`rustup run` also searches PATH

Nobody has claimed this yet.

Problem

; rustup run 1.69 whoami
jyn

This seems ... undesirable. In particular, it means that rustup which and rustup run differ in behavior:

; rustup which whoami
error: not a file: '/Users/jyn/.local/lib/rustup/toolchains/1.69-aarch64-apple-darwin/bin/whoami'

As far as I can tell, the only difference is that rustup run sets RUSTUP_TOOLCHAIN and DYLD_FALLBACK_LIBRARY_PATH:

; rustup run nightly python3 -c $'import os \nfor k,v in os.environ.items(): print(k, v)'  | sort -u | diff - <(python3 -c $'import os \nfor k,v in os.environ.items(): print(k, v)' | sort -u)
8d7
< DYLD_FALLBACK_LIBRARY_PATH /Users/jyn/.local/lib/rustup/toolchains/nightly-aarch64-apple-darwin/lib:/Users/jyn/lib:/usr/local/lib:/usr/lib
29,30d27
< RUSTUP_TOOLCHAIN nightly-aarch64-apple-darwin
< RUST_RECURSION_COUNT 1
49c46
< _ /Users/jyn/.local/lib/cargo/bin/rustup
---
> _ /Applications/Xcode.app/Contents/Developer/usr/bin/python3

my original use case, although it's pretty complicated

i am writing a new rustc_driver tool that goes in the sysroot. i want to be able to run it as cargo +nightly foo instead of PATH=$PATH:$(rustc +nightly --print sysroot)/bin cargo +nightly foo. to that end, i've put a shell script in ~/.cargo/bin/cargo-foo that emulates a rustup proxy:

#!/bin/sh
me=$(basename $0)
exec rustup run "$RUSTUP_TOOLCHAIN" $me "$@"

This works fine if the tool is actually present in the sysroot. However, if it's not present, rustup falls back to path and re-executes this same script, eventually leading to a recursion error that it's nested too deeply. I would like to instead give a hard error. Today, i have to workaround rustup's behavior with more calls:

# Make sure this is actually installed for the given toolchain. `rustup run` falls back to PATH,
# which will recursively invoke this script; that's not what we want.
if ! rustup which $me --toolchain $RUSTUP_TOOLCHAIN 2>/dev/null; then
    printf "\033[31;1merror:\033[0m '$me' is not installed for the toolchain '$RUSTUP_TOOLCHAIN'\n"
    exit 1
fi

it would be nice to be able to avoid that.

Steps

rustup run 1.69 whoami (or any installed toolchain)

Possible Solution(s)

Break hard; only look in toolchains/1.69/bin
Add a feature flag to remove the PATH lookup
Document the difference somewhere

Notes

No response

Rustup version

rustup 1.26.0 (5af9b9484 2023-04-05)

Installed toolchains

Default host: aarch64-apple-darwin
rustup home:  /Users/jyn/.local/lib/rustup

installed toolchains
--------------------

nightly-2022-12-07-aarch64-apple-darwin
nightly-2023-03-14-aarch64-apple-darwin
nightly-2023-04-12-aarch64-apple-darwin
nightly-aarch64-apple-darwin (default)
1.60-aarch64-apple-darwin
1.64-aarch64-apple-darwin
1.65-aarch64-apple-darwin
1.68-aarch64-apple-darwin
1.69-aarch64-apple-darwin
stage1
stage2
1.60.0-aarch64-apple-darwin

installed targets for active toolchain
--------------------------------------

aarch64-apple-darwin
aarch64-unknown-linux-gnu

active toolchain
----------------

1.69-aarch64-apple-darwin (overridden by '/Users/jyn/src/redacted/rust-toolchain.toml')
rustc 1.69.0 (84c898d65 2023-04-16)

Start with the rustup run and rustup which command paths and reproduce rustup run 1.69 whoami using a command found on PATH but absent from the toolchain bin directory. Compare the lookup behavior and existing tests, then establish the intended policy among hard failure, an opt-out, or documentation; done means the selected behavior is implemented and covered by regression tests.

Written by the indexing model from the issue text.

Description

Problem

Steps

Possible Solution(s)

Notes

Rustup version

Installed toolchains

Contributor guide

First steps

Research direction

Assessment

`rustup run` also searches PATH

Description

Problem

Steps

Possible Solution(s)

Notes

Rustup version

Installed toolchains

Contributor guide

First steps

Research direction

Assessment

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