Regression on nightly: doctest on recursion_limit crashes rustdoc

[nodakai]

```compile_fail
#![recursion_limit="2"]

struct Foo;

impl Foo {
    fn foo(&self) {}
}

fn main() {
    let foo = Foo;
    let ref_foo = &&Foo;

    // error, reached the recursion limit while auto-dereferencing &&Foo
    ref_foo.foo();
}
```

It was extracted from doc/error-index.md.

$ multirust run stable rustdoc --test ~tmp/reclim.md

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured

$ multirust run stable rustdoc --version
rustdoc 1.7.0 (a5d1e7a59 2016-02-29)
$ multirust run beta rustdoc --test ~tmp/reclim.md

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured

$ multirust run beta rustdoc --version
rustdoc 1.8.0-beta.1 (facbfdd71 2016-03-02)
$ multirust run nightly rustdoc --test ~tmp/reclim.md

running 1 test
thread panicked while panicking. aborting.
zsh: illegal hardware instruction  multirust run nightly rustdoc --test ~tmp/reclim.md
$ multirust run nightly rustdoc --version
rustdoc 1.9.0-nightly (21922e1f4 2016-03-21)

Due to this crash, I couldn't run make check-docs on Git HEAD. (related: #31587 )

[nikomatsakis]

cc @brson

[nikomatsakis]

This is presumably specific to compile-fail?

[nikomatsakis]

triage: P-high

[nikomatsakis]

The illegal instruction is worrisome, might be stack overflow or something though?

[nikomatsakis]

Ah, I see it a double panic.

[alexcrichton]

To me this may not necessarily warrant P-high, the only way to trigger this is using #![recursion_limit] in a documentation test which seems... quite unlikely.