linux-gnu
1.84-x86_64-unknown-linux-gnu
1.85-x86_64-unknown-linux-gnu
1.86-x86_64-unknown-linux-gnu
1.86.0-x86_64-unknown-linux-gnu
1.88-x86_64-unknown-linux-gnu
1.89-x86_64-unknown-linux-gnu
1.90-x86_64-unknown-linux-gnu
stage1

active toolchain
----------------
name: stable-x86_64-unknown-linux-gnu
active because: it's the default toolchain
installed targets:
  aarch64-apple-darwin
  i686-unknown-linux-gnu
  i686-unknown-linux-musl
  powerpc-unknown-linux-gnu
  wasm32-unknown-unknown
  x86_64-pc-windows-gnu
  x86_64-pc-windows-msvc
  x86_64-unknown-linux-gnu
  x86_64-unknown-linux-musl
  x86_64-unknown-none

OS version

Linux dross 6.16.3-76061603-generic #202508231538~1758561135~22.04~171c8de SMP PREEMPT_DYNAMIC Mon S x86_64 x86_64 x86
_64 GNU/Linux