`rustup +nightly run cargo` seems like it could work but doesn't

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

In rust-lang/cargo#16149 a user was trying to figure out how to do rustup run with nightly and tried:

$ rustup +nightly run cargo miri test
error: toolchain 'cargo' is not installed

From the user:

... etc. I found many mentions of cargo +toolchain on the net, but it was really quite hard to find the syntax to actually get this to run. One reason is that as a beginner I really had no idea if I was looking for an option to run, not rustup or cargo..

Made worse by the fact that rustup accepts a toolchain parameter like cargo does

Steps

1. Run rustup +nightly run cargo -h

Possible Solution(s)

Option 1: Improve the diagnostic by checking if the toolchain name is a command and suggesting:

• rustup run nightly cargo -h (if they specified a toolchain)
• rustup run <default> cargo -h (if they didn't specify a toolchain)

Option 2: Hack up the command-line in such a way as to make this work

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

Start by reproducing rustup +nightly run cargo -h and compare it with rustup run nightly cargo -h. Read the rustup command-line parsing and diagnostic paths to determine whether the intended outcome is improved guidance or support for the alternate syntax. Done means the chosen behavior is implemented and covered by command-line tests, though no specific test file is named in the issue.