<p dir="auto">(Not exactly a show stopper here, but the message did say to file a bug report.)</p>
<p dir="auto">I tried this code:</p>
<div class="highlight highlight-source-rust notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="macro_rules! mack_row {
    hello =&gt; ()
}"><pre class="notranslate"><span class="pl-k">macro_rules!</span> mack_row <span class="pl-kos">{</span>
    hello =&gt; <span class="pl-kos">(</span><span class="pl-kos">)</span>
<span class="pl-kos">}</span></pre></div>
<p dir="auto">I expected to see a syntax error happen. Instead, this happened:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="hello.rs:1:1: 3:2 error: internal compiler error: wrong-structured lhs for follow check (didn't find a TtDelimited or TtSequence)
hello.rs:1 macro_rules! mack_row {
hello.rs:2  hello =&gt; ()
hello.rs:3 }
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 'Box&lt;Any&gt;', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:129"><pre class="notranslate"><code class="notranslate">hello.rs:1:1: 3:2 error: internal compiler error: wrong-structured lhs for follow check (didn't find a TtDelimited or TtSequence)
hello.rs:1 macro_rules! mack_row {
hello.rs:2  hello =&gt; ()
hello.rs:3 }
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 'Box&lt;Any&gt;', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:129
</code></pre></div>
<h2 dir="auto">Meta</h2>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="$ rustc --version --verbose
rustc 1.0.0-nightly (3b3bb0e68 2015-03-04) (built 2015-03-05)
binary: rustc
commit-hash: 3b3bb0e682c2d252e9f62dd9df5cff9552af91ad
commit-date: 2015-03-04
build-date: 2015-03-05
host: x86_64-unknown-linux-gnu
release: 1.0.0-nightly"><pre class="notranslate"><code class="notranslate">$ rustc --version --verbose
rustc 1.0.0-nightly (3b3bb0e68 2015-03-04) (built 2015-03-05)
binary: rustc
commit-hash: 3b3bb0e682c2d252e9f62dd9df5cff9552af91ad
commit-date: 2015-03-04
build-date: 2015-03-05
host: x86_64-unknown-linux-gnu
release: 1.0.0-nightly
</code></pre></div>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="$ RUST_BACKTRACE=1 rustc hello.rs 
hello.rs:1:1: 3:2 error: internal compiler error: wrong-structured lhs for follow check (didn't find a TtDelimited or TtSequence)
hello.rs:1 macro_rules! mack_row {
hello.rs:2  hello =&gt; ()
hello.rs:3 }
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 'Box&lt;Any&gt;', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:129

stack backtrace:
   1:     0x7feb32680f1f - sys::backtrace::write::hf79a3da4fdecb8a0OBA
   2:     0x7feb326abc32 - panicking::on_panic::h9f64f4c69e19f194hHJ
   3:     0x7feb325e0eda - rt::unwind::begin_unwind_inner::h37f4496c980fe936knJ
   4:     0x7feb2fa4babd - rt::unwind::begin_unwind::h8320268356453106285
   5:     0x7feb2fa4ba63 - diagnostic::SpanHandler::span_bug::h83c8af232eaba6a9h0D
   6:     0x7feb2fb8b4cf - ext::base::ExtCtxt&lt;'a&gt;::span_bug::h57adb13cf16fc804GA5
   7:     0x7feb2fb8afa2 - ext::tt::macro_rules::compile::h873b32c3d4368ef635f
   8:     0x7feb2fb8a373 - ext::base::ExtCtxt&lt;'a&gt;::insert_macro::haaccd9e27f928901yy5
   9:     0x7feb2fbf64ce - ext::expand::expand_item_mac::hd3b165c5aff186110rb
  10:     0x7feb2fbe444a - ext::expand::expand_annotatable::h378d1c124c198a916Ub
  11:     0x7feb2fbe05e1 - ext::expand::expand_item::h8a962e5f8fafea81Wlb
  12:     0x7feb2fbee09f - ext::expand::MacroExpander&lt;'a, 'b&gt;.Folder::fold_item::h556c21a26b784fb1hcc
  13:     0x7feb2fbede2f - iter::FlatMap&lt;I, U, F&gt;.Iterator::next::closure.60138
  14:     0x7feb2fbedbac - iter::FlatMap&lt;I, U, F&gt;.Iterator::next::h6910105070373715398
  15:     0x7feb2fbed817 - vec::Vec&lt;T&gt;.FromIterator&lt;T&gt;::from_iter::h8529730263212573627
  16:     0x7feb2fbed43e - fold::noop_fold_mod::h13574430500869506708
  17:     0x7feb2fbe9a48 - ext::expand::expand_item_underscore::h04b059e96977a16fBpb
  18:     0x7feb2fc484f2 - fold::noop_fold_item_simple::h11525352532286500933
  19:     0x7feb2fc4799e - ptr::P&lt;T&gt;::map::h14542742277292937439
  20:     0x7feb2fbe5f47 - ext::expand::expand_annotatable::h378d1c124c198a916Ub
  21:     0x7feb2fbe05e1 - ext::expand::expand_item::h8a962e5f8fafea81Wlb
  22:     0x7feb2fbee09f - ext::expand::MacroExpander&lt;'a, 'b&gt;.Folder::fold_item::h556c21a26b784fb1hcc
  23:     0x7feb2fc53954 - ext::expand::expand_crate::hbbabb26fe27c96b6Xhc
  24:     0x7feb32cec980 - driver::phase_2_configure_and_expand::closure.18893
  25:     0x7feb32ca3ca3 - driver::phase_2_configure_and_expand::h1b105a3027b471ffDsa
  26:     0x7feb32c94888 - driver::compile_input::h3913ff7013f0c056Iba
  27:     0x7feb32d5dcb7 - run_compiler::h28a4446bae1034e7H5b
  28:     0x7feb32d5b829 - thunk::F.Invoke&lt;A, R&gt;::invoke::h6503055919709693733
  29:     0x7feb32d5a4a0 - rt::unwind::try::try_fn::h1384674024000742916
  30:     0x7feb3271bde8 - rust_try_inner
  31:     0x7feb3271bdd5 - rust_try
  32:     0x7feb32d5ac3f - thunk::F.Invoke&lt;A, R&gt;::invoke::h5780663349966142752
  33:     0x7feb32696965 - sys::thread::thread_start::h4ab695857833a5dar8E
  34:     0x7feb2c52bb4f - start_thread
  35:     0x7feb3226ce6c - &lt;unknown&gt;"><pre class="notranslate"><code class="notranslate">$ RUST_BACKTRACE=1 rustc hello.rs 
hello.rs:1:1: 3:2 error: internal compiler error: wrong-structured lhs for follow check (didn't find a TtDelimited or TtSequence)
hello.rs:1 macro_rules! mack_row {
hello.rs:2  hello =&gt; ()
hello.rs:3 }
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 'Box&lt;Any&gt;', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:129

stack backtrace:
   1:     0x7feb32680f1f - sys::backtrace::write::hf79a3da4fdecb8a0OBA
   2:     0x7feb326abc32 - panicking::on_panic::h9f64f4c69e19f194hHJ
   3:     0x7feb325e0eda - rt::unwind::begin_unwind_inner::h37f4496c980fe936knJ
   4:     0x7feb2fa4babd - rt::unwind::begin_unwind::h8320268356453106285
   5:     0x7feb2fa4ba63 - diagnostic::SpanHandler::span_bug::h83c8af232eaba6a9h0D
   6:     0x7feb2fb8b4cf - ext::base::ExtCtxt&lt;'a&gt;::span_bug::h57adb13cf16fc804GA5
   7:     0x7feb2fb8afa2 - ext::tt::macro_rules::compile::h873b32c3d4368ef635f
   8:     0x7feb2fb8a373 - ext::base::ExtCtxt&lt;'a&gt;::insert_macro::haaccd9e27f928901yy5
   9:     0x7feb2fbf64ce - ext::expand::expand_item_mac::hd3b165c5aff186110rb
  10:     0x7feb2fbe444a - ext::expand::expand_annotatable::h378d1c124c198a916Ub
  11:     0x7feb2fbe05e1 - ext::expand::expand_item::h8a962e5f8fafea81Wlb
  12:     0x7feb2fbee09f - ext::expand::MacroExpander&lt;'a, 'b&gt;.Folder::fold_item::h556c21a26b784fb1hcc
  13:     0x7feb2fbede2f - iter::FlatMap&lt;I, U, F&gt;.Iterator::next::closure.60138
  14:     0x7feb2fbedbac - iter::FlatMap&lt;I, U, F&gt;.Iterator::next::h6910105070373715398
  15:     0x7feb2fbed817 - vec::Vec&lt;T&gt;.FromIterator&lt;T&gt;::from_iter::h8529730263212573627
  16:     0x7feb2fbed43e - fold::noop_fold_mod::h13574430500869506708
  17:     0x7feb2fbe9a48 - ext::expand::expand_item_underscore::h04b059e96977a16fBpb
  18:     0x7feb2fc484f2 - fold::noop_fold_item_simple::h11525352532286500933
  19:     0x7feb2fc4799e - ptr::P&lt;T&gt;::map::h14542742277292937439
  20:     0x7feb2fbe5f47 - ext::expand::expand_annotatable::h378d1c124c198a916Ub
  21:     0x7feb2fbe05e1 - ext::expand::expand_item::h8a962e5f8fafea81Wlb
  22:     0x7feb2fbee09f - ext::expand::MacroExpander&lt;'a, 'b&gt;.Folder::fold_item::h556c21a26b784fb1hcc
  23:     0x7feb2fc53954 - ext::expand::expand_crate::hbbabb26fe27c96b6Xhc
  24:     0x7feb32cec980 - driver::phase_2_configure_and_expand::closure.18893
  25:     0x7feb32ca3ca3 - driver::phase_2_configure_and_expand::h1b105a3027b471ffDsa
  26:     0x7feb32c94888 - driver::compile_input::h3913ff7013f0c056Iba
  27:     0x7feb32d5dcb7 - run_compiler::h28a4446bae1034e7H5b
  28:     0x7feb32d5b829 - thunk::F.Invoke&lt;A, R&gt;::invoke::h6503055919709693733
  29:     0x7feb32d5a4a0 - rt::unwind::try::try_fn::h1384674024000742916
  30:     0x7feb3271bde8 - rust_try_inner
  31:     0x7feb3271bdd5 - rust_try
  32:     0x7feb32d5ac3f - thunk::F.Invoke&lt;A, R&gt;::invoke::h5780663349966142752
  33:     0x7feb32696965 - sys::thread::thread_start::h4ab695857833a5dar8E
  34:     0x7feb2c52bb4f - start_thread
  35:     0x7feb3226ce6c - &lt;unknown&gt;
</code></pre></div>
<p dir="auto">A simple malformed macro makes the compiler crash.</p>
<p dir="auto">Code:</p>
<div class="highlight highlight-source-rust notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="macro_rules! code_block {
    $c:block =&gt; $c
}"><pre class="notranslate"><span class="pl-k">macro_rules!</span> code_block <span class="pl-kos">{</span>
    $c<span class="pl-kos">:</span>block =&gt; $c
<span class="pl-kos">}</span></pre></div>
<p dir="auto">Playpen error:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="&lt;anon&gt;:1:1: 3:2 error: internal compiler error: wrong-structured lhs for follow check (didn't find a TtDelimited or TtSequence)
&lt;anon&gt;:1 macro_rules! code_block {
&lt;anon&gt;:2     $c:block =&gt; $c
&lt;anon&gt;:3 }
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 'Box&lt;Any&gt;', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:130"><pre class="notranslate"><code class="notranslate">&lt;anon&gt;:1:1: 3:2 error: internal compiler error: wrong-structured lhs for follow check (didn't find a TtDelimited or TtSequence)
&lt;anon&gt;:1 macro_rules! code_block {
&lt;anon&gt;:2     $c:block =&gt; $c
&lt;anon&gt;:3 }
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 'Box&lt;Any&gt;', /home/rustbuild/src/rust-buildbot/slave/nightly-dist-rustc-linux/build/src/libsyntax/diagnostic.rs:130
</code></pre></div>
<p dir="auto">Playpen: <a href="http://is.gd/COoZe7" rel="nofollow">http://is.gd/COoZe7</a></p>