Yet another thread 'main' panicked Rust gotcha -- mind your build.rs

Picking up the little Rust project I'd set aside in late summer, with the latest release, I got

thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Error { repr: Os { code: 2, message: "The system
 cannot find the file specified." } }', ../src/libcore\result.rs:837

Setting environment variable RUST_BACKTRACE to 1, and retrying gave

stack backtrace:
   0:   0xf314b7 - std::panicking::default_hook
                at C:\bot\slave\stable-dist-rustc-win-msvc-32\build\src\libstd\panicking.rs:263
   1:   0xf31caa - std::panicking::rust_panic_with_hook
                at C:\bot\slave\stable-dist-rustc-win-msvc-32\build\src\libstd\panicking.rs:451
   2:   0xf31b47 - std::panicking::begin_panic<collections::string::String>
                at C:\bot\slave\stable-dist-rustc-win-msvc-32\build\src\libstd\panicking.rs:413
   3:   0xf31a86 - std::panicking::begin_panic_fmt
                at C:\bot\slave\stable-dist-rustc-win-msvc-32\build\src\libstd\panicking.rs:397
   4:   0xf319e2 - std::panicking::rust_begin_panic
                at C:\bot\slave\stable-dist-rustc-win-msvc-32\build\src\libstd\panicking.rs:373
   5:   0xf36f82 - core::panicking::panic_fmt
                at C:\bot\slave\stable-dist-rustc-win-msvc-32\build\src\libcore\panicking.rs:69
   6:   0xf22402 - core::result::unwrap_failed<std::io::error::Error>
                at C:\bot\slave\stable-dist-rustc-win-msvc-32\build\src\libcore\macros.rs:29
   7:   0xf21d0f - core::result::Result<std::process::ExitStatus, std::io::error::Error>::unwrap<std::process::ExitStatu
s,std::io::error::Error>
                at C:\bot\slave\stable-dist-rustc-win-msvc-32\build\src\libcore\result.rs:737
   8:   0xf23b5e - build_script_build::main
                at c:\Users\steve\hg\Projects\rust\HelloWorld\build.rs:8
   9:   0xf3556b - panic_unwind::__rust_maybe_catch_panic
                at C:\bot\slave\stable-dist-rustc-win-msvc-32\build\src\libpanic_unwind\lib.rs:97
  10:   0xf322f0 - std::rt::lang_start
                at C:\bot\slave\stable-dist-rustc-win-msvc-32\build\src\libstd\rt.rs:51
  11:   0xf23d01 - main
  12:   0xf39768 - __scrt_common_main_seh
                at f:\dd\vctools\crt\vcstartup\src\startup\exe_common.inl:253
  13: 0x75168e93 - BaseThreadInitThunk
  14: 0x772d9bc2 - RtlDestroyQueryDebugBuffer

which wasn't particularly helpful.

Resolution was an inspired guess -- my build.rs contained a path which ran a command by explicit path into the windows SDK for an x64 machine, when I was trying on an old 32-bit box.

Fixing the path to be selected according to whether the file actually exists resolved the issue.