ICE with *-*-ios targets without ios developer stuff installed

[wthrowe]

$ echo 'fn main() {}' | rustc - -Z unstable-options --pretty=normal --target=x86_64-apple-ios
error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
note: run with `RUST_BACKTRACE=1` for a backtrace
thread 'rustc' panicked at 'failed to get iphonesimulator SDK path: No such file or directory (os error 2)', src/librustc_back/target/apple_ios_base.rs:59

I don't expect to be able to actually compile anything with those targets, but it shouldn't ICE and most of the --pretty modes should probably work. (--pretty works with all other targets, except for --pretty=typed because that needs libstd.)

[cardoe]

This should likely improve with #34980.

[japaric]

Triage: Still happens with 1.11.0:

$ rustup run stable rustc -V
rustc 1.11.0 (9b21dcd6a 2016-08-15)

$ rustup run stable rustc --target aarch64-apple-ios --print cfg
error: internal compiler error: unexpected panic
note: the compiler unexpectedly panicked. this is a bug.
note: we would appreciate a bug report: https://github.com/rust-lang/rust/blob/master/CONTRIBUTING.md#bug-reports
thread 'rustc' panicked at 'failed to get iphoneos SDK path: No such file or directory (os error 2)', ../src/librustc_back/target/apple_ios_base.rs:59

Copying Alex's comment on a duplicate issue here so it doesn't get lost:

I believe this has essentially always been the case, but I'd love to have a fix! I think the best choice here is to delay the pieces of the iOS target that are fallible until we actually need them, for example at link time.

[cardoe]

@japaric The change I mentioned didn't land until 1.12.0. I've removed the explicit panic!() call and now its returning a Result which I believe is now handled gracefully. I'd love if you could test it for me.

$ rustup run beta rustc --target aarch64-apple-ios --print cfg

My Mac has Xcode installed now and I haven't had the time to uninstall it to test this.

[cardoe]

This is confirmed to not ICE and can be closed.

[alexcrichton]

Yay!