stable: Give a better error message for `rustc +nightly -Zunstable-options`

[jyn514]

Code

$(rustup which rustc --toolchain 1.68) +nightly -Zunstable-options

Current output

error: the option `Z` is only accepted on the nightly compiler

Desired output

error: the option `Z` is only accepted on the nightly compiler
error: couldn't read +nightly: No such file or directory (os error 2)
|
= note: selecting a toolchain with `+toolchain` arguments require a rustup proxy; see <https://rust-lang.github.io/rustup/concepts/index.html>

Rationale and extra context

Distros often install rustc directly, without a rustup wrapper. In that case, people using the distro rustc may see suggestions on stack overflow or elsewhere that won't work with their toolchain. We should improve the error message for them.

Originally reported in #104785.

Other cases

No response

Anything else?

Note that to test your change for this issue, you'll have to set channel = "stable" in config.toml; nightly reports a different error:

; $(rustup which rustc --toolchain nightly) +nightly -Zunstable-options
error: couldn't read +nightly: No such file or directory (os error 2)

error: aborting due to previous error

[jyn514]

It would also be nice to point people to how to install a nightly compiler; maybe something like

error: the option `Z` is only accepted on the nightly compiler
|
= help: consider switching to a nightly toolchain: `rustup default nightly`
= note: for more information about Rust's stability policy, see <https://doc.rust-lang.org/book/appendix-07-nightly-rust.html#unstable-features>

[jyn514]

This is non-trivial to fix because we don't have a handler this early in the compiler, so there's no record that the error was emitted; right now you'd have to juggle around an ErrorGuaranteed between rustc_session/interface/driver. I think I'd suggest implementing this by having a single handler used throughout rustc_driver::run_compiler, and calling abort_if_errors() just before the call to interface::run_compiler:

rust/compiler/rustc_driver_impl/src/lib.rs

Line 329 in 8ec84dd

interface::run_compiler(config, |compiler| {