Beta regression: Option::unwrap() on a None value during macro expansion

[iliekturtles]

This issue started around the same time as #40631 which I initially thought was the real cause. Problem exists in beta and nightly on both TravisCI and AppVeyor.

/d/Code/Test/rust/uom (master)$ rustc +beta -vV
rustc 1.17.0-beta.2 (b7c276653 2017-03-20)
binary: rustc
commit-hash: b7c27665307704a9b158fe242e88e83914029415
commit-date: 2017-03-20
host: x86_64-pc-windows-msvc
release: 1.17.0-beta.2
LLVM version: 3.9
/d/Code/Test/rust/uom (master)$ RUST_BACKTRACE=1 \cargo +beta build --verbose
       Fresh typenum v1.6.0
   Compiling uom v0.11.0 (file:///D:/Code/Test/rust/uom)
     Running `rustc --crate-name uom 'src\lib.rs' --crate-type lib --emit=dep-info,link -C debuginfo=2 --cfg 'feature="f64"' --cfg 'feature="si"' --cfg 'feature="default"' --cfg 'feature="f32"' -C metadata=1ce58
2e5bebbe787 -C extra-filename=-1ce582e5bebbe787 --out-dir 'D:\Code\Test\rust\uom\target\debug\deps' -L 'dependency=D:\Code\Test\rust\uom\target\debug\deps' --extern 'typenum=D:\Code\Test\rust\uom\target\debug\de
ps\libtypenum-9b41dcbaf77f6ff6.rlib'`
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 'called `Option::unwrap()` on a `None` value', src\libcore\option.rs:323
note: Some details are omitted, run with `RUST_BACKTRACE=full` for a verbose backtrace.
stack backtrace:
   0: <std::time::SystemTimeError as core::fmt::Display>::fmt
   1: std::panicking::Location::line
   2: std::panicking::Location::line
   3: std::panicking::rust_panic_with_hook
   4: std::panicking::begin_panic_fmt
   5: std::panicking::begin_panic_fmt
   6: rust_begin_unwind
   7: core::panicking::panic_fmt
   8: core::panicking::panic
   9: syntax::ext::placeholders::PlaceholderExpander::add
  10: <syntax::ext::placeholders::PlaceholderExpander<'a, 'b> as syntax::fold::Folder>::fold_expr
  11: syntax::feature_gate::UnstableFeatures::is_nightly_build
  12: syntax::feature_gate::UnstableFeatures::is_nightly_build
  13: syntax::feature_gate::UnstableFeatures::is_nightly_build
  14: <unknown>
  15: <syntax::tokenstream::TokenStream as core::convert::From<syntax::tokenstream::TokenTree>>::from
  16: syntax::feature_gate::UnstableFeatures::is_nightly_build
  17: syntax::feature_gate::UnstableFeatures::is_nightly_build
  18: <unknown>
  19: <unknown>
  20: <syntax::tokenstream::TokenStream as core::convert::From<syntax::tokenstream::TokenTree>>::from
  21: syntax::feature_gate::UnstableFeatures::is_nightly_build
  22: syntax::feature_gate::UnstableFeatures::is_nightly_build
  23: <unknown>
  24: <unknown>
  25: <syntax::tokenstream::TokenStream as core::convert::From<syntax::tokenstream::TokenTree>>::from
  26: syntax::feature_gate::UnstableFeatures::is_nightly_build
  27: syntax::feature_gate::UnstableFeatures::is_nightly_build
  28: <unknown>
  29: <unknown>
  30: <syntax::tokenstream::TokenStream as core::convert::From<syntax::tokenstream::TokenTree>>::from
  31: syntax::feature_gate::UnstableFeatures::is_nightly_build
  32: syntax::feature_gate::UnstableFeatures::is_nightly_build
  33: syntax::feature_gate::UnstableFeatures::is_nightly_build
  34: <syntax::ext::placeholders::PlaceholderExpander<'a, 'b> as syntax::fold::Folder>::fold_item
  35: <unknown>
  36: syntax::ext::expand::Expansion::make_impl_items
  37: syntax::ext::placeholders::PlaceholderExpander::add
  38: syntax::ext::expand::MacroExpander::expand_crate
  39: syntax::ext::expand::MacroExpander::expand_crate
  40: rustc_driver::driver::count_nodes
  41: rustc_driver::driver::count_nodes
  42: rustc_driver::driver::compile_input
  43: rustc_driver::run_compiler
  44: <unknown>
  45: _rust_maybe_catch_panic
  46: <unknown>
  47: std::sys::imp::thread::Thread::new

error: Could not compile `uom`.

Caused by:
  process didn't exit successfully: `rustc --crate-name uom src\lib.rs --crate-type lib --emit=dep-info,link -C debuginfo=2 --cfg feature="f64" --cfg feature="si" --cfg feature="default" --cfg feature="f32" -C m
etadata=1ce582e5bebbe787 -C extra-filename=-1ce582e5bebbe787 --out-dir D:\Code\Test\rust\uom\target\debug\deps -L dependency=D:\Code\Test\rust\uom\target\debug\deps --extern typenum=D:\Code\Test\rust\uom\target\
debug\deps\libtypenum-9b41dcbaf77f6ff6.rlib` (exit code: 101)

[arielb1]

cc @jseyfried

[jseyfried]

@iliekturtles Thanks for the report!
This was caused by #40220, fixed in #40813.