Suggest use of --print target-list when target is not found.

[ghost]

If given target could not be found suggest using --print target-list.
Previously, error has been reported as:

$ rustc --target 86-unknown-linux-gnu
error: Error loading target specification: Could not find specification for target "86-unknown-linux-gnu"

After changes it looks as follows:

$ rustc --target 86-unknown-linux-gnu
error: Error loading target specification: Could not find specification for target "x86-unknown-linux-gnu". Use --print target-list for a list of supported targets

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @arielb1 (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[nagisa]

This needs a more careful wording. --print target-list does not print all targets that would be accepted in any given environment. There’s custom json targets, RUST_TARGET_PATH and similar stuff, all of which are not covered by --print target-list.

What this should be saying instead is:

error: Error loading target specification: Could not find specification for target "x86-unknown-linux-gnu". Use `--print target-list` for a list of built-in targets.

Better off you could make the note/help an actual note:

error: Error loading target specification: Could not find specification for target "x86-unknown-linux-gnu". 
help: Use `--print target-list` for a list of built-in targets.

For that you’ll need to convert this code to use the DiagnosticBuilder. Namely the code would end up looking something like

panic!(sp.struct_fatal("Error loading…").help("Use `--print target-list`…").emit());

[ghost]

Thanks @nagisa. I updated the message and moved it to help sub-section. Though, I had to manually panic with errors::FatalError, otherwise main driver does not understand that panic is not unexpected (this happens because emit() doesn't return anything).

[nagisa]

r=me

[eddyb]

@bors r=nagisa

[bors]

📌 Commit 443f1ca has been approved by nagisa

[bors]

⌛ Testing commit 443f1ca with merge 5556554...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-cargotest, auto-linux-64-cross-freebsd, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-opt-mir, auto-linux-64-opt-rustbuild, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-opt, auto-mac-64-opt-rustbuild, auto-mac-cross-ios-opt, auto-win-gnu-32-opt, auto-win-gnu-32-opt-rustbuild, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-cargotest, auto-win-msvc-64-opt, auto-win-msvc-64-opt-mir, auto-win-msvc-64-opt-rustbuild
Approved by: nagisa
Pushing 5556554 to master...