<div class="highlight highlight-source-rust notranslate position-relative overflow-auto" dir="auto" data-snippet-clipboard-copy-content="trait Foo { fn foo(&amp;self); }

macro_rules! foo {
    () =&gt; ();
    ($a:expr, $($n:expr,)*) =&gt; (
        impl Foo for [(); $a] {
            fn foo(&amp;self) {
                // Okay
                let baz: [i32; $a] = [$($n),*];
                // ICE
                let bar = [$($n),*];
            }
        }
        foo!($($n,)*);
    )
}

foo!(3, 2, 1, 0,);

fn main() {}"><pre class="notranslate"><span class="pl-k">trait</span> <span class="pl-smi">Foo</span> <span class="pl-kos">{</span> <span class="pl-k">fn</span> <span class="pl-en">foo</span><span class="pl-kos">(</span><span class="pl-c1">&amp;</span><span class="pl-smi">self</span><span class="pl-kos">)</span><span class="pl-kos">;</span> <span class="pl-kos">}</span>

<span class="pl-k">macro_rules!</span> foo <span class="pl-kos">{</span>
    <span class="pl-kos">(</span><span class="pl-kos">)</span> =&gt; <span class="pl-kos">(</span><span class="pl-kos">)</span><span class="pl-kos">;</span>
    <span class="pl-kos">(</span>$a<span class="pl-kos">:</span>expr, $<span class="pl-kos">(</span>$n<span class="pl-kos">:</span>expr,<span class="pl-kos">)</span><span class="pl-c1">*</span><span class="pl-kos">)</span> =&gt; <span class="pl-kos">(</span>
        <span class="pl-k">impl</span> <span class="pl-v">Foo</span> <span class="pl-k">for</span> <span class="pl-kos">[</span><span class="pl-kos">(</span><span class="pl-kos">)</span>; $a<span class="pl-kos">]</span> <span class="pl-kos">{</span>
            <span class="pl-k">fn</span> foo<span class="pl-kos">(</span>&amp;<span class="pl-smi">self</span><span class="pl-kos">)</span> <span class="pl-kos">{</span>
                <span class="pl-c">// Okay</span>
                <span class="pl-k">let</span> baz: <span class="pl-kos">[</span><span class="pl-smi">i32</span>; $a<span class="pl-kos">]</span> = <span class="pl-kos">[</span>$<span class="pl-kos">(</span>$n<span class="pl-kos">)</span>,<span class="pl-c1">*</span><span class="pl-kos">]</span>;
                <span class="pl-c">// ICE</span>
                <span class="pl-k">let</span> bar = <span class="pl-kos">[</span>$<span class="pl-kos">(</span>$n<span class="pl-kos">)</span>,<span class="pl-c1">*</span><span class="pl-kos">]</span>;
            <span class="pl-kos">}</span>
        <span class="pl-kos">}</span>
        foo!<span class="pl-kos">(</span>$<span class="pl-kos">(</span>$n,<span class="pl-kos">)</span><span class="pl-c1">*</span><span class="pl-kos">)</span>;
    <span class="pl-kos">)</span>
<span class="pl-kos">}</span>

<span class="pl-en">foo</span><span class="pl-en">!</span><span class="pl-kos">(</span><span class="pl-c1">3</span>, <span class="pl-c1">2</span>, <span class="pl-c1">1</span>, <span class="pl-c1">0</span>,<span class="pl-kos">)</span><span class="pl-kos">;</span>

<span class="pl-k">fn</span> <span class="pl-en">main</span><span class="pl-kos">(</span><span class="pl-kos">)</span> <span class="pl-kos">{</span><span class="pl-kos">}</span></pre></div>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="&lt;anon&gt;:11:27: 11:36 error: internal compiler error: cat_expr Errd
&lt;anon&gt;:11                 let bar = [$($n),*];
                                    ^~~~~~~~~"><pre class="notranslate"><code class="notranslate">&lt;anon&gt;:11:27: 11:36 error: internal compiler error: cat_expr Errd
&lt;anon&gt;:11                 let bar = [$($n),*];
                                    ^~~~~~~~~
</code></pre></div>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="rustc 1.0.0-nightly (91bdf23f5 2015-03-09) (built 2015-03-09)"><pre class="notranslate"><code class="notranslate">rustc 1.0.0-nightly (91bdf23f5 2015-03-09) (built 2015-03-09)
</code></pre></div>
<p dir="auto">Test case:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="fn main() { []; }"><pre class="notranslate"><code class="notranslate">fn main() { []; }
</code></pre></div>
<p dir="auto">Output:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="$ RUST_BACKTRACE=1 rustc test.rs 
test.rs:1:13: 1:15 error: internal compiler error: cat_expr Errd
test.rs:1 fn main() { []; }
                      ^~
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:         0xcc029720 - sys::backtrace::write::h66f3f2784d0f6c5ddgA
   2:         0xcc051b80 - panicking::on_panic::h6fdde9e4e9166fe6jaJ
   3:         0xcbf91c30 - rt::unwind::begin_unwind_inner::hba10c536ef2aae75MQI
   4:         0xc92a10c0 - rt::unwind::begin_unwind::h11287547919384241778
   5:         0xc92a1050 - diagnostic::SpanHandler::span_bug::h551f703868554ecfUBD
   6:         0xc9e32fe0 - session::Session::span_bug::h10f6b5fdbea6485fbjp
   7:         0xcb6c5920 - check::regionck::visit_expr::hc78138689559ecf8Yrd
   8:         0xcb6c35c0 - check::regionck::Rcx&lt;'a, 'tcx&gt;::visit_fn_body::h75e2fac33dc50b43a4c
   9:         0xcb75ee50 - check::check_bare_fn::hf42e5aabaa5cb880Oan
  10:         0xcb756f40 - check::check_item::hdbe569c101f96905stn
  11:         0xcb822f10 - check_crate::closure.36015
  12:         0xcb81d9c0 - check_crate::h9d44bf3fde687089EXB
  13:         0xcc6a38c0 - driver::phase_3_run_analysis_passes::h36c633d41f11ca74wFa
  14:         0xcc689ec0 - driver::compile_input::h0de317319ab5c701Hba
  15:         0xcc754e20 - run_compiler::hffa0c0711e361de8E5b
  16:         0xcc753720 - thunk::F.Invoke&lt;A, R&gt;::invoke::h12673221910893163557
  17:         0xcc752610 - rt::unwind::try::try_fn::h15130568597725528481
  18:         0xcc0bd740 - rust_try_inner
  19:         0xcc0bd730 - rust_try
  20:         0xcc752910 - thunk::F.Invoke&lt;A, R&gt;::invoke::h3924406537784056782
  21:         0xcc03d990 - sys::thread::thread_start::h3734be2229bd3edegEE
  22:         0xc5f650c0 - start_thread
  23:         0xcbc0a449 - __clone
  24:                0x0 - &lt;unknown&gt;"><pre class="notranslate"><code class="notranslate">$ RUST_BACKTRACE=1 rustc test.rs 
test.rs:1:13: 1:15 error: internal compiler error: cat_expr Errd
test.rs:1 fn main() { []; }
                      ^~
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:         0xcc029720 - sys::backtrace::write::h66f3f2784d0f6c5ddgA
   2:         0xcc051b80 - panicking::on_panic::h6fdde9e4e9166fe6jaJ
   3:         0xcbf91c30 - rt::unwind::begin_unwind_inner::hba10c536ef2aae75MQI
   4:         0xc92a10c0 - rt::unwind::begin_unwind::h11287547919384241778
   5:         0xc92a1050 - diagnostic::SpanHandler::span_bug::h551f703868554ecfUBD
   6:         0xc9e32fe0 - session::Session::span_bug::h10f6b5fdbea6485fbjp
   7:         0xcb6c5920 - check::regionck::visit_expr::hc78138689559ecf8Yrd
   8:         0xcb6c35c0 - check::regionck::Rcx&lt;'a, 'tcx&gt;::visit_fn_body::h75e2fac33dc50b43a4c
   9:         0xcb75ee50 - check::check_bare_fn::hf42e5aabaa5cb880Oan
  10:         0xcb756f40 - check::check_item::hdbe569c101f96905stn
  11:         0xcb822f10 - check_crate::closure.36015
  12:         0xcb81d9c0 - check_crate::h9d44bf3fde687089EXB
  13:         0xcc6a38c0 - driver::phase_3_run_analysis_passes::h36c633d41f11ca74wFa
  14:         0xcc689ec0 - driver::compile_input::h0de317319ab5c701Hba
  15:         0xcc754e20 - run_compiler::hffa0c0711e361de8E5b
  16:         0xcc753720 - thunk::F.Invoke&lt;A, R&gt;::invoke::h12673221910893163557
  17:         0xcc752610 - rt::unwind::try::try_fn::h15130568597725528481
  18:         0xcc0bd740 - rust_try_inner
  19:         0xcc0bd730 - rust_try
  20:         0xcc752910 - thunk::F.Invoke&lt;A, R&gt;::invoke::h3924406537784056782
  21:         0xcc03d990 - sys::thread::thread_start::h3734be2229bd3edegEE
  22:         0xc5f650c0 - start_thread
  23:         0xcbc0a449 - __clone
  24:                0x0 - &lt;unknown&gt;
</code></pre></div>
<p dir="auto">Compiler version:</p>
<div class="snippet-clipboard-content notranslate position-relative overflow-auto" data-snippet-clipboard-copy-content="rustc 1.0.0-nightly (b47aebe3f 2015-02-26) (built 2015-02-27)
binary: rustc
commit-hash: b47aebe3fc2da06c760fd8ea19f84cbc41d34831
commit-date: 2015-02-26
build-date: 2015-02-27
host: x86_64-unknown-linux-gnu
release: 1.0.0-nightly"><pre class="notranslate"><code class="notranslate">rustc 1.0.0-nightly (b47aebe3f 2015-02-26) (built 2015-02-27)
binary: rustc
commit-hash: b47aebe3fc2da06c760fd8ea19f84cbc41d34831
commit-date: 2015-02-26
build-date: 2015-02-27
host: x86_64-unknown-linux-gnu
release: 1.0.0-nightly
</code></pre></div>
<p dir="auto">I modified the test case in <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="59341575" data-permission-text="Title is private" data-url="https://github.com/rust-lang/rust/issues/22894" data-hovercard-type="issue" data-hovercard-url="/rust-lang/rust/issues/22894/hovercard" href="https://github.com/rust-lang/rust/issues/22894">#22894</a> by moving the <code class="notranslate">&amp;*""</code> into <code class="notranslate">fn main()</code>, and I saw the same ICE.  I then changed the <code class="notranslate">""</code> to the similar <code class="notranslate">&amp;[]</code>, and I saw an ICE with a different error message and a different stack trace (e.g. phase 3 instead of phase 4).  The <code class="notranslate">&amp;</code> and <code class="notranslate">*</code> are unnecessary.</p>
<p dir="auto">I also see the <code class="notranslate">cat_expr Errd</code> message in <a class="issue-link js-issue-link" data-error-text="Failed to load title" data-id="56894364" data-permission-text="Title is private" data-url="https://github.com/rust-lang/rust/issues/22037" data-hovercard-type="issue" data-hovercard-url="/rust-lang/rust/issues/22037/hovercard" href="https://github.com/rust-lang/rust/issues/22037">#22037</a>, but there are differences -- this issue happens in phase 3 rather than 4, and there is no "fictitious type" message.</p>